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35 comments
Commentator

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Good Answers: 1

94 GMC 4.3

07/25/2010 4:40 PM

Have owned this truck a couple years and made all necessary repairs since but have encountered a challenge on this one.

It's a 94 GMC Sierra. 4.3 Liter Throttle Body Injected Automatic. Mileage is 200+k and usually rides like a new one.

Everything seems 100% functional in that the truck will start right up, idle down to about 500 RPM like normal and then, after a varying time period (say 1 to 3 minutes) surges to 2,000 (if it doesn't flood out and die). Sometimes, it will begin "feathering" back down towards the 500 mark, sometimes not...

I have tested all the sensors, egr, pvc, etc. and have no fault codes indicated.

Lacking a OBD scanner but have all the usual shade tree tools.

Was looking for the ladder logic of the controller for possible conditions that would "flood" engine with fuel and open the IAC.

Dang... just occurred to me that I did NOT check the CPS (crankshaft position sensor) and the CMP (camshaft position sensor). My shade tree logic is telling me that if the controller sees less than target RPM = then more air/fuel same as it would when it sees more target RPM = less air/fuel. (Closed loop PID)

Anyone else "nail" a similar problem with this truck's drivability problem?

Too bad we can't troubleshoot in reverse! Last things first that is...

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#1

Re: 94 GMC 4.3

07/25/2010 5:57 PM

Many times a questionable coolant temp sensor will fail or give intermittent readings and tell the ECM the engine is COLD and needs more fuel and higher warm up RPM. A problem like this will usually turn on the C/E lite , but not always, depending on degree of fault.

Also , a questionable alternator will not output enough and the ECM will raise the RPM in an attempt to compensate.

I'm sure you are going to get plenty of help here, its a good place to start.

h

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#2
In reply to #1

Re: 94 GMC 4.3

07/25/2010 6:37 PM

already tested... sensor (thermistor) OK... simulated temperature with potentiometer to rule out loose connections.

Note: unplugging the temperature sensor while idling has the same effect on the circuit that I'm trying to eliminate.

Corroded or loose connection somewhere? Where (on the input side) to look?

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#3

Re: 94 GMC 4.3

07/25/2010 7:54 PM

There are 2 different fuel injection systems used on the 4.3 during these years. On is the simple throttle body fuel injection system. The second, and better performing system uses a large intake manifold, and a fuel manifold inside it. It is commonly called a spider, because of the lines running away from the single inlet point. These spiders are prone to leakage. This will cause the system to go very rich. If you do not have the throttle body injection system, and you pull the plugs out, and they are covered in wet fuel, you may have a leak inside the intake manifold. he fuel mileage will drop like crazy if you do. It is not a terrible job to replace, but it is harder than changing a Holly 4BBL. Don't buy parts till you test. Many auto parts stores will test your computer for codes for free. Good luck.

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#4
In reply to #3

Re: 94 GMC 4.3

07/25/2010 8:37 PM

Nope...throttle bodied. Has been running great untill this gremlin moved in. Thanks though!

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#6
In reply to #4

Re: 94 GMC 4.3

07/26/2010 2:12 AM

Since on first cold start all control engine control inputs are relegated to the fuel map by the initial reading of the coolant temp sensor most surging is due to mechanical problems:

poor or inadequate injector spray pattern

miniscule niggling little vacuum leaks (get access to a smoke machine to detect)

deposits in the throttle plate/bore

A weak or erratic signal from the CPS or CKS will likely cause -a no start or intermittent stall while driving.

Without looking at the live data stream from the OBD port you can never be certain of the signals the control unit sees. Depending on location and construction of connectors you can sometimes back probe the connection for gross errors (it's open/short fo voltage/ground, or signal/no signal is present) but if you're looking for an intermittent open or a signal discrepancy of 10 mV then just disturbing a connection can put you in the weeds.

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#7
In reply to #4

Re: 94 GMC 4.3

07/26/2010 8:51 AM

"It's a 94 GMC Sierra. 4.3 Liter Throttle Body Injected Automatic." I guess I should have read more carefully? Sorry.

If you are good when cold, but get bad when warmed up, I can think that the closed loop fuel monitoring may be off.

Some of the things that will cause an engine to go rich are the temperature sensors detecting a colder environment than is actually there. But if one of the temp sensors was giving a signal that the computer was colder than a car could be, it would trip the coolant sensor out of range code, and check eng light would be on.

The O2 sensor could be bad, or covered with soot causing a lean reading, which would send a signal to fuel to richen. But that also should set the check eng light.

A fuel injector that is dirty, and giving a poor spray pattern can end up dumping extra fuel, but it should trip the engine over rich code, and bring on that light again.

If you have a timing light, you can set it up as normal, and shine it on the injector. If you look at the injector pattern, you can see an injector that is dripping, or has a poor pattern.

Other than that, I think you are going to need to read the trouble codes in the memory.

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#5

Re: 94 GMC 4.3

07/26/2010 12:38 AM

Check the catalytic converter - mine went west around 200K and emptied itself into the muffler - partially plugging it. Had some very strange symptoms until we disconnected the cat!

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#8

Re: 94 GMC 4.3

07/26/2010 11:01 AM

Thanks to all for replying so promptly. All good information but still looking for the guy who has been down this road before. (been there, done that)

Would love to take it to a shop for diagnosis but lacking money and being ten miles away from the nearest shop is a problem. I could "limp mode" drive it to town but then how to pay for service? Unemployment can be inconvenient at times.

Just started it up and it idled right down to 5-600 RPM. Wiggled wiring harnesses looking for loose connections with negative result. After a minute or so, fault occurs causing idle air & fuel pulse jump to 1500 -2000 RPM and then gradually idle back down to the normal idle. Sometimes it will idle back down, sometimes not. %$#%$$@!

Loose terminations somewhere keeps coming to mind as you can simulate the same anomaly by unplugging the temperature sensor.

Wish me luck!

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#9
In reply to #8

Re: 94 GMC 4.3

07/26/2010 11:42 AM

How about a no cost test? while the truck is cold, crawl under and disconnect the O2 sensor. That should force it to stay in open loop. If the condition does not repeat, you might have a bad O2 sensor. Your truck may have more than one of them. Pull the wire on both for testing. If that works, replug one of them, and retest. Process of elimination.

I don't think that looking for the guy that had the same problem, and copying his repair is a good way for you to go. As I said, there are many things that can cause the poor running when in closed loop mode. It John had a bad temp sensor, and Frank had a bad injector, both could have the same symptoms, yet John's repair would not have fixed Frank's car. Same thing if reversed. I will try to see if there is a way to read codes with no reader. Older vehicles could do this.

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#10

Re: 94 GMC 4.3

07/26/2010 1:51 PM

There have been cases where a good size vacuum leak will cause a similar result. A thorough check of entire the intake system is in order. Picture this, progress to closed loop, o2 sensor picks up lean condition, ECM corrects it with more fuel higher RPM, TPS says reduce RPM by its signal, ECM again responds and cycle repeats. Sounds logical from where I'm sittin ,but I could be out on the end of the branch too!

h

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#13
In reply to #10

Re: 94 GMC 4.3

07/26/2010 10:12 PM

Guest H

The problem I see with the vacuum leak theory is that the OP claims the engine is acting correct while warming up. If it was bad enough to run that fast in closed loop, it should be running fast on open look. IMHO.

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#18
In reply to #13

Re: 94 GMC 4.3

07/27/2010 8:07 PM

Mr. Bob, I just know you will correct me if I am wrong. When the Ecm is in open loop there is no automatic control of fuel mixture other than what is programed into the prom. This is why I felt once it goes into closed loop the ECM will try to respond to its input signals which could be affected by a vacuum leak. Thanks for making me think , it's so exhausting these days.

Also I believe the mixture in open loop is possibly determined by input from map and tps sensors. I have to quit now , I'm exhausted.

h

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#20
In reply to #18

Re: 94 GMC 4.3

07/27/2010 10:24 PM

"I'm exhausted." I had a muffler with that same problem.

Before closed loop, the fuel and timing are determined by the ECM, based on predetermined information fed to the ECM at build time. The ECM will collect information from the throttle position sensor, MAP sensor, coolant temp sensor, air charge sensor, vehicle speed, gear position, brake pedal position and whatever else I forgot. The ECM will compare all that information, and compare it to what it was told it should set fuel and timing at. When the engine warms up sufficiently, The ECM will do exactly the same things as in open loop, but, will add into the info mix, what the oxygen sensor reads, and make fine adjustments accordingly to maintain desired exhaust emission levels.

So if there is a vacuum leak when in open loop, the engine will act as if the throttle is opened farther than at idle. The ECM will notice the increased engine speed, and restrict the throttle bypass valve in an attempt to slow the engine speed down to where it knows it should be. If the vacuum leak is small enough, the engine may come back down to the desired speed. But, the Throttle position sensor signal, and the engine speed will not be within what the ECM knows they should be. This should trip the Check Engine light to come on with the throttle position sensor out of range as a code, or the throttle bypass valve out of range.

So, the same things that caused the engine to speed up in closed loop, should have caused the same speed increase in open loop. At least, that is how I would expect the system to respond to a vacuum leak. Only time and the OP will tell. Cheers.

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#11

Re: 94 GMC 4.3

07/26/2010 9:36 PM

i have a 92 blazer 4x4 4.3 vortec w/ air plentum and spider .what controls drivability after 2 min. or when o2 sensor gets to 600 f. is as follows ...

coolent temp sensor, map sensor ,air intake temp sensor,o2 sensor,throttle position sensor a couple of these share ground or ref. voltage .....look for prob. at tps or map connectors.... also veh. speed sensor at tranny( but you will notice drivability issues pulling hills passing )....... i did get a bad tps new, took a week to trouble shoot . it is possible that the egr valve is sticking intermitenly....i did find 2 vac lines had rubbed holes in them by distributer (spelling?) if its 4x4 my front axle is engaged w/ vac from under batt. tray relay+switch. hope that helps ... pulled my hair out after 500.00 spider didnt fix it..and i trouble shoot for a living ... they are known for leaking... runnin rich

a bad connection at map,tps.. try sliding them just partway apart and back together when prob. is active.....

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#12
In reply to #11

Re: 94 GMC 4.3

07/26/2010 10:08 PM

After going through the spider replacement, How would suggest that another person diagnose a leaking spider? Is there a vacuum port that might allow one to cover with clear plastic and see the fuel leaking?

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#14

Re: 94 GMC 4.3

07/27/2010 3:44 PM

Please check the mounting gasket under the throttle body, this is what I was referring to re; vacuum leak. Have had bad experience with these mounting gaskets leaking after many miles. I don't mean a visual check because you can't see it , you need to use carb/chole cleaner to see if engine speed changes when that area is sprayed.Thanks

h

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#15
In reply to #14

Re: 94 GMC 4.3

07/27/2010 6:05 PM

You are very right. just last month, we had a F150 with a vacuum leak under the carb spacer. Back in the 70s the GM 4 barrel engines were very prone to this. I thought they had solved that problem by now.

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#16

Re: 94 GMC 4.3

07/27/2010 7:37 PM

if you remove the vortek emblem cover plate from the intake plentum there is a round valve thats called the varible intake tunning valve it changes the manifold from duel plane to single plane for torque at diffrent driving conditions. anyway remove it and you can see in .... youll see top of injector w/ elec.plug and you can see around if you climb all the way in the eng. compartment.... if its leaking usually it will be washed clean most find it half clean shiny and half dirty black.. good autoparts(napa) stores can sell you,,,,,, reg, inj.,o rings in pieces alltho most say its an unservicable unit sold as an asembly.................i think my problem was mor likly the spider tubes leaking w/ maybe stuck poppit,,,or reg oring leak

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#19
In reply to #16

Re: 94 GMC 4.3

07/27/2010 9:59 PM

Good answer.

I have a friend that has had the spider replaced once, and repaired again after that. I will look into exploring the access through the tuning valve. If his goes bad again at least I will be able to look for the problem while still assembled.

I have looked into the parts from NAPA for these spider setups and thought they could be repaired also. Thanks.

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#17

Re: 94 GMC 4.3

07/27/2010 7:42 PM

sorry i just remembered vac leak is what everyone thinks ok i found holes rubbed into vac line to tranny theres three lines that run over top of tranny need a flashlight and mirror..... on drivers side....good luck

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#21

Re: 94 GMC 4.3

07/28/2010 12:49 PM

Good morning all! It's only fair that I update my progress for those who replied. At 242567 miles, I guess something SHOULD go wrong. Between grandchildren, wife and other distractions, managed to look at it again this morning.

Found the ecm and cps - disconnected & reconnected each (with no power). Pulled distributor cap & checked connections to ignition module & cam sensor and reassembled.

On initial crank, rpm goes to around 1000 and idles down to 5-600. Sometimes it will idle right down and at other times it will stumble before getting to the idle setpoint and then rev to 1500 to 2000 rpm.

Unless I disconnect something, the ses light doesn't come on and no hard faults are stored.

While trying to get this typed, it occurred to me that my problem may lie in the ignition circuit. The cap, rotor, & pickup sensor connections look good and do function but when it's idling down it stumbles then revs to 15 - 2000.

Arghhh, grandkids at it again so I gotta be the referee for a while. Guess I'll poke around that distributor some. Wish I had an OBD-1 scan tool!

Again, I WILL post the solution when (if?) I get a nail in it.

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#22
In reply to #21

Re: 94 GMC 4.3

07/28/2010 1:20 PM

Have each of the grandkids hold a spark plug wire and start the engine. Perhaps they will decide to pursue careers in the automotive field.

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#23

Re: 94 GMC 4.3

07/31/2010 12:10 PM

OK, pulled the ignition and wrung through everything. Thought I had it nailed by the broken pickup coil connector because when i first started it, ran fine & idled right down but then, the dreaded stumble & rev to 2000 rpm. Turning it off & re-starting brings rpm back to 500 again for a while. Looks like I'll have to bite the bullet and drive it to town in limp mode to hookup a scan tool. BBL.

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#24
In reply to #23

Re: 94 GMC 4.3

07/31/2010 6:46 PM

Have you tried spraying WD 40 all over the intake manifold area? If there is a vacuum leak, it should find that and run faster as long as the WD lasts.

Did you try pulling the wires on the O2 sensor and try that?

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#26
In reply to #24

Re: 94 GMC 4.3

08/01/2010 10:03 AM

Yep. It causes the ecm to set the o2 sensor fault.

Hooked up a timing light yesterday evening to see if the ignition was cutting out and observed the firing & timing to be right on target when it idles down beu when it decides to idle up, the timing goes way reatarded, iac opens up and fuel pulses increase.

Sounds like the knock sensor but running with it disconnected has no effect. I read that the knock sensor is a piezo-electric crystal that emits a small voltage when "knocked" so an open circuit there shouldn't affect the ecm.

I wonder if there is a "ladder logic" of the ecm anywhere?

Oh, if I had a vacuum leak, would the engine be able to idle at 500 rpm? Any time I pull a vacuum hose, the idle increases.

My problem seems to be something (intermittent) that causes the ecm to go into "limp" mode.

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#27
In reply to #26

Re: 94 GMC 4.3

08/01/2010 2:58 PM

Take your timing light, and watch the spray pattern of the injectors. If the injector is not giving the pattern as described above by wrench in # 25.

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#28
In reply to #27

Re: 94 GMC 4.3

08/03/2010 4:43 PM

Howdy Bob C. Your advice seems mostly on target but I still haven't "nailed" my problem. Yanked the distributor, tested everything & put it back. Thought I had found the bugger because it ran normally a couple minutes and then, same symptom - 1500 to 2000 rpm idle. Found my timing light and checked the injectors - OK. Looking at the timing mark while running noticed when the engine goes into high idle that the timing retards probably 20 degrees. When I disconnect the battery to reset the ecm, it'll start up and idle right for a while but then it "stumbles", sometimes stalls & goes right back to 1500-2000. I've "wiggled" all the connectors while running looking to localize the fault to no avail. I Haven't opened the ecm yet but I did wiggle & tap all the connectors with no luck.

I spoke to three different auto mechanics today on the subject with no new advice other than the OBD-1 scanner's usefulness on finding the fault. Virtually useless they tell me.

When it cools off this evening I'll "probe out" the ecm inputs. Especially the inputs from the coolant thermostat and tps. When it's idling correctly, unplugging the coolant thermostat has the same effect. Gotta give that tps a little closer look as well. Thanks again for everyone's time & thoughts. Check back as I will post the solution (no matter how humbling) if and when I find it.

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#29
In reply to #28

Re: 94 GMC 4.3

08/03/2010 7:41 PM

When it's idling correctly, unplugging the coolant thermostat has the same effect.

By this I am guessing you mean the coolant temp sensor. If this is correct, Spring for the coolant sensor. It is probobly the cheapest item there is on tour truck.

It may be possible to check the sensor, with an ohm meter at specific temperatures.

Coolant Temp Sensor
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Per Car Qty.: 1
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Product Line: Echlin Ignition
Per Car Qty.: 1
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#30
In reply to #29

Re: 94 GMC 4.3

08/03/2010 7:45 PM

That was redundant again and again and again. There are just two different sensors listed. Same sensor, just better quality. Look at the picture carefully especially the wire connector. If you are sure that you are unplugging this sensor from your truck, I would say change the sensor, and see what happens. God luck.

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#25
In reply to #23

Re: 94 GMC 4.3

08/01/2010 9:37 AM

When you depress the throttle under load it is the responsibility of the computer to supply additional air and additional fuel while first decreasing and then advancing ignition timing all choregraphed to allow the motor to accelerate to the desired speed.

In order to do this, the IAC flies full open to allow off idle air control to be regulated by the throttle plate position. This varies the throttle position switch output, read as a percentage open, which increases the injector open time along a curve similar to the advance curve, but based on the fuel map segment as determined by the ECT sensor.

The TPS grounds through or shares a ground point with the IAC. The map sensor trims the fuel curve by reading the vacuum pulse as the throttle plate opens and the manifold vacuum goes towards atmosphere.{p} So if we assume that the electronics are working correctly, then the only remaining glitches to be examined that would can a lean stumble are a poor injector spray pattern (streaming instead of a well formed conical shape) or a weak coil or ignition wires that can't distribute a viable spark during a lean condition. The surge then becomes a natural correction, where the fuel quantity is increased by feedback from the 02 sensor seeing a lean condition then trimming it back as the exhaust flow goes rich.

If you remove the plug for the injector and connect two jumper wires to the injector, one to B+ and one FOR ground, you can see the operation of the injector. Jumper the fuel pump relay so that the pump runs continuously. Touch the injector ground wire to ground and observe the spray pattern, If the pattern is good, replace the plugs and ignition wires unless they are absolutely known good parts, otherwise either replace the throttle body injector (good idea anyway since at 200:+ miles the pintle spring is probably close to worn out) or start treating the gas with a strong detergent additive (BG 44K, Chevron Techron) that will, noticeably improve driveability, after several tanks of fuel and short trip driving while dissolving and flushing both the fuel injector deposits and intake valve deposits that disrupt and reduce fuel flow.

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#31

Re: 94 GMC 4.3

08/10/2010 7:14 PM

IT'S FIXED!!! YAY!!!

I would like to humbly thank everyone for their advice and offer the final FIX. Sometimes, all one needs to do is follow directions to the letter and not ass-u-me. I checked the TPS early on in this exercise per the instructions in my Chilton's and dismissed it being the culprit because it showed such a smooth, linear response across the full scale with an ohmmeter. I even checked for 5.0 vdc supply and for reference voltage 0.5 - 1.2. I was reading from 0.5 wide open to 1.7 closed. Paragraph 7 states "if the sensor voltage IS NOT as specified, replace the sensor. I dismissed this advice, thinking I knew what a good potentiometer "feels like" and it was ONLY 0.5v out of spec.

So, after having checked every sensor, wiring, connectors, distributor, ignition components, timing, ECM board, prom, 1 new IAC, and much frustration, Spent the $32.00 for new TPS and now she runs like a cowboy-Cadillac is supposed to.

GM certainly has better onboard diagnostics now to lead BIG DUMMY'S like me to faulty components. Then again, if it were too easy, there'd be no need for a GOOD mechanics/technicians.

Some of my best lessons are the ones I learned hardest. For those future queries on GM OBD-1 owners experiencing the problem listed in the original post, the TPS voltages must read as stated in the manual. Also, disconnecting the temperature sensor will exhibit the same symptom (but it won't be intermittent).

Humbly yours,

Steve - (AKA duh big dummy)

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#32
In reply to #31

Re: 94 GMC 4.3

08/10/2010 8:09 PM

As usual, the answer to the most puzzling diagnosis is hit me in the head simple.

Must have been a cold solder joint at the ground connector.

Bob

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#33
In reply to #31

Re: 94 GMC 4.3

08/10/2010 10:24 PM

Good for you. And thank you for posting the solution to the problem. Would having bought a scan tool have saved you money?

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#34
In reply to #33

Re: 94 GMC 4.3

08/11/2010 2:56 AM

I don't think so. The IAC was the only bad "jump to conclusion part" I purchased. Located and spoke with 3 pro (grey haired) mechs in town about "borrowing" their OBD-1 scan tool and was told that the OBD-1 diagnostics menu is very limited and only displays basic info and actual fault codes stored. Virtually useless one gentleman replied. Yes, I'm prejudiced. I earned my grey hair the "old fashioned" way and tend to dismiss the "newbies" fresh out of trade school. My error in nailing this issue was dismissing the observed 0.5v deviation of the specified TPS signal. I didn't think half a volt deviation was enough error to "throw" the ecm into "limp mode". I was wrong.

Lessons learned:

1) Follow pre-engineered instructions (manuals) to the letter! (or .5v in this case)

2) There's some GREAT minds at work on this site.

3) The collective mind is the most powerful tool!

to be continued....

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#35
In reply to #34

Re: 94 GMC 4.3

08/11/2010 1:24 PM

OK. 34 comments. 9 were yours. So you now need to suggest to the next 25 automotive problems that they change their TPS. It does not matter their problem, just tell them to change the TPS.

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