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Accelerator Loses Function in Jeep

10/02/2012 6:19 PM

As I'm driving down the road, suddenly my accelerator becomes useless, the Jeep jerks as it drastically slows down and I begin to coast. Pushing on the gas pedal does nothing. The engine continues to run as if in neutral, I still have my power steering and radio. The check engine light goes on only during the incident, then goes off again as soon as the engine "engages" again. There is no other unusual mechanical noise. Actually, I just said it seems in neutral, but upon further thought, it seems to decelerate even quicker than it would, if I had just put it in neutral. It feels more like a downshift from 4th to 3rd gear... but the RPMs don't increase. They decrease.

For the past six weeks or so, it would occasionally happen for about half a second, and the vehicle would jerk (my body would jerk forward). It happened maybe one or two times a week, sometimes less. One time it happened while I was at a red light, but only for that half second; so I couldn't really tell if the engine was going to die.

But today it got worse, and twice, within three minutes, it happened again... but it lasted a good 4 or 5 seconds. It was quite alarming, as it was on a single lane road with no shoulder, during evening rush hour. Luckily they guy behind me wasn't quite close enough to rear end me during my quick deceleration.

Had it checked right away with a meter at the auto parts store. They found no bad codes.

I put in some fuel injector cleaner and crossed my fingers for the last 6 miles home. Nothing happened during that time.

The vehicle is a '99 Jeep Wrangler 4x4, 6 cyl., 4.0

The guy at the auto parts had some suggestions, but anything he could think of, he says would have showed up as an error code on the meter. (ie fuel pump) I've considered the fuel filter (it's the internal gas tank, sock type). Is there anything I've described that might eliminate the fuel filter as the culprit?

FYI, not sure if it's relevant, but I'll throw it out there... my driveway is EXTREMELY steep, and I drive down it in 1st gear, with considerable strain on the tranny every morning. It's a manual trans.

The situation is somewhat alarming, as there is nowhere to pull over during most of my daily ride. If the fuel injector cleaner didn't fix it, it could eventually end badly.

Suggestions?

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#1

Re: Accelerator Loses Function in Jeep

10/02/2012 7:10 PM

Power to the fuel injectors is good? Sounds like your engine is shutting down, but the clutch is still engaged so you do slow down, maybe?

I'm betting 10:1 that the cleaner won't help. I doubt that it's the fuel filter, unless the sock is so dirty that it is getting sucked into the filter inlet?

You don't say how many miles, under what conditions driven or if the fuel filter has ever been changed.

And it's a Jeep. It loves steep. Forget about the driveway.

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#2
In reply to #1

Re: Accelerator Loses Function in Jeep

10/02/2012 7:25 PM

Yes, it's as you described... the engine is shutting down, it seems. Although, as I said, the last two episodes this evening, it lasted 4 or 5 seconds... and the engine didn't completely shut off in that time. The injectors seem to be working, when I check them... but it's intermittent, so can't really tell.

The engine block is 10,000 miles old. Everything else is about 190,000 miles old. Fuel pump was checked about 8000 miles ago. Was fine, so wasn't replaced. And they couldn't help but check filter at same time, since it's right there. It's in the tank with the pump so it's a pain to get to, with my resources. Since the new block was put in, it's mostly been highway and country road miles.

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#3
In reply to #2

Re: Accelerator Loses Function in Jeep

10/02/2012 7:36 PM

I'm having trouble with fuel starvation. It isn't necessarily instantaneous, as an electrical problem would be.

You say, "the engine didn't completely shut off in that time". Is that because the drive train was still engage and the tires were driving the engine?

What makes the long intervals different from the .5 second ones?

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#4
In reply to #3

Re: Accelerator Loses Function in Jeep

10/02/2012 7:45 PM

Yes, I suppose the tires were driving the engine.

Short vs long... before today, it was always short... half second or so. So I couldn't really tell what was happening. It was just a jerk while driving. Could have been many different reasons. But with it finally lasting several seconds, I realized the engine was shutting down, in one sense or other... seemingly fuel related. But I still cant be sure. Could be some type of sensor problem?

Let me mention once again, that it did happen while idling at a red light once. But also very quick, and couldn't really tell what was happening, but seemed to be shutting down for that bit.

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#5
In reply to #2

Re: Accelerator Loses Function in Jeep

10/02/2012 9:26 PM

Fuel filter needs a look-in.

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#17
In reply to #2

Re: Accelerator Loses Function in Jeep

10/03/2012 11:05 PM

In motor vehicles is there a meter to count the number of hrs the engine has operated similar to the counter for mileage?.

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#18
In reply to #17

Re: Accelerator Loses Function in Jeep

10/03/2012 11:26 PM

I've never seen an hour meter in a passenger type vehicle here, only industrial and farm equipment type machinery and vehicles. I would assume that everything related to the passenger vehicles here could be monitored be the odometer milage. (service periods, etc.)

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#6

Re: Accelerator Loses Function in Jeep

10/02/2012 11:44 PM

Dirty fuel filter problems usually manifest themselves at high speed when the demand for fuel is greatest and the clogged filter just won't let anymore through, the engine bucks because it's running too lean, then as the speed drops and the flow re-establishes itself, the engine runs fine as long as that threshold speed isn't reached, plus the low fuel rail pressure and/or lean running would set a trouble code.

What you're describing is more closely aligned with the ignition system or the computer, reason being that these are short duration events that quickly clear themselves. If it were the fuel system the times would be longer since it takes time for the pressure to bleed down.

I had a Volvo 740 Turbo that exhibited a similar problem, random cut-outs with quick recovery, they replaced the spark control module 3 times until they discovered that one of the wires leading to the module was routed by a sharp piece of metal that slowly chewed through the wire. The wire grounded and the module and cut out the ignition, absolutely terrifying at highway speeds to push on the accelerator and have the engine not respond. Once the engine untorqued the wire would move and the engine could be restarted, it got progressively worse and was found when the wire welded to the sharp edge.

I've worked on the Jeep 6, start checking all the wiring to and from the ECM and the ICM paying particular attention to where the wiring is flexed or near any body or engine metal and, of course, check all the connectors and ground connections, especially since the engine was replaced.

As I recall on the Grand Cherokee the "coil on plug" system is used and there is a connector that should have a captive anti-release tab on it, if it's not reinstalled properly the connector will separate and the ignition is lost but the dumb ECM doesn't know it so no code is set.

In your case with a manual transmission when the engine stops vibrating and the contact remakes the forward motion restarts the engine. Not so in most automatics because once the engine stops hydraulic pressure is lost so there's no coupling from the wheels through the transmission back to the engine.

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#9
In reply to #6

Re: Accelerator Loses Function in Jeep

10/03/2012 10:42 AM

RAMconsult, thank you for all that info. It could be useful as I work to try to get this figured out.

Additional info:

Most of the time I got the short hiccup, or the 5 second shutdown, I was traveling at 45 mph or 35 mph. Except the one time while I was idling.

The block replacement was almost two years ago.

Let me ask this question... what would the reaction of the vehicle be if particles of debris were getting momentarily caught in the injection nozzle, and then breaking free? Would I get the quick jerk I described? And the longer incident... as I described? Or would the engine completely shut off within those 5 seconds?

Quite honestly, I'm hoping the fuel injector cleaner solved the problem (I drove 15 miles this morning with no incident, but as I said, this only happens a couple times a week so far).

But as you and lyn and others have suggested... electrical issues would cause the instantaneous power-down and power-up, and a fuel related would not be so instantaneous? Is that correct?

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#13
In reply to #9

Re: Accelerator Loses Function in Jeep

10/03/2012 12:14 PM

I'm with RC on this one. You have six injectors, so debris in one or two might cause a miss, but not a complete shutdown. The fuel pressure regulator would be a single-point failure, but I don't think it would give your symptoms. I think you're looking for one of those wicked intermittent electrical problems.

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#14
In reply to #9

Re: Accelerator Loses Function in Jeep

10/03/2012 1:51 PM

More than likely the fuel is not the problem, as others have pointed out a single injector failure will result in lower power, not a loss of power. However there is the possibility that the connector that powers the injectors is intermittent, you'll get a total loss of fuel until it reconnects.

You state that the engine doesn't quit entirely so I'm going to assume that it drops to idle speed. That probably rules out a lot of the intermittent stuff except for one thing that was mentioned previously, the TPS (Throttle Position Sensor). When you press on the accelerator pedal you move a cable that is wrapped around the throttle valve. Attached to that you'll see a round black housing, inside the housing is the TPS. It's purpose is to tell the ECM how far down you're depressing the pedal, it's nothing more than a rotary variable resistor. If it gets dirty, loose, or worn badly then the ECM doesn't get the proper signal so the engine "doesn't follow your foot". It's relatively easy to check but you'll need a simple Volt-Ohm-Meter (VOM) to check the smoothness of the resistance change as the pedal moves. You may also find that the TPS is mechanically loose or binding when you start exploring. Either way you'll get symptoms like you describe.

One more thing about the computer, we'd like to believe that it monitors "everything" about the engine but it doesn't. Many of the codes are set by indirect observation of the result, not the action, of a part. For example there is no direct measurement of individual sparkplug voltage, but if a plug is misfiring the knock sensor will "hear" the difference in the fuel burn and the computer will "know" which cylinder is having the problem. If it persists for a preset time then it will set the trouble code, it may even clear it if the computer determines that it was only a transient problem, that's why you need a OBD reader that has a snapshot function to tell you in real-time what's going on throughout the system. They're inexpensive and will save you much more than their cost when you go to a mechanic.

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#15
In reply to #14

Re: Accelerator Loses Function in Jeep

10/03/2012 2:48 PM

Does this sound like the OBD Reader that will do what you're describing? So you're saying I just plug it in and drive with it in place until the issue happens again?

Actron Auto Scanner OBD II Code Reader

Part No. CP9575

$129.99

Auto Scanner is a trilingual (select English, Spanish, or French display) scan tool that will read and erase Diagnostic Trouble Codes as well as pending codes, freeze frame engine data and live engine data and the VIN number. The tool allows the user to view engine-sensor, switch and relay inputs in real time to help pinpoint problem components faster.

Product Features:

  • Recommended for use with OBD II compliant vehicles made in 1996 or later; scan tool is CAN compliant
  • Large, 8-line LCD screen displays Diagnostic Trouble Codes (DTCs); retrieves all generic OBD II codes and their descriptions, contains over 15,000 generic and manufacturer-specific code definitions
  • User-friendly menu with trilingual display (English, French and Spanish)
  • Reads and erases DTCs; turns off check engine light; monitors status and I/M readiness to determine whether the vehicle is ready for an emissions test
  • Displays a freeze frame data snapshot of vehicle's operating conditions at time of fault
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#46
In reply to #15

Re: Accelerator Loses Function in Jeep

10/05/2012 7:46 PM

The price may seem high for this, but trust me that if you were to try half of the suggestions you have heard here, you would be out 10 times that price. Just try not to become fixated with it, and its results. Remember that it only reads sensors and other electrical items. A coil that fails will only read as a random engine misfire. A vacuum leak will read lean engine mixture, rich fuel setting, and may have random engine misfire. You must still interpret the results. But, it is an invaluable tool for todays cars.

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#43
In reply to #9

Re: Accelerator Loses Function in Jeep

10/05/2012 7:20 PM

Yes. Fuel injector clogged will not clear itself without help. One injector failing or clogging will just yield one dead cylinder.

If the car had a failed fuel pump, and the car lost power for 4-5 seconds, it would have shut off, not remained at an idle. Try driving at 45-50, and turn the ignition off and then back on as fast as you can. If you do try this, please do it somewhere that is void of other cars. The recovery time is longer than "just a hiccup".

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#7

Re: Accelerator Loses Function in Jeep

10/03/2012 12:24 AM

You stated that it has happened while at a stop once. Does the engine quit, and need to be restarted with the key, or does the engine just go to idle speed?

Are you handy, or rich?

A decent scanner will be able to read soft codes that disappear after the cause goes away. The story of the shorted wire on the Volvo is an example that should have been in the computer's memory even after the wire moved away. The computer should have interpreted it as low voltage to that component, and set that code in its memory.

Because you never mentioned restarting, I am guessing that it only returns to an idle speed, so here goes. You have a throttle position sensor on your Jeep. Do you see anything that could cause an interruption in the signal it sends to the computer? Loose connector, loose mounting screws (If the sensor is not mounted tight against the moving throttle shaft, it could become separated from the throttle shaft, and spring back to the idle position. When you return the gas pedal to the idle position, it would again be aligned, and might work again till it works loose. Fits your clues.) Any harness connections that may have been disturbed at engine change. Some of these harness connectors have retaining tabs of plastic that get brittle with age. If one of these retaining tabs broke during the block replacement, the harness may separate under driving vibrations. One cure for this is to buy 6" long wire ties in the thinnest size. Use the tie to secure the connectors together.

If these DIY efforts do not yield any results, find a repair facility that has a scanner with "snapshot" function. When connected to your vehicle, and driven, it will constantly monitor all functions present in the computer. When a code is tripped, it will take a snapshot of the functions to compare to what should have been there.

Any of the engine speed sensors could also give the instant idle condition. Vehicle speed sensors could also trigger it under certain conditions, but not if vehicle is not moving. Good luck. Do some looking before throwing parts at it. Let us know as testing continues.

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#8
In reply to #7

Re: Accelerator Loses Function in Jeep

10/03/2012 10:30 AM

bob c, thanx for the well thought out reply.

Unfortunately, automotive electrical harnesses are not my strong point, and a DIY on checking what you suggested is unlikely. But I do like the snapshot sensor suggestion you made. I will be looking into that, to see who might have one of those. That would be ideal.

No, the engine has never quit entirely, but I don't know if it would have if the condition lasted longer than the 5 seconds. Both times, I was concentrating more on not getting rear-ended, and wasn't focused on all the observations that I wish I could have made.

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#42
In reply to #8

Re: Accelerator Loses Function in Jeep

10/05/2012 7:13 PM

Come on. Take a shot at it. Just look at the throttle body. It is what is moving under the hood as the gas pedal is moved. Look for a small switch that would be able to see the throttle being opened. See if it is loose.

I would not expect the fuel to be an issue. If the fuel pump was bad, and the engine starved for fuel, it should sputter out as fuel pressure lowers. If the fuel pump started to work again, it would come back to life like yours does. But, how many times do you think that that pump would come back to life by itself just before the engine stalled? From your description, it has happened to you every time.

Fuel filter? Nope. Once it is clogged, what would ever allow it to become clear?

Clogged or failed injector? NOPE. Look at the chances of the fuel pump coming back to life each and every time. Now multiply that by 6 for each injector. If it was only one injector failing, it would just be a miss, and a loss of 1/6 of its power.

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#10

Re: Accelerator Loses Function in Jeep

10/03/2012 11:31 AM

I'm reaching here...but isn't the throttle body mechanically controlled? If it were electronically controlled you may have air starvation. The symptoms are nearly identical to fuel starvation.

Does this year have a limp mode? If the computer is throwing it in and out of limp it would exhibit similar symptoms.

You could also be losing spark due to occasional flashover on the coil wire. Check for burn or pit marks on the wire. The coil itself could also be suspect. Actually the distrubutor setup could also be faulty with intermittent interruption on the low voltage side. It could be something as simple as loose wires. They don't necessarily show any fault when they aren't being bounced around (i.e., when using a code reader).

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#11
In reply to #10

Re: Accelerator Loses Function in Jeep

10/03/2012 11:35 AM

Good suggestions. Distributor and coil wires are something I can check on my own. Will do. Thanx.

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#12
In reply to #11

Re: Accelerator Loses Function in Jeep

10/03/2012 12:05 PM

You can throw a timing light on it and drive around with it. If the problem occurs and you see a loss of spark signal...

That would be kind of annoying, so you could just loop a coil and an led driver...same effect. Anyway, a simple way to check for spark while you are driving down the road.

After looking at the ignition wiring diagram, I would check out the fuel pump relay. The ignition control module feeds the low voltage to the coil and sets the relay. Check the plugs on that module, it controls both paths...spark and fuel.

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#45
In reply to #12

Re: Accelerator Loses Function in Jeep

10/05/2012 7:33 PM

That would be kind of annoying?????

I suppose you never got pulled over for having a flashing light inside your car at night.

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#54
In reply to #45

Re: Accelerator Loses Function in Jeep

10/10/2012 10:54 AM

It's not permanent.

And I wouldn't make the recommendation to troubleshoot a stalling vehicle when it's dark, either.

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#44
In reply to #10

Re: Accelerator Loses Function in Jeep

10/05/2012 7:30 PM

Flash-over, or failure of the high voltage coil system would be load sensitive. And if he were to keep his foot in the throttle, as the load fell off from loss of spark energy, the engine will sputter back to life, and stay at that sputtering point till a move is made in the throttle. That is, if the throttle is released, the car will idle as the OP 's does. But if the throttle is pushed, the engine will just take longer till it reaches the stutter point. And at some point, the unburned fuel will probably ignite, causing an exhaust explosion. And how do you explain the same thing happening at an idle?

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#55
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Re: Accelerator Loses Function in Jeep

10/10/2012 10:58 AM

The load does not drop off from high speed to idle, only the frequency. The stoichiometric ratio does not change enough for the engine to manage spark in a vehicle from 1999.

Grounding out a circuit does not necessarily remove all spark energy from the cylinder.

Corona discharge from wet wires often causes this type of issue.

Your responses seem rather trollish since you are offering criticism on so many posts here, but don't offer much in the solution area.

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#57
In reply to #55

Re: Accelerator Loses Function in Jeep

10/11/2012 12:04 AM

I would rather help the OP test to isolate the problem than to just quote past results other people have had. What fixed one person's problem may not the cure for another person's problem. On this forum, I have always tried to isolate and test.

As far as the load not changing when backing off of the throttle. Well, I may have every term wrong, but I can assure you that an ignition coil, spark plug, spark plug wire , rotor, distributer cap, condenser, or ignition control module is sensitive to the amount of throttle opening. The lower the manifold vacuum , the more likely the ignition component is to break down, or miss-fire.

And to all that I may have offended with my "trollish" answers, I apologize. While I of course do not know everything there is to know about every problem, I can often recognise a poor path for the poster to follow. Just as a noise that is heard when stopped at a light will not be a driveshaft, or wheel bearing.

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#59
In reply to #57

Re: Accelerator Loses Function in Jeep

10/16/2012 3:54 PM

Manifold vacuum in a '99 Jeep with the 258 (4.0) will have no effect on spark strength. Ported vacuum is used to control spark advance and retard, not manifold vacuum.

Breaker point setting on much older vehicles may affect it, but not in the case of an electronically controlled ignition system.

Older vehicles with a variable ballasting resistor can be used to control spark strength vis-a-vis duration, but this is not the case here either.

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#60
In reply to #59

Re: Accelerator Loses Function in Jeep

10/16/2012 4:37 PM

Manifold vacuum in a '99 Jeep with the 258 (4.0) will have no effect on spark strength. Ported vacuum is used to control spark advance and retard, not manifold vacuum.


You still seem to be missing Bob's point. He was not talking about ignition timing; he was talking about load and cylinder pressures.


Every spark ignition engine experiences varying amounts of manifold vacuum, and manifold vacuum has been used as an indication of engine load for about a century. Bob is using manifold vacuum as an indication of load: low manifold vacuum translates to high manifold absolute pressure. Load, in turn, affects cylinder pressure, and cylinder pressure affects plug firing voltage*. That last relationship is so well-established that there are efforts underway to use spark voltage to measure cylinder pressures in the interests of more precise engine control.


His point has absolutely nothing to do with how spark timing is controlled.


*and therefore, as he says, increasing load shows up weaknesses in the entire ignition system, because the breakdown voltage is higher.

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#58
In reply to #55

Re: Accelerator Loses Function in Jeep

10/11/2012 2:51 PM

The load does not drop off from high speed to idle, only the frequency.


I gather you've never used a Sun scope.


The stoichiometric ratio does not change enough for the engine to manage spark in a vehicle from 1999.


I wonder what you meant here. The engine does not manage its own spark in any car from 1990 on (and many from long before then). The spark timing is managed by the ECU, and is managed for things like idle speed control, avoiding detonation, optimization for warmup, optimization for efficiency, etc., etc.


In any spark ignition engine (old and new) spark plug firing voltage varies with cylinder pressure (which depends upon compression ratio and cylinder filling... which in turn depends upon throttle position, induction efficiency, air pressure and temperature, etc.)


Your responses seem rather trollish since you are offering criticism on so many posts here, but don't offer much in the solution area.


Bob is helping to weed out the implausible solutions. Often eliminating the implausible causes of a problem can streamline troubleshooting and diagnoses in any system. ("Doctor, I have a cough. Is it lung cancer?" "No, probably not." ) This is a technique used in expert systems and often taught in troubleshooting courses.


For example, a clogged injector can obviously not cause the described symptom. Having the OP get his injectors tested would be waste of time and money. Had the OP instead complained that his engine sometimes seems a little rough, throws codes, and has slightly lower power than normal, then a clogged injector could be a possible cause.


Many years ago, I built an expert system for a very large candy company that would troubleshoot a packaging machine, based mainly on the answers a tech would provide to video prompts ("Do the panel lights look like this.... or this.) The techs were reluctant to use "the machine" because they feared it would put them out of work. One night, three techs worked for an entire 8 hour shift (on a machine for which downtime was in the many thousands of dollars per hour range) attempting to fix the machine, using (obviously) poor troubleshooting techniques, jumping from one wrong conclusion to the next. The next morning, the manager came in, fired up "the machine" and had the packaging machine running in 15 minutes. This machine had hundreds of inputs and outputs to several PLCs, and a shutdown might have 30 plausible causes (and 50 implausible but not impossible causes). The expert system was effectively an idiot, but a very methodical and logical idiot, checking the most probable causes first.

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#16

Re: Accelerator Loses Function in Jeep

10/03/2012 10:38 PM

Had a similar problem witha 1997 Range Rover. Engine light did not come on and it turned out to be an intermiitant crankshaft position sensor, I should have just changed anyway but I used an oscilloscope and sure enough when fault was there the signal from the sensor just dissappeared.

Unfortunatley tracking down faults like this are really tricky, I sometimes wire in multimeters to a suspect area and then drive vehicle until fault occurs and hope I get readings that confirm or eliminate the area. Note Computer systems will not pick some faults depends on the progarmming of the engine management systems.

good luck on your fault finding

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#19

Re: Accelerator Loses Function in Jeep

10/04/2012 12:02 AM

I haven't had a chance to read all the replies on this topic, so I apologise if someone else has come up with this solution already.

There may be an intermittent fault in the temperature thermistor that will cause a cut in the power to the injectors. It most probably wouldn't show up in any diagnostics and would only show as a reduction in the injector power when the thermistor goes on the blink.

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#20
In reply to #19

Re: Accelerator Loses Function in Jeep

10/04/2012 12:30 AM

Had the same problem with my ford, after spending $600.00 at ford dealer, I fixed it my self with a fifteen dollar temp sensor.

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#21

Re: Accelerator Loses Function in Jeep

10/04/2012 12:34 AM

Throttle position sensor is my guess...

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#22

Re: Accelerator Loses Function in Jeep

10/04/2012 2:11 AM

One possibility not yet suggested is a small split in the intake hose between the airflow meter and the manifold. Occasional bypassing of the airflow meter will shut don engine revs. This will get progressively worse as the split enlarges. These are usually concertina-type hoses, and the split can be invisible until removed and stretched a bit - the split occurs in the bottom of a groove. A loose jubilee clip at either end of it has the same effect.

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#23

Re: Accelerator Loses Function in Jeep

10/04/2012 4:28 AM

Have you checked the vacuum hoses. All of them?

The MAP sensor on the firewall is prone to failure on Jeeps. (vacuum device)

There are two fuel filters, the "sock" in the tank and an inline filter can...have another look.

The work you had done on your fuel pump recently may have developed an intermittent ground.

Check the pump's connectors.

Intermittent faults are not easy to pin down.

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#24

Re: Accelerator Loses Function in Jeep

10/04/2012 6:47 AM

Have you considered a open in the tps reference wire back to the CPU. In 1996 the government mandated that all cars have an OBD2 CPU program. It has a limp home, and a block learn. If the open was on the reference back to the comp it could think you took your foot off the gas which would give the symptoms you describe. Because the car does not die ,and if the CPU has compensated for ware is the tps(block learn), then the open would have the same effect at idle.

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#26
In reply to #24

Re: Accelerator Loses Function in Jeep

10/04/2012 7:16 AM

I would check the fuel pump ground left rear quarter panel 4.0 l and computer grounds rear of engine. The police package also has a ground near glove box lamp. I would clean them with sand paper reinstall and coat with grease.

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#25

Re: Accelerator Loses Function in Jeep

10/04/2012 7:15 AM

One thing no one has mention is water in the gas.

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#47
In reply to #25

Re: Accelerator Loses Function in Jeep

10/05/2012 7:51 PM

Water in the gas is not going to come and go. When the engine is supplied water instead of fuel, it dies. What are the chances that there was just enough water to make it come to an idle, but not stall every time that it has happened to him?

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#27

Re: Accelerator Loses Function in Jeep

10/04/2012 7:19 AM

I personally would look in to the (TPS) throttle position sensor I had one on a van that went bad not totally out mind you just intermittent. Was a real pain to track down, on these jeeps they also use wide band O2 sensors that also can go intermittent, it can not determine the fuel to air ratio and it will also cause this problem. the (TPS) is usually $15-$45 dollars and about 10-15 minutes to change the O2 sensor is another story anywhere form $50-$400 just because it is a wide band sensor but it can be tested with a volt ohm meter as it warms up to operating temp and also while you drive you can set the meter in the passenger seat for a quick look when it happens the next time. Not to say this is your issue but a good place to begin and rule out before you spend a lot of time and money with a dealer or repair shop. One last thing there are several good Jeep forums that someone there might have already had this problem and done the legwork for you. Duke

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#28

Re: Accelerator Loses Function in Jeep

10/04/2012 7:25 AM

could be water in your gas, you can get additives at the auto parts store to remove it, or the more drastic drain the gas tank and refill.

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#29

Re: Accelerator Loses Function in Jeep

10/04/2012 10:00 AM

The crankshaft position senor will do these things and it doesn't show up on a scan unless it does it while the scanner is hooked up.

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#65
In reply to #29

Re: Accelerator Loses Function in Jeep

09/30/2015 7:47 PM

h2om, you were 100% correct! And also correct that it didn't show up unless it happens when scan is hooked up, which I wasn't able to make happen until 3 years later when it got more and more common.

GA long time coming!

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#70
In reply to #65

Re: Accelerator Loses Function in Jeep

09/30/2015 9:33 PM

Wow! We rarely get feedback at all - but after 3 years, that's exceptional! GA to h20m and thanks for the feedback.

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#30

Re: Accelerator Loses Function in Jeep

10/04/2012 10:04 AM

I appear to be getting a lot of sensor related suggestions.

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#31
In reply to #30

Re: Accelerator Loses Function in Jeep

10/04/2012 10:32 AM

Well, you can know for sure. Using your repair manual, connect the probes to the tps sensor as specified. Manipulating the throttle by hand, slowly give it gas (engine off). You are looking for a reading that suddenly jumps OR the reading going to overload. It should be be smooth and linear like a regular potentiometer.

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#32
In reply to #31

Re: Accelerator Loses Function in Jeep

10/04/2012 10:52 AM

This will work, even though the problem is intermittent? It hasn't happened for two days now. Usually once or twice every week or two. But as I said, up until two days ago, it was only a hiccup, and the jeep bucked. But the other day it lasted for many seconds and I lost all acceleration, in the middle of traffic.

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#48
In reply to #32

Re: Accelerator Loses Function in Jeep

10/05/2012 7:56 PM

Yes, that will work, but it will only show up while the malfunction is present. Will you be able to see it when it happens if you are driving, and watching the road and finding that emergency switch? Not a safe plan in my eyes if it is that hard to make it happen. JMHO.

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#33

Re: Accelerator Loses Function in Jeep

10/04/2012 10:52 AM

Ok so most of what has been said so far is good. I would bet you its not your fuel pump or filter. What you are describing sounds very electrical to me. So I then see a post that leads me to believe you have rebuilt or had the engine rebuilt

If this is the case please go through your wires very carefully and be sure one did not get pinched, crushed, damage during this process. It may not even be visible from outside the wiremolding, Look for evidence of pinching and crushing or pulling on the wire mold, insulation, and tape. Even if it does not look like it broke the outerlayer the wire inside could have broken and as you drive with vibration and heat the two ends ocassionally disconnect shutting off your ECM/PCM or even just the TPS or MAF momentarily. This will not throw code nor will it avail itself at a dealership.

I had a similar problem on an 08 town and country. First it would not start. I would shut it off to gas up or run into a store and come out and it would not even crank. no code, no fault indication. Wait 10 minutes to an hour and it would start right up.This condition got slowly worse over time untiil it started shutting down while driving. At this point it spent three weeks at the dealership where they drove it daily until it finally failed for them. But by the time the towed it back to the dealership it started right up no problem. After several expensive component replacements our service manager made a few calls and found out the van had been in frontal collision that was repaired in a questionable shop and so it did not make it to Carfax. Once we learned that I sold the van. There are so many places in the front of a new Caravan that a wiring harness could have been damaged or pulled or chaffed its crazy. We figured that the cost pulling all of the harnesses and subharnesses and checking them out would exceed the value of the vehicle.

Wiring problem is where I would look.

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#34

Re: Accelerator Loses Function in Jeep

10/04/2012 11:11 AM

I once had a Voyager minivan that behaved the same way. I also owned an OBDII tool at the time & it always reported nothing. I eventualy solved the problem by discovering the heavy guage ground wire lead from the battery to the chassis was corroded at the chassis connection. Once I cleaned that up I never had the problem again.

Insofar as OBD codes were concerned, none would be left since the computer was in reality, just being turned on & off.

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#37
In reply to #34

Re: Accelerator Loses Function in Jeep

10/04/2012 12:30 PM

I don't have total electrical failure, as in a shorting battery cable... only insofar as it applies to my accelerator pedal. But as you and others have mentioned, it may be a grounding wire somewhere. That's the last thing I want it to be. My skill-set doesn't include knowledge to track that kind of thing down.

I'll see what I can find with the snapshot function of an OBD scanner first, and go from there, I guess.

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#49
In reply to #34

Re: Accelerator Loses Function in Jeep

10/05/2012 8:01 PM

An OBD system uses multiple power and ground connections. If one of them goes bad, usually the codes will read a few items with a low battery supply voltage( when one power wire is lost) or high battery supply on a few items.(if loss of ground)

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#35

Re: Accelerator Loses Function in Jeep

10/04/2012 11:15 AM

The check engine light goes on only during the incident, then goes off again as soon as the engine "engages" again.

Good info. This can be captured with the freeze frame mode that even pretty cheap scanners have. This will probably tell you fairly clearly what the problem is.


A failing O2 sensor should not give you this symptom. Most cars will run OK with the O2 sensor disconnected. They run in open loop instead of closed loop -- meaning that fuel efficiency will suffer.


The fact that your driveway is steep means that a lot of water could accumulate in the tank, and once the truck is more or less level this could get sloshed into the fuel pump intake. You can usually find water in the tank (if there is not an obvious way to drain it from the lowest pont) with a long vinyl tube and a means for drawing fuel into it. By fiddling with truck positioning you can often get the tube to the lowest point in the tank.


Often, with a failing TPS, the engine will idle fine, but not respond to throttle. Many TPS sensors are potentiometer type (rather than hall sensors) so their contacts can wear.

A clogged injector will not cause this symptom, but instead will cause a slight reduction in power (from the one cylinder not running). But definitely getting freeze frame info would be a huge help.

I assume you Jeep has cable throttle, but it is possible that it has electronic throttle -- very easy to check, of course.

You may want to be prepared to immediately hit the four way flashers when this happens. With a sensitive foot you can also tap the brakes enough that the lights come on without causing further deceleration. Definitely leave plenty of room when getting into or crossing traffic.

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#36
In reply to #35

Re: Accelerator Loses Function in Jeep

10/04/2012 12:24 PM

Good stuff. I think I'll just go ahead and buy a scanner. That will always be useful. So just to be clear... I can plug it in, turn it on and just drive with it like that for however long it takes for it to happen again?

Regarding the driveway... I wasn't clear. I park in the garage at the top of that steep driveway. The garage is level. So I'm not parked on the incline.

And yes, I've already zeroed in on the 4-way flasher tab. Will be interesting driving till I fix this thing.

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#39
In reply to #36

Re: Accelerator Loses Function in Jeep

10/04/2012 3:40 PM

I think I'll just go ahead and buy a scanner. That will always be useful. So just to be clear... I can plug it in, turn it on and just drive with it like that for however long it takes for it to happen again?


Yep. I have a ScanGauge II which is fun, because it also gives you lots of mpg functions. For your current use, you could set one of the 4 "gauges" that it displays to TPS, so you could see if the TPS is giving an odd reading at the time of the malfunction. Another could be set to ignition timing. Another to MAP. Etc.


Of course the usual disclaimers apply -- it is easy to put all your attention on the scanner, causing you to drive off the road, taking out an entire elementary school, a church, two banks and yourself.

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#40
In reply to #39

Re: Accelerator Loses Function in Jeep

10/04/2012 3:56 PM

Does your SGII have the "snapshot" function that has been mentioned?

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#41
In reply to #40

Re: Accelerator Loses Function in Jeep

10/04/2012 5:34 PM

The "snapshot" is usually called "freeze frame", I believe. The ScanGauge supports freeze frame.

Access to some runtime data (such as reading O2 sensor voltage as you drive) requires setting up "X Gauges" a term unique to the ScanGauge, as far as I know. These are software gauges beyond the 12 (MAP, TPS, intake air temp, load, ignition timing, etc,) that are readily available just by pushing a button to cycle through them. I have not yet used the X gauge functions, but apparently, they do not give you complete access to everything going on with the ECU. Here is a list for Chrysler products.

The ScanGauge is more expensive than some scanners, and not because it is a robust tool, but more so because it does the fuel economy and HP calculations. Mine has worked fine and has been very useful, but I imagine that something from Snap-on (etc) would be more robust.

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#38

Re: Accelerator Loses Function in Jeep

10/04/2012 1:59 PM

This old geezer has been messing with cars, computers, and all sorts of mechanical stuff for most of my 76 years. SO, start with the idea that your old pump is in partial failure due to stiffened (leaking) gaskets, membranes, and perhaps more than a bit of water and rust/sand crud at the bottom of the tank. Now, if YOU opened things up to have a look-see, that introduces a whole new area of uncertainty. Human error and old parts are a risky combo. Often messing with this kind of old car parts introduces more errors. I'm guessing that the pump is failing. I have the same intermittent "konk-out" with my '98 Infinity and gave the gas tank a double dose of "fuel system cleaner". So far only one repeat "die, or almost die" event, so stay tuned for future conclusion...

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#50
In reply to #38

Re: Accelerator Loses Function in Jeep

10/05/2012 8:09 PM

Why out of all the possibilities do you focus only on the fuel pump? And if you are that convinced of that, why not test with a pressure gauge, or better yet a STDP adjustable pressure switch to watch under and over pressure on the fuel system?

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#51

Re: Accelerator Loses Function in Jeep

10/05/2012 8:11 PM

Thanx bob c. Good feedback on the various comments. That's what I need.

I can do a brake job, or replace a radiator or linkage... but electronics and even electrical systems are usually Greek to me. But the weekend is here and I'll try whatever suggestions that I can do, alone in my garage. I like the idea of owning an OBD scanner, and will probably order or buy one tomorrow. It seems a useful device to have in any case.

I welcome any more feedback from anyone, and will keep you all updated if I find a solution.

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#56
In reply to #51

Re: Accelerator Loses Function in Jeep

10/10/2012 1:22 PM

I previously told you where the ground wires terminate. A loose ground can cause all these problems. You pulled the engine and could easily have not tightened the computer grounds on back of the engine. Also check the fuel pump grounds I talked to one man who had two fuel pumps replaced at the dealer because of a loose ground wire. And he didn't get a refund.

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#52

Re: Accelerator Loses Function in Jeep

10/05/2012 11:09 PM

Why didn't I think of this before? It's your ground wire for your fuel pump inside the fuel tank. I repaired it for a friend once, soldered it back on. It was so frustrating until we dropped the gas tank and saw the intermittent short. Did it at a gas station, actually, in their parking lot. You can check to see if you have fuel pressure by pressing the schrader valve located on one of the fuel rails to see if any juice comes out. But you must immediately turn the ignition off and coast to a stop. If you cycle the key you could actually re energize the fuel pump if the wire makes contact, and it will seem like you have fuel pressure.

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#53
In reply to #52

Re: Accelerator Loses Function in Jeep

10/10/2012 7:53 AM

CR4 Admin: Spam: This post was modified because it contained advertising outside the Commercial Space forum. Please review Section 14 of the CR4 Site FAQ about advertising.

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#61

Re: Accelerator Loses Function in Jeep

10/16/2012 6:30 PM

Boy faults like this are a real pain and in the last few months I have had two faults that were similar to complete loss of power. And being intermittent . They were electric fuel pumps. The strange thing was both faults were intermittent. Even closing a door was enough to send a shock through the car that would disturb I guess the contact of the brushes with the commutator.

The resistance reading of both pumps would vary from hundreds of ohms eg 200 Ohms to 600 Ohms ,the 2k then O/C, the trouble is when benching testing contact resistance with the test probes makes confirming faults like this really difficult. But sure enough when I tapped the pumps they would come good and then a jolt again the pump would fail. I was going crazy. It was only when I got two brand new pumps one for a four wheel drive and the other for a passenger car that showed a good pump measured less than a few Ohms. Of course your particular fault or faults for that matter could be something else. But a check of the DC resistance of your pump may point you in the right direction and hopefully not that hard to eliminate.

All the best. Alan

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#62

Re: Accelerator Loses Function in Jeep

01/04/2013 6:57 PM

Wow! How old is this Jeep? I would believe that your spark plugs were missing under load. Most new automobiles compensate by increasing the voltage to that plug. But in years gone bye, plugs, coil, condenser, or distributer would be suspect. I would look for an ignition problem today also even in todays high fangled Jeeps.

I rebuild a carburetor several times, and it ran like a top each time for a little time, then bam - the engine acted like it was starved for fuel! - It was the coil wire. So even though it seems to be a fuel problem don't overlook the other possibilities!

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#63

Re: Accelerator Loses Function in Jeep

09/30/2015 6:59 PM

SOLVED

SUCCESS! Finally... 2 days short of 3 years since I asked this question, and I finally have the answer. I've browsed through the 62 previous comments, suggestions and helpful ideas, and as much as I greatly appreciated everyone's efforts, I don't believe anyone hit the mark (although I may have missed it). And neither did several garages that tried many things.

The culprit?...... Faulty Crankshaft Position Sensor. It was putting the lobe of the cam in the wrong position. Quite honestly, I've never heard of a Crankshaft Position Sensor.

I spent $173.30 today, and my Jeep purrs like a kitten. No issues. No jerking, stalling, slowing. I'm guessing my mpg will also increase, and I have no more worries of hiway speed stalling or conking out at the red lights. Best money I ever spent. Oh, and she just turned over 200,000 miles last month. I just can't believe none of the mechanics working on it have solved this mystery until now.

Happy day!

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#64
In reply to #63

Re: Accelerator Loses Function in Jeep

09/30/2015 7:41 PM

Well now that the problem is solved, it seems that post 29 was the first to mention the crank sensor. He deserves the GA. And I feel I owe you an apology. I have known the crank sensors to be questionable, and I did not mention it. But it does fit the symptoms as it does not set hard codes unless the ohms readings are wrong. Did you put a shim on the new one when installing it? It looks like a round sticker. It hits the flywheel and is rubbed off, leaving the correct air gap.

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#67
In reply to #64

Re: Accelerator Loses Function in Jeep

09/30/2015 7:52 PM

Oops, my bad... I didn't notice that h2om had indeed mentioned the crank sensor 3 years ago. He absolutely gets a GA. I have to go back and see if I missed others with that opinion. But, as h2oh stated... it wasn't able to be found unless it happened while a scan was hooked to it, which was the problem. It took all this time for the problem to get bad enough that it was able to be caught during an actual scan. He probably deserves a double GA for that fact as well.

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#68
In reply to #67

Re: Accelerator Loses Function in Jeep

09/30/2015 8:10 PM

Actually, that was the clue. When no code is set, suspect the crank sensor. It does not set codes. That is why I owe you the apology, I should have picked up on that.

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#66
In reply to #63

Re: Accelerator Loses Function in Jeep

09/30/2015 7:48 PM

In my first post #7, I mentioned the engine speed sensors as possible, and then left the subject. Well, the engine speed sensors are the camshaft sensor, and the crankshaft sensor.

The crankshaft sensor tells the ECM the speed the engine is at so the injectors, and the spark plugs fire at the same speed as the engine is turning.

The camshaft sensor tells the ECM where the engine is in the firing order so the correct spark plug and injector fire on the compression stroke near TDC.

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#69
In reply to #66

Re: Accelerator Loses Function in Jeep

09/30/2015 8:16 PM

Eh.... that's close enough for #7 to get a GA as well. It was the right answer in a sense. I wasn't able to check that though, for the reasons I'd mentioned.

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