Is This Bizarre Behavior Related to Premium-Grade Gasoline?

Thats a strange dilemma, I drive a 2000 Jeep Wrangler Sport with the 4.0L six Cyl. It is my daily driver. I have never burnt premium fuel in it. I drive approx 50 miles a day to commute, but frequently do long trips. Next time I fill it up, I'll put in some premium and see what happens.

I have been told by a few mechanic types that switching fuel octane is generally not a good idea. If you have been running on regular for most of the vehicle's life, then switch to supreme, it may cause issues. I have nothing to base this claim on other than hearsay from some good wrenchers. but I'm willing to see what happens when i make a switch, for science. (However i think I'll see what others have to say before dumping extra hard earned cash into the oil pits)

Good idea, fellow Jeeper

But I suspect, if this is indeed the situation, it may more likely be due to some particular condition with my particular engine. But I'm looking forward to comments.

Hmmmm, you wanted out of the box

I had a Volvo with a similar problem. Do you also get a progressively louder humming sound? Will it eventually stall?

The Volvo was older than your Wrangler, but probably not too different. Fuel is pulled from the tank, sometimes with a high-volume low pressure "lift pump". Then run into a high pressure fuel injection pump, then an accumulator to prevent air introduction or pressure loss. It is then routed to the equivalent of a high pressure "rail", mine wasn't but it worked the same, after which unused fuel at the engine is then routed back to the engine.

On the Volvo as one went for a drive, a progressively louder humming could be heard, followed by loss of power, eventually stalling with no restart. This was slightly dependent on outside air temp, but it never got cold enough for it not to happen. It was closely correlated to how much fuel was in the tank.

The humming was the sound of fuel cavitating in the high pressure pump. The pump is destroying itself due to cavitation. The fuel was cavitating because it was too hot. The fuel was too hot because too much fuel was being sent to the engine and then cycled back to the tank because the pump was mis-sized to begin with. The engine compartment then heated the fuel on each trip, these trips are more frequent when total fuel is low. It heats it up much faster.

The tests for this are easy; low fuel in the tank, does onset happen faster? Pull in and pour cold fuel in the tank, symptoms gone? Keep driving, do they come back? Run a hose at the station over the top of the tank for a few minutes, what happened to the symptoms?

Hope your Wrangler is cheaper to work on than my sister's Cherokee. the tank pump replacement (for other reasons) is a substantial pile of green.

Do some tests, I won't waste air if it doesn't fail the way my problem did, and you won't need my fix.

Well. we're both in the desert. I live in Queen Creek. SO it's a very interesting idea you have there. Will explore it.

And the fuel pump "issue" is the same on the Wrangler as on the Cherokee. Completely asinine design.

I have had a Ford and a GM do this, once the gas in your tank heats up it expands your fuel pump leaving it very ineffeciant. You will find in time just hot weather with a short trip will cause the same effects. Replace the fuel pump and run regular fuel, you don't need premium unless your engine is pinging.

Does premium grade gasoline burn hotter, by chance? Might this even accentuate the problem? That was my original line of thinking.

I concur with edigan in relation to pumps. Just a note in regards to premium fuel. If your engine does not have 9:75 compression or better using premium is a total waste. Simply, a higher octane was developed for more "anti-nock" in cars with higher compression. Todays cars will compensate in timing. A higher compression engine will run better with higher octane so that more timing can be implemented usually providing more mechanical output from the engine. Your Jeep (googled it) has 8:8 compression. Not a power house ratio. If the cpu is pulling back on your timing, you will feel the power loss. It's a possibility but I would venture more to what edigan says. Try running just regular 85 octane for a length of time. Continue researching the pump and measuring line pressure when it shows those symptoms. It may be worth running computer diagnostic just to rule out any other issues regarding your O2, Map etc.

Dear Box,

This sounds like it might be a fuel vapor lock in the fuel line. Try wrapping the hottest section of your fuel line with some heat resistant material. You can start with aluminum foil as a test.

Rock1

Ohhh, I do like the cat possibility!

And we have a Cherokee sitting in New Mexico that periodically strangles and dies, doesn't want to restart and leaves no codes behind.

Unfortunately troubleshooting starts with dropping the tank and replacing the pump - just to "see". I recognize the need, but I DO hate the design.

edignan, I don't believe you have a fuel problem & premium fuel is a total waste for your jeep. It sounds to me that the ignition coil is failing. As the temperature in the coil builds the insulation is breaking down causing a reduced spark output. If you have an ohm meter you can check the resistance cold and then again when things heat up and the problem appears. Considering the price of a coil I think I would just replace it now because if it fails completely a tow could be quite expensive. J.Conway

If the coil is breaking down as you suggest, an easy test would be to allow the vehicle to warm sufficiently to detect the problem. Return to the driveway, and squirt with tap water from a spray bottle. If the wet coil makes the car run better it is a heat sensitive coil that is bad. If the wet coil makes the Jeep run worse, or die, the coil was bad, and needs to be replaced. Isolate the water to the coil to be sure your test is accurate.

Excellent, excellent information from each and every one of you!

Several very possible scenarios there. I agree, the cat sounds very possible. As well as the vapor lock, and the other things that I cant remember, as I cant see the answers while I'm replying. The off-on test is a great bit of advice, with the webpage you provided. I will have a good weekend of playing in the garage coming up. And I can start with some optimism for a change. Yes, it's extremely annoying.

I'll have to fit this all in around the Superbowl, though... as I plan on thoroughly enjoying watching the Arizona Cardinals pull off a most unlikely miracle year and become champions for the first time! Go CARDINALS!!!

I know this will probably be shot down but from a layman could it be that the breather for the fuel(gas) tank is blocked or partially blocked? This might cause a vacuum effect in the fuel system on long trips. The carby might not be getting enough fuel as presuure builds up. This would not happen on short trips as the pressure would not have time to build up and air might seep in through the fuel cap or the partially blocked breather.

Side issue: I had trouble a few years ago. Everytime i tried to fill up the tank, fuel would spurt out of spout if I tried to fill it too fast. I finally checked the breather tube and found it was kinked. I spent 10 min straghtening it and have not had trouble since. Did not go on any long trips until after problem.

Most if not all modern vehicles keep the fuel tank under a slight negative pressure. If you have a problem with say the filler cap gasket or a vapor line you will get a code for for it "massive vacuum leak" is one I got some time age because the pump jockey did not tighten the cap. The days of a tank that is vented to the atmosphere are long past because of hydrocarbon issues.

Edmund

carby - I think I saw one in a museum once

geomech:

I searched the thread to see if anyone else thought of this.

my thoughts exactly. "crack open" the gas cap when power drops and see if it gets better

I'm taking notes...

GA from me.

A quick test for proper venting is to stop and open the filler cap and reclose it, did it suck in air? Was the problem fixed after letting air into the tanK? that might be the problem....

In a relatively modern car this probably means that the charcoal filter system (that prevents ptrol vapour exitting) has a problem...

these vintages have a sophisticated evaporative control system.

most likely, blocked a vent valve, allowing vacuum to build up, or a weak fuel pump.

though unnecessary, premium will not affect the performance.

the O2 sensors, and the knock sensors take care of those issues.

I haven't run thru the suggestions yet. Will do this weekend. But I will add one more symptom that I believe is related, and may give a better clue as to the problem.

I simply have no power on inclines. At the slightest incline, it appears to have trouble and I can't accelerate. And if it's a real hill... well, on a single lane road, I get a lot of angry motorists in Cavaliers and Vega's and pretty much any old peice of crap, lining up behind me. I can only gain speed, and the engine relaxes when I'm going downhill and I gain momentum. This engine should do far far better than that. Obviously a big problem. And this symptom is all the time, not sporadic like the other. I should have mentioned this earlier.

I had a Nissan Sentra that had old plug wires and they caused the distributor to heat up until it shut down when running at highway speeds for 20 to 45 minutes. The distributor would get so hot you couldn't hold your hand on it. Let it sit and cool off and it would start and go again. I replaced the fuel pump twice. Changed the distributor twice thinking I got a bad one. Replaced the wires and it ran fine. Look at your plug wires in the dark with it running and see if they have little blue arcs coming off them. If so they're bad.

I have seen similar behavior from two sources: vapor lock and fuel pump problems.

Does your engine trouble light come on? Actually, whether it does or not, go to an auto parts store and borrow their OBD2 unit and check whatever error codes may be in your computer. If your auto parts store does not have one, go to another one. Any good parts store will have a loaner for you to check your car. It will also allow you to monitor engine parameters as you motor along. You can record values when the problem does not occur and then record again when it arises and by noting the difference figure out what is happening if there are no error codes, or the error codes are no help.

In general, it is a good idea to own an ODB2 unit. Mine has certainly saved me more than $5,000 over the past 10 years.

Concerning premium fuel, don't use the stuff. It won't do your Wrangler a bit of good. All gasoline has the same energy per gallon, about 115,000 BTUs, regardless of the grade. For automobiles designed for regular fuel, it is impossible to get better mileage with premium. To get better mileage, you would need more energy per gallon.

The only thing that premium does is resist knocking for high compression high performance engines in cars purchased by yuppies. Never buy a car that requires premium, never use the stuff unless you are a millionaire and into conspicuous consumption.

Okay, it's obviously not the premium gas. ANd everyone has been saying not to use it. I rarely did. Only on long trips. Once every month or two. But... point taken

So, I didn't know I could borrow an OBD2 unit from parts stores, to actually take along with me. That's very interesting. Will have to locate one.

My first thought was the pump. It was such a hassle to replace the fuel filter that I didn't want to do it again to get the pump out unless I had a better idea that that was the issue. Wanted to try other things first. But if need be.. I'll have to.

But I'm getting a sense that it is a vapor lock problem, after reading all of this. What is the remedy, if that turns out to be the case?

The catalytic also sounds like a prime suspect.

If you have an in tank electric fuel pump, you do not have vapor lock. Vapor lock happens when the fuel gets hot from whatever reason. Then the mechanical fuel pump applies suction to it. Lower pressure=lower boiling point. Now the fuel pump is pumping fuel vapors. The car will not run on fuel vapors.

I am very suspicious of your exhaust system. They can cause the same problems.

Yes it is an in-tank fuel pump. But don't know ifit's electric on a jeep.

Yes, it is electric.

If it's in the tank it has to be electric. No mechanical fuel pumps are mounted in production cars in the tank.

At this point, I would suggest that you beg, borrow, or steal a fuel pressure tester. After the engine has warmed up, install the gauge, and drive up a hill. If the fuel pressure remains in the acceptable range while the engine is clearly lacking power, test for exhaust blockage. If you search for exhaust problems here, it should show up. If you can't find it let me know, and I can walk you through it. Good luck.

"It is possible that the ECU is still heating up after an hour, I suppose, so maybe an a-d converter in the ECU is failing and causing an incorrect input: for example, the ECU thinks the O2 sensor says the engine is running rich (when it is not) and the ECU keeps trimming the fuel back."

You know what, Blink? This sounds familiar. I did have a friend read a code for me once, for another reason, and I believe something along this line was mentioned. Would this give me problems on all inclines though? Long inclines... not just going up an overpass. That's when it is very noticable.

What is the fix for this, it this turns out to be the culprit? Where is the a-d converter located? Is it just a matter of replacing it? Is this one of the cheap fixes, or the expensive?

The a/d (analog to digital) sections are an integral part of the ecu. The ecu receives many signals that are analog (varying voltages) and the ecu first converts these to digital, and then does its processing. Unfortunately if this is the problem, the ecu must be replaced (one of the expensive fixes) -- so I'd try other things first, and also do a really thorough scan.

If the engine is running lean, the problem is most noticeable under load, i.e when going uphill, and would tend to be more noticeable when vehicle inertia has bled off a little and the engine is really working to move the vehicle up hill. But again, you'd want to do some test to be sure that the ecu is bad before replacing it (unless you have a junk yard around kind enough to let you swap it out temporarily).

I am mechanically inept, so am saying this in almost total ignorance, but doesn't the color of the deposits on the spark plug change with lean running? This should show up after a long trip.

Examples of varied plug conditions can be found, here ...

http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm

I think it is due to malfunctioning of the fuel pump ; when it gets heated up after one hour of driving . The rubber kit of the pump is affected by the heat and so it fails to deliver sufficient quantity of fuel . You may try by changing the fuel pump .

Sounds like a O2 sencer failer over time with heated exhaust heat, witch in turn will cause the motor to run rich. This kind of condition will cause the catilitic converter to over heat and lower its ability to "breath".

This condition will trip at least 1 obd trouble code. The OP did not mention a "check eng light"

The next time this condition occurs, suggest removing and evaluating the condition of a spark plug or two. This will tell you if the engine is running too lean, too rich, or just right. Perhaps, this will help lead you to finding the cause.

Your problem is unlikely due to the grade of gasoline.

Gasoline octane ratings were originally used for leaded fuels. Tetraethyl lead was added to reduce the rapid explosive effect in cylinders. Leaded fuels leveled the rate of burning in cylinders and was recommended for high compression engines.

Many people think that buying higher rated fuels give them better mileage,cleans their engines, etc., etc... Nothing but a waste of money.unless you have a high performance racing engine... If your engine does not "knock" using regular fuel... don't waste your money.....Additionally, When the corn growers were promoting "green energy" and got this country's politicians on board to promote a shortage of corn that would get subsidies to add to gasoline.... Those who were aware of the consequences were ignored (I was one of those who predicted the damage that would occur in cattle food shortages from the inevitable shortages of cattle feed and increased meat prices)...In fact, when the federal government subsidizes ethyl alcohol addition to fuel, it not only affects corn production, but decreases the BTU value of the gasoline, hence, it adds about six cents in actual cost (due to BTU reduction) of a gallon of gas....But this has not news value and will not get anyone a Nobel (Gore) Prize.

If the jeep has a steel tank, it is possible that washed of rust has partially blocked the simple "Sieve" that most in tank pumps have to prevent small rocks getting into them. If all else fails, you will need to run the tank dry, drop the tank and have a look inside (without a cigarette please!)

I had a similar problem with a car. It would go about a mile or so, then peter out. A couple of minutes later it would start and go another few miles.

At the time I was a long way from anywhere in the outback of Australia.

I limped the car into the nearest town, where it turned out to be both the sieve blocked and the petrol pump wrecked.

The actual cause was traced, eventually, as being probably due to water in the tank making rust which clogged the sieve and then wrecked the pump.

The problem only showed up on a long trip, but then escalated.

If your problem is the same, the tank needs to be dropped and the sieve and pump checked as soon as possible before the problem escalates to a ruined pump.

Good luck

Strewth, I had the same problem that happened to my '96 Pajero but it happened just out of Roma, on the way to Mt Isa.

I got back to Roma and went to a Mechanic and he told me it was the petrol/air gauge that's located near the air filter. Well, it told 5 days and $800 to get the Part. 5 days at a hotel in roma $650. Wife and kids were NOT happy. Got about 20 KLM out and it happened again. Limped to Charleville and the RACQ local mechanic replaced the "fuel pump" in 1hr costing $130.

Anyone one want a air/fuel monitor for a '96 Pajero NJ 3.5lt? Used for just 200km. Going cheap.

I suspect the higher octane premium contains a higher percentage of ethanol. This fuel has a lower vapour pressure and, when the engine heats up, you are creating a vapour lock somewhere in you fuel delivery system. As other posts have stated, the low pressure lift pump in the tank has too low of a discharge head (requires changing) and you are cavitating the high pressure rail delivery pump once engine temperature warms up. Eventually, you will likely see these conditions with all grades of fuel as the first pump continues to degrade.

I had a similar issue maybe 10 years ago. I thought I would put a K&N air filter in my '94 Ford Explorer. After a few thousand miles it would lose power during long highway drives and the gas mileage suffered. Turns out I over oiled the filter which contaminated the MAF sensor. Cleaning the MAF with "MAF cleaner" fixed my problem.

I would have never guessed this was my problem. My first guess was a bad ignition coil.

Ford and Volvo for years used two pumps an internal primer pump and an external pump. If this is the case disconnect the inlet to the external pump. Use a coffee can have somebody try to start you should get fuel. If you don't change the in tank pump. Take a pressure gage and monitor the pressure on the system. If its low clamp off the return line if it comes to Jeep specifications. Change the pressure regulator otherwise install a new pump. A bad converter will over heat get red look at it after a long drive. This be going on during normal use not only on trips. If it don't ping stay with low octane fuel.

I have used premium grade fuel in every vehicle I've ever owned with no exceptions. The reason for this is that when I was quite young I worked as a part time mechanic in a foreign car shop and assisted the master mechanic rebuilding engines. This fellow was trained in Great Briton in a formal school - more like a college than an American Trade School - and over the course of several months I learned quite a lot from him.

Quite a few of the foreign car owners at that time had the opinion that a foreign car was more of a toy than a serious vehicle and would feed it the least expensive diet possible. About an equal number of owners realized that they had a sensitive machine and bought the best for their vehicles.

From the first engine we dismantled, Harold would point out that if the engine had deposits caked on everything inside the engine, the owned used regular fuel. Those that were relatively clean inside used premium fuel without exception. We also rebuilt eight or ten filthy engines to one clean engine. I made a point of asking a few of the owners of the rebuilt engines what their fuel preferences were and their responses supported Harold's claims to a "T".

In my opinion, there is enough things in an automobile to go wrong without adding another item to the list. The few cents difference that premium fuel costs more than offsets any repairs that a dirty engine might cause.

By the way, I have owned most of my vehicles for several years - usually a minimum of six or seven years - and when we parted ways, the engine in the vehicle was still running well.

That's my story, and I'm sticking to it...

By using the term "quite young" I am venturing a guess that these wonderful vehicles that you tutored around still had 6 volt batteries, generators, and a set of points so small that they would fit inside a USB adaptor. But those manufacturers never did do anything to change the fuel delivery system did they?

The driving habits of the drivers would have more to do with carbon build up on pistons than the grade of gasoline. Do you remember the phrase "take it on the highway, and blow out the carbon"?

Take it up on the highway for an Italian tune-up! I tell that to my son when I drive my mustang, gotta blow the dust out of the supercharger!

Dave

Your mechanic was misinformed. Higher octane fuels create more deposits and soot during combustion. This is why it's usually offered with a greater volume of detergents. It's not a big deal because the deposits are so minimal. It's "only" advantage is a higher flash point which will eliminate detonation in engines that run hotter. Engines with high HP to liter ratios usually require high octane because they release more energy within a given volume than that typically found in basic transportation.

If you don't need it don't use it. It costs more and isn't going to help your engine. On the contrary, if your engine needs it, you "should" run it.

Also, I know some regions sell gas in the 83-86 octane zone. This is too low for nearly all gasoline IC engines. It might be ok if you own a gas turbine, but it's below anything an auto manufacturer would recommend.

You will find that PretendEngineer is saying just the opposite, that premium causes more carbon and soot (a contention which would be hard to support with real evidence). When in doubt, perhaps it is best to rely on the gasoline companies who have everything to gain by selling you more profitable high test instead of the cheap stuff. If you read up on the BP site, you will find that 1. Today, all fuel grades use equal amounts of detergents, and 2. that there is no benefit whatsoever in using high test in an engine that does not require it. This from the people who would profit more from selling you premium.

I too worked for a old codger mechanic (this one from Germany) when I was a kid. He too was full of absolutes, many of which turned out to be completely incorrect. He also schooled me to do practices that were just plain wrong, such as using an oversized stud in a stripped-out hole in a VW block (a hole stripped out, I might ad, by his refusal to use a torque wrench) when a helicoil should have been used. Customers thought he was great, because he had a German accent.

Actually, the engines that had stuff caked on them in the old days (in places other than in the combustion chambers) suffered from refusal to use detergent oils, which also stemmed, mainly, from old wives' tails.

On the subject of stripped studs; I herd this one recently from a friend. "A cross threaded bolt is better than Locktite". I was speechless.

...now we know how to keep you quiet!!

Not so. All fuel must meet EPA detergent standards for gasoline, which provides cleaning action.

If you want to maintain as clean an engine as possible, here is what you have to do: Run for 5000 miles exclusively on one brand, say Exxon, and then switch to another brand exclusively for the next 5000 miles, etc. Although the base gasoline is the same for all brands, the additives are different and consequently different patterns of buildup occur in the engine.

Running one brand for 5000 miles will cause their pattern of buildup, and then the next brand for the next 5000 miles will tend to destroy this buildup and create its own pattern.

Now, you don't need to do this unless you are really obsessive about this. Modern gasolines have very good detergent action so that if you use a major brand exclusively, you should not experience any additional problems for the life of the car.

Personnally, what I do is randomly use Exxon or Shell depending on who has the best prices because both have issued me credit cards that provide discounts for their fuel which makes them the cheapest fuel for me to purchase even when their posted prices are higher.

I just sold my 1996 Oldsmobile after 225,000 miles, and much of that on gravel roads, on regular gasoline exclusively (3.1 liter V6) and it was running little changed from the day it was new, still able to get 30 mpg on the highway (did 31 - 32 when new) with the original injectors and the engine never opened up and on only its second set of platinum spark plugs.

Premium gasoline is a total and complete waste of money UNLESS you have a high compression engine that needs it to prevent knocking. No one should ever buy any highway vehicle that requires premium fuel. If you are in the show room falling in love such a vehicle, run away from it immediately when you realize that it requires anything but regular.

If you must have a high compression engine, buy a diesel and help save the planet.

Could this problem possibly be related to altitude?

Ah, once again, the slogan "it's a jeep thing, you wouldn't understand" comes to mind, but beyond that, using premium in a jeep four or six is a waste of resources, both your money and the extra refining,etc. that it requires to produce premium. Also, regarding the Brit that was "old school", deposits occur from driving slowly and never really heating the engine up to the point where the carbon deposits were burned off. I have heard that water injection prevents this, but no personal case studies.

Good luck, I think fuel delivery issues are the target.

When I worked at an Oldsmobile dealership, we would occasionally get a car with carbon built up on the piston to an extent that it would contact the cylinder head. The sound was just like a rod bearing loose. Wynn's Spitfire and water would clean them up without removing anything more than the air filter. Bring the car up to temp pour the Spitfire slowly down the carburetor while holding about 2000 RPM. Allow the engine to stall by slowing the engine speed a little at a time. Let it soak foe an hour, and restart. Finish the Spitfire and then run water the same way till all of the smoke was gone. The car was good for another couple of years.

Sounds like your catalytic converter is failing. I've had a similar experience, except that the problem was much more dramatic - my car had very little power and speed after driving for an hour, and I was not able to go faster than 10-15 mph after I pulled it over. The engine idled fine, but would not allow speed >15mph. The catalytic converter had partially collapsed.

the weight of evidence seems to be against better performance from higher grade fuel.

i suggest being more systematic about your observations - is it truly only with higher grade fuels, is the loss of performance predictable as to amount of time the engine is running at high rpm, is it indeed running at higher rpm on the highway than around town, what happens if you drive for an extended period - such as you do on the highway - at 35 mph? there are only two possibilities that immediately come to mind, one of which is that your spark advance is set for a quicker-igniting lower grade fuel, and with the slower burning premium, the ignition is too late in the cycle. that would be exacerbated when you try to accelerate. the other is that you are getting some kind of partial vapor lock. you might want to run an hvac temperature sensor to any point in the fuel system where that might occur and see if you find a correlation.

jhw

the answer, don't use premium fuel. Premium fuel is allowed to have a higher vapour pressure and you are getting to much flashing as everything heats up.

I would put premium in an engine that didn't require it thinking about the old days where going over the mountain pass the old car would knock and just make it over. But even on a cold day at 11,000 feet, the iso butanes would come boiling out, the tank would pressure up and the vent relieve would open making the inside of the car smell of gasoline. The engine would not run well until I'd stop, and slowly relieve the pressure on the tank. Let it boil away and cool the gasoline. Then jump in and the performance of the engine would increase (no, not to sealevel performance).

I quit the premium thing and never had problems again.

You have secondary spark breakdown. Replace your sparkplugs. If it's not cured, see if spark will jump an inch off the wire. If it won't, the coil could be arcing internally. Plug wires, cap, or rotor are also possibility, if so equipped. I use the platinum or iridium plugs because they should last around 100,000 miles. That way you can get alot more time to neglect maintenece.

Thank you. I did recently replace my plugs with the best quality available. No change. I will check the other things you mentioned.

The plugs are good, but loosen and tighten them every 20,000 miles of so or they can sieze into the heads, especially aluminium heads!

Andy Germany That's why we apply anti-size to the threads when we install the plugs. J.Conway

Perfect, but nobody mentioned it before.....better late than never I suppose!.....I used to use a smear of Copper paste, but some mechanics frown upon that.....

When GM started to use aluminum heads it seemed that every car had stripped heads from cross-threading them. Threaded inserts were very common. I don't know weather the mechanics got smarter, or the manufacturers made better products, but it just never seems to happen anymore. (even with the diminished vision)

if you stop and let the car "cool off "' does the problem go away for a while? i other word what "fixes" the problem?

i doubt that the fuel octane rating has any thing to do with it. a couple of people have mentioned the screen that is on the fuel pump in the tank. this screen, sock or sieve can gather loose rust particles and dirt that get into the tank over time and on a long trip will finally plug the screen enough that it limits the amount of fuel availible to the engine which would cause the loss of power.

try to get as large a diameter of hose that will fit in the tank and try to get it to the bottom of the tank and siphon out some fuel into a CLEAN can. check for particles visible in the can. be sure to smoke a ciggarett and have some type of open flame near by while doing this procedure. it make things much more exciting. yes i am kidding be careful doing this.

how many miles are on your car? it is rare that the fuel pump may be failing slowly. usually electric fuel pumps fail quickly and the diagnosis is easy. 100,000 - 150,000 miles is good life for these pump. preventive maintainace would be to change the pump to avoid being completely stranded by the side of the road if your mileage figures are near those numbers. there is also a short, one inch long piece of hose that connects the actual pump to the metal tubing that goes up and out of the tank that may be failing. the new fuel formulations seem to be eating this hose by turning it into mush, easy to split type of rubbery mess that although the pump has not failed lets the fuel be put back into the tank rather than pushing it out into the fuel line with a resulting loss or pressure and the loss of power that you describe.

please answer back. i am curios about the final diagnosis. wrench

There sure are alot of suggestions! Not sure I'll be able to get to them all this weekend.

I'm suspecting that although it may be fuel related, it isn't an actual fuel pump failure, as this problem has existed for 8 or 10 months, and the pump would have simply failed completely by now. As far as what fixes the problem... pulling over for 5 or 10 minutes sometimes fixes the problem, other times it doesn't. The jeep never dies, it just gets to that point where it just won't go faster than 45 mph. And it feels as if it's struggling. And even when it is at its worst, if I hit a downhill, the engine will run perfectly again. So it is probably not a mechanical failure. But I always have trouble going up hills. Especially steep hills. It will slow down to a ridiculous speed when it is under strain like that.

So downhill is always good. Extended up hill is always bad. And on level ground it will happen sometimes just out of nowhere if it has been a long trip of more than a couple hours at highway speeds.

I apologize for adding this new info so late into the thread, but this has been going on so long and so many times that I'm just now remembering all the different situations where it has happened.

Oh, one more thing that fixes the problem. If I have the slowdown, and keep the accelerator pushed down, the problem continues. It wants to go faster, but just can't. I can feel it. I can have it floored and it has no effect except for maybe even going slower. If I slowly let up on the accelerator, there will be a point where I can feel the engine suddenly "relax", as if I have reached a point of equilibrium. It's the point where the gas pedal loses effectivness if I push it down any further, but does control my speed if I let up any further from that point. Am I making myself clear? If I do this enough times... let up on the accelerator and coast momentarily... then push down till it strains and let up and coast... etc... often this will fix the problem and then eventually one of the times when I push down again, it will behave as it should, and I will be able to accelerate past that equilibrium point once again, and it is fine... for a while at least.

I'm thinking that this may be a key point, and will allow someone to come closer to the true problem.

I'll make a couple of statements that I feel might be accurate and helpful:-

1) its not a premium grade problem, you might actually get the problem even more pronounced with the correct (lower) grade of petrol

2) Its a fuel injected or electronic carburettor engine, not an old fashioned carburettor.

3) the plugs, if checked when the problem has happened say for 5 minutes, would be very lightly colored, the ceramic may even have a faint blue tinge to it possibly.

Your engine is simply not getting enough fuel and your plugs may now be damaged due to slight overheating.

I and several other posters postulate that a possible reason could be a blocked sieve and or damaged pump in the tank (I believe the external pump has already been changed, if not, it could be that also!) or that the tank cannot get air to replace the used petrol.

I actually gave you a simple test of when the problem happens, which was - to just unscrew the filler cap and then replace it while the engine is running with the problem.....but you haven't tried that yet I see!!!!

If that does not help, then sieve is possibly blocked or pump(s) is damaged.....the engine is simply not getting enough fuel.

It could be a fully blocked air filter, but I am sure that you have checked and replaced that already......

Well I haven't tried that yet Andy, because as I said, it only happens on long trips. I can't take a 100 mile road trip during the week, hoping it's going to happen at some point. And the deserts of Arizona are not known for long uphill climbs unless I take a long trip to the high country. Again, not something I can do during the work week.

And yes, I have replaced everything that is accessable from under the hood.

Understood, sorry, I thought it happened more often than that!!

From, this it sure sounds like it must be a fuel delivery problem, or a wacko engine computer that is playing mind games with you. (Screwed up timing caused by the engine computer can cause your symptoms in addition to fuel delivery problems.)

Is this temperature related?

In the winter when it is cold, does it still happen?

Or is it equally likely to occur whether whether the weather is hot of cold outside?

Did you monitor with an OBD 2? We would sure like to know what it says, both in terms of error codes and the detailed data.

Knowing what your spark advance, O2 sensor, etc., are doing would be a big help.

There is no cold weather where I live. But it's happened when it's 70 and when it's 115.

Have not had the opportunity to locate and access an OBD2 since I started this thread a couple days ago. My first efforts will be simpler things that I can check, as was suggested.

And first I will have to know what a spark advance even means, and understand what an OBD2 does and how to read it. I'm not an automotive mechanic, nor even a hobbyist. Hence, my questions to those that are, here on CR4.

An OBD 2 is a simple thing to use. You plug it in and start punching buttons to get what you want. Nothing to be scared of.

You can even read the instructions if you want. It tells you error codes and displays all of the parameters of your power train that the computer can access.

Spark advance is the number of degrees of rotaion before top dead center when the spark plugs fire. When the RPMs increase, the spark advance has to increase. If it does not, lack of power and poor performance are the result. With higher RPM, the spark advance has to be higher to give the ignition enough time to ignite the fuel so that when top dead center is reached, there is enough time for all that combustion to occur before the cylinder bottoms out. Bad timing also results in poor fuel economy.

Oh, spark advance means timing. Yes I understand timing. But no I haven't confirmed my timing lately. But I do know what an improperly timed engine sounds like, and I don't hear that. Also, the plugs I replaced recently, with higher quality plugs, actually looked pretty good when I took them out.

I have never used a timing light. And although that is important, and there are a dozen things I need to check... alot of these things would require me to take to a mechanic. And I just have to keep my budget in mind. I have had mechanics work on my engine since this trouble began, but evidently it was nothing obvious, as whatever was tried evidently didn't address the problem.

I am a mechanical engineer. So I'm not totally lost in the world of mechanical systems. I prototype custom machines, in fact. But automotive mechanics in particular are not one of the specialties that I have had experience in.

You bring up some good points, but if there are no OBD 2 messages, its basically got to be either a lack of fuel, or a lack of air......the modern computer always keeps the mix correct, so if it doesn't get enough of one, it also allows less of the other - if you get my drift!

Before computers, you got great black clouds of smoke when the air filter was blocked and explosions in the silencer or blowing back through the carb when the fuel line was blocked!!!!

It was always very clear where the problem lay......

And no, I have had no check engine warnings.

Then it must be air or fuel, with a tiny possibility of plug damage as a result if fuel......

Air volume sensor is another but fairly remote possibility, as is the oxygen sensor....

As far as spark plugs, I replaced them a few months ago with the best plugs I could find. The old ones were not particularly fouled or discolored. But there was no change in the symptoms I described.

The problem I believe you have MIGHT damage the plugs, but changing them would not change the problem!!!! You appear to have missunderstood me...

The COLOR of the plugs, if looked at with the problem, would have given us a clue as to what the problem is, I strongly believe.... if very light colored or even clean looking, this indicates that a lack of fuel is the problem.....if a medium tan color or darker, then too little fuel is not the problem..

But with computer controlled modern engines, the color changes are far less dramatic and therefore much harder to differentiate between.......

Does the method I described, about quickly letting off the accelerator repeatedly until it returns to normal, and that equilibrium point that I mentioned in post #63... suggest anything in particular?

could be that by the time you have driven for and hour or so the fuel level has dropped somewhat and the rapid throttle movement will dislodge some of the stuff that accumulated on the fuel pickup screen from the rapid sloshing that will occur in the tank. when you stop for awhile the lack of fuel flow that grabs and holds the particulate matter to the screen will be gone and good old gravity will allow some of the stuff to fall off the screen and return to solution. then you are able to drive for awhile again before the stuff returns to plug things up. hope you find the trouble. wrench

Worth trying, but loosen the pipe before the cat, up there at the engine block could be too dangerous with a lot of very hot gas....

Personally I do not think its the cat as there are generally up to full temperature after 10 minutes, if blocked, the symptoms should show up much earlier, but I could be wrong, been wrong before.......

Hi just going thru old threads and saw this one about your jeep. I was wondering if you found the trouble? I am curious at to what the fix was.

hey, I was checking back too, and wondering what the resolution turned out to be...I still go with fuel delivery problems

Hope you got it dealt with, it can be maddening

WOOOHOOOO!!!!!! FINALLY FIXED!!! Three years and several thousand dollars later... I just drove my Jeep home from the shop and she fly's like she's high on oxygen!

What a crazy trip this has been. I tried many of your suggestions. Thank you all for your input and interest. I even have a new engine last month, and that didn't fix the problem. Some time ago I did a back pressure check, and found that there was a serious back pressure problem. I put in a brand new high performance cat converter, sure that that would finally be the fix. No dice!

I can't tell you how many mechanics I've talked to about this. So many ideas. So much time. Well finally, I got a tip to go to one particular shop. A muffler shop. Dark and dank and dirty. He said my cat was good... muffler was good. He was stumped. When his assistant took a look, he noticed something. In the exhaust system, up in front, right under the engine, the pipe made an oxbow and doubled back toward the converter. Right in the middle of that wide 180 degree bend there was a small swelling of the pipe. About the size of a baseball. This evidently is the PRE- catalytic converter! He stuck a scope with a camera in there... and there it was! The honeycomb inside this little SOB was broken loose from the housing and flopping around in there as it pleased! THERE'S my PROBLEM!

The reason that it was an intermittant problem was because this cartridge sometimes lined up with the flow and sometimes it didn't. We cut that thing outta there as fast as I could get them to put a torch to it... put a straight pipe section in its place... and Voila! She now purrs like a kitten, climbs the steepest hills and hits 80 on the freeway, no problem! What an ordeal this was! Evidently some jeeps have this little pre-cat. Who knew!

Okay, so I'm now back in business and have a proper Jeep. Can take long trips and can do what a Jeep is supposed to do. I'm one happy mo fo :)

Thanx again folks. And remember. A 1999 Jeep Wrangler with a 4.0 has a little pre-cat, way up front. And it's as important as the big one.

I'm going Jeepin'!.............

First of all, THANK YOU, for being good enough to let us know what curred your Jeep. Most never do.

I am disturbed that so many people could have sold you parts, and never tested the car for restrictive exhaust system. If you, as a novice auto repairman did not test, that is your prerogative. But for an exhaust repair facility to sell you something that did not fix your problem, and then not to do the test afterword, is just wrong.

Happy motoring.

Well after the back pressure test, it was almost assured that it was the main cat. So i bought a converter "off market" from a guy I knew, and had it put in. But as I said, it wasn't the main converter. And nobody had heard of or even noticed that there was a second one further up the line. It's hardly noticable even as you look at it. It could have just as easily been a pipe connection. I was at the point where i was willing to try absolutely anything, and encouraged anyone to try anything. I tested everything possible. But I also took some long shots. This was a three year ordeal, remember. I was grasping at straws.

I've got a lot of new parts now on a vehicle that had nearly 200,000 miles. So I'm not too worried about that aspect. It's fixed and that trumps everything else.

thanks for the reply about the cure!!!

Just to clarify... I didn't put in a new engine in a bid to try to fix this. I broke a wrist pin. But I had hopes that when the new block was put in, the problem would be gone. There came a point when there was simply nothing left to check, so I was hopeful about everything.

Again, thanx to everyone's interest and input to this two year long thread. Some of you put a lot of thought and effort into helping me solve this. I appreciate that, and that's why I love you guys Pleutonically, of course

But I've gone thru all the posts and given everyone who mentioned the catalytic converter, a GA. Although no one mentioned a possibility of there being a SECOND catalytic converter !!! , it still contributed to the final solution.

Comment #81, by Bob C, actually came closest to describing what was truly going on, albeit, the location of the problem was about 30" further up the exhaust line. There are no Grand Prize winners, but Bob C, you get First Prize in my opinion.

Alright, so now I could use some assistance with a welding/warping problem I've described over in the "Manufacturing" section . It's called "Help w/ Welding Distortion is Sheet Metal". Anyone have any IQ left over after all of this?

Does that qualify me to move from platonic, to a weekend in the mountains?

Ha! Well, we'll save that one... but it does qualify you to remind me how to spell platonic