rough idle

hey all,


I drove to work today and was sitting parked after a 30 min drive. right before i was going to get out of the jeep i heard the engine sputter, rpm tac dove a little and idled like this for a while 650-1000rpm. It got a little worse as i was studying it and when throttle was applied it really sounded like crap. I shut the car off and popped the hood for a visual witch showed nothing obvious. I turned the jeep back on and it really sounded much worse (up idle and fall... but not slow. not stalling but horrible idle. i offed it and went to work.

At break i push every electrical and vacuum connection in, that i could see, grounds look good. turned it on drove a lap and it seemed to run fine plus it got me home with no issue.
today was the first time this has ever happened
Fairly new: cap & rotor, plugs, wires, Fuel pump, battery, alt, cps MOPAR .

ignition coil probably going to replace for good measure (only because its cheap and ive seen its a possible culprit). IAC is stock but im not convinced its a culprit after driving home. Im leaning coil or vacuum.

what do you all think? 99xj auto 185,300

I have been experiencing problems with our "oxygenated" fuel in our warmer than normal temperatures.

If there is an idle issue that comes and goes randomly, the first thing to suspect is the IAC.

Hypoid said:

I have been experiencing problems with our "oxygenated" fuel in our warmer than normal temperatures.

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Is this controlled during seasons?

Tom the iac is stock so perhaps. Is this a sensor that I should only go stealer ship like the cps?

4x4JeePmaNthINg said:

Is this controlled during seasons?

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Oh boy... buckle in...

"Oxygenated" fuel is just regular gasoline with alcohol added to it (Ethanol, that good old stuff you can get drunk on to be exact.) Ethanol is similar to ethane (a gas heavier than methane, but lighter than propane) but with an oxygen molecule attached to it. Hence, the term "oxygenated fuel." Part of the reason for adding ethanol to gasoline is to increase the amount of oxygen going into the engine, which can help reduce carbon monoxide emissions (which typically are highest during rich conditions, such as a cold start) If you're worried about ethanol in your fuel, it's pretty easy to find out how much is in it at home- http://www.waldeneffect.org/blog/How_to_measure_the_ethanol_content_of_gasoline/ For what it's worth, I've done tests like this before and have never seen an ethanol level above 8 or 9%, even in the cheap 85-octane stuff where I live.

Gasoline on the other hand, has no specific chemical formula, but is a mixture of hydrocarbons that have more or less the same properties. The most important one for this discussion is the boiling point of the gasoline and yes, that is controlled during the seasons and from location to location. For example, the "winter blend" gasoline is Minnesota is going to be different than the "winter blend" gasoline in Texas. This is because the "winter blend" gasoline for vehicles in Minnesota needs to vaporize easier at low temperatures than the gasoline in Texas does. However, the "summer blend" gasoline for these two locations is likely to be pretty similar. Vaporization isn't much of an issue because of the higher temperatures in the summer however, we don't want the gasoline to vapor TOO EASILY and cause a vapor lock condition either. I should add that testing the boiling point of gasoline is a bit more involved and expensive. Part of the reason for this is because gasoline is a mixture of hydrocarbons and those different hydrocarbons boil off/vaporize at different temperatures. As a result, you're basically looking at what percentage of the gasoline vaporized at what temperature and graphing that. The only reliable way I know of to test this is by doing a Reid vapor pressure test, the equipment for which, is not cheap. OTC makes a kit that costs over $400. The really good, laboratory-grade stuff starts at about $6,500 and goes up from there...

Sounds like bad fuel to me.Might try a can of sea foam in the tank.

JeepNoob said:

The most important one for this discussion is the boiling point of the gasoline and yes, that is controlled during the seasons and from location to location. For example, the "winter blend" gasoline is Minnesota is going to be different than the "winter blend" gasoline in Texas. This is because the "winter blend" gasoline for vehicles in Minnesota needs to vaporize easier at low temperatures than the gasoline in Texas does.

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That's it, "winter blend!"

My '90 has a fuel return line. If I drive highway speed (drive it like I stole it ) everything stays in a state of equilibrium. I don't have problems after a hard run, I have problems trying to restart 10 or 15 minutes later. When I do get it running, I have to keep the RPMs up for a half-minute before the fuel system reaches equilibrium again.

Hypoid said:

That's it, "winter blend!"

My '90 has a fuel return line. If I drive highway speed (drive it like I stole it ) everything stays in a state of equilibrium. I don't have problems after a hard run, I have problems trying to restart 10 or 15 minutes later. When I do get it running, I have to keep the RPMs up for a half-minute before the fuel system reaches equilibrium again.

Click to expand...

Hmm... I'd keep an eye on that... I've seen legitimate problems cause hard starts like that (bad fuel rail check valve, fuel pump on it's way out, leaky injector O-rings, etc.) I've also seen oil-coated or greasy cam and crank sensors cause the same problem on my 4.0. (My theory is that at higher engine speeds, the flywheel/flex-plate/cam sensor target has more centrifugal force and thus, keeps itself cleaner by slinging more oil from whatever leak off.)

Also, don't bother running anything other than cheap, low-octane gas in a stock 4.0. I've actually seen old manufacturer service messages advising against it. Out in Colorado, that means 85-octane. I know the owners manual says 87, but because there's less/thimner air in the mountains, you're going to have less engine vacuum and less cylinder pressure (compression) which means you can get away with lower octane fuel. One of those things gasoline producers know about along with seasonal blends.

Hypoid said:

That's it, "winter blend!"

My '90 has a fuel return line. If I drive highway speed (drive it like I stole it ) everything stays in a state of equilibrium. I don't have problems after a hard run, I have problems trying to restart 10 or 15 minutes later. When I do get it running, I have to keep the RPMs up for a half-minute before the fuel system reaches equilibrium again.

Click to expand...

Hmm... I'd keep an eye on that... I've seen legitimate problems cause hard starts like that (bad fuel rail check valve, fuel pump on it's way out, leaky injector O-rings, etc.) I've also seen oil-coated or greasy cam and crank sensors cause the same problem on my 4.0. (My theory is that at higher engine speeds, the flywheel/flex-plate/cam sensor target has more centrifugal force and thus, keeps itself cleaner by slinging more oil from whatever leak off.)

Also, don't bother running anything other than cheap, low-octane gas in a stock 4.0. I've actually seen old manufacturer service messages advising against it. Out in Colorado, that means 85-octane. I know the owners manual says 87, but because there's less/thimner air in the mountains, you're going to have less engine vacuum and less cylinder pressure (compression) which means you can get away with lower octane fuel. One of those things gasoline producers know about along with seasonal blends.

Bobbie said:

Sounds like bad fuel to me.Might try a can of sea foam in the tank.

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why would the jeep start fine later in the night if this was the case? and i drove home fine today. btw what is seafoam added to the fuel going to do?

JeepNoob said:

Hmm... I'd keep an eye on that... I've seen legitimate problems cause hard starts like that (bad fuel rail check valve, fuel pump on it's way out, leaky injector O-rings, etc.) I've also seen oil-coated or greasy cam and crank sensors cause the same problem on my 4.0. (My theory is that at higher engine speeds, the flywheel/flex-plate/cam sensor target has more centrifugal force and thus, keeps itself cleaner by slinging more oil from whatever leak off.)

Also, don't bother running anything other than cheap, low-octane gas in a stock 4.0. I've actually seen old manufacturer service messages advising against it. Out in Colorado, that means 85-octane. I know the owners manual says 87, but because there's less/thimner air in the mountains, you're going to have less engine vacuum and less cylinder pressure (compression) which means you can get away with lower octane fuel. One of those things gasoline producers know about along with seasonal blends.

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I think we are on the same page. I tried cycling the key (weak pump), I tried clearing the cylinders (leaky injectors), I tried opening the hood after a run on the freeway, all in the same afternoon. Circumventing heat soak produced the results I wanted.

Before anyone starts talking about hood vents, ambient temperatures have been in the high 60's to low '70's... "Shorts, T-shirts, and flip-flops" weather in Colorado.

4x4JeePmaNthINg said:

why would the jeep start fine later in the night if this was the case? and i drove home fine today. btw what is seafoam added to the fuel going to do?

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Sea-foam is good for lots of things. Please Google it and read up on it.You may have water in the fuel that will Seattle after it sets and when you drive it the water mixes with the fuel.

well i bought some foam and filled up with e85 just to see how she would do. ive run 85 before but never really paid attention to how she drove/mpg. 87 has always run well and 91 is pointless.

I wish there was somewhere i could run not ethanol gas just to see. everywhere in CO i know of sells race fuel so thats beyond DD use.

If its not the fuel and the IAC, is this something that can be checked or do the just die?

thanks for everyones help thus far theres been some good reading here

4x4JeePmaNthINg said:

well i bought some foam and filled up with e85 just to see how she would do.

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DOH!!! :gee:

I helped a guy who took everyone's advice and ran E85 + I forget how many fuel additives + ran his XJ on the highway at high speeds, ALL AT THE SAME TIME!

When we took the CAT off his Jeep, the front of the core was melted into a blob. I wish I took pictures of that one!

Granted, he was trying to pass emissions, but you need to apply a little methodology and try things one at a time.

4x4JeePmaNthINg said:

well i bought some foam and filled up with e85 just to see how she would do. ive run 85 before but never really paid attention to how she drove/mpg. 87 has always run well and 91 is pointless.

I wish there was somewhere i could run not ethanol gas just to see. everywhere in CO i know of sells race fuel so thats beyond DD use.

If its not the fuel and the IAC, is this something that can be checked or do the just die?

thanks for everyones help thus far theres been some good reading here

Click to expand...

Give time to work.

Hypoid said:

DOH!!! :gee:

I helped a guy who took everyone's advice and ran E85 + I forget how many fuel additives + ran his XJ on the highway at high speeds, ALL AT THE SAME TIME!

When we took the CAT off his Jeep, the front of the core was melted into a blob. I wish I took pictures of that one!

Granted, he was trying to pass emissions, but you need to apply a little methodology and try things one at a time.

Click to expand...

haha unfortunate, but im not going to crazy with it. I added the seafoam to the TB & gas tank last night. Not really the smoke show I thought I would have from the videos ive seen. The injectors are fairly new BWD 4 hole as of last summer so i dont think ill notice any difference there. seafoam says its a fuel stabilizer but as ive stated above i filled up with 85 so maybe it was bad gas and i save $$$. I really dont know as random as this issue was. one thing i didnt metion was i had a 1/4 tank left when this problem occurred.

Give it time i shall! thank you for everyones input. as the miles climb i keep learning lots about my jeep.

That sounds more like a heat soak and or fuel pressure leak down issue after the pump/engine is turned off problem to me.

In over 40 years I have never seen a real fuel problem from bad or winter/summer fuel blends. I think the fuel blend may just expose an underlying issue that may get worse over time.

Hypoid said:

That's it, "winter blend!"

My '90 has a fuel return line. If I drive highway speed (drive it like I stole it ) everything stays in a state of equilibrium. I don't have problems after a hard run, I have problems trying to restart 10 or 15 minutes later. When I do get it running, I have to keep the RPMs up for a half-minute before the fuel system reaches equilibrium again.

Click to expand...

Yes it is a heat soak issue, on days when the ambient temps are in the high 60's to low 70's. If the ambient temps are in the 40's or 50's, or I leave the hood open when I park, the Jeep starts and runs like it should.

Hypoid said:

Yes it is a heat soak issue, on days when the ambient temps are in the high 60's to low 70's. If the ambient temps are in the 40's or 50's, or I leave the hood open when I park, the Jeep starts and runs like it should.

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I need to upload and share the photos of the last 89 Renix gas tank and fuel pump I just replaced. Looked like someone put paint stripper and tar in the tank. Hoses were shot, the rubber had all turned to mush tar. Most ghastly site anyone had seen. The crazy part was it ran fine till the pump motor finally died. The output hose was even leaking, broken partly.

Mine fuel hoses on my diesel also turned to mush last year. The biodiesel in the current diesel fuels eats the old rubber lines, gotta use Viton lined injector fuel hoses now.

How about troubleshooting instead of replacing expensive stuff?

First, pull the plugs and take a look at them. Is one worse than the others?

Go out at night without any lights around and start the engine, watch for sparkplug arcing.

Pull the distributor cap and look inside for carbon arc tracks.

One cylinder at a time, unplug the fuel injectors and watch for the amount of rpm differences. If one is significantly different from the rest, zero in on that cylinder.

Depending on the age of the engine, I would do a compression test, preferably wet and dry.