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Unread 11-28-2007, 12:31 PM   #1
tjfun
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rough idle

My 1998 Cherokee has a slight miss, most noticable at idle. The tail pipe is sooty black and mileage isn't that great. Was getting about 20mpg and now more like 15-17. At idle, the miss is barely noticable but when accelerating after a stop is quite noticable, especially when any load is on the vehicle. Once underway can't seem to notice any issues or miss. I removed all injectors and had them cleaned and tested, all ok. New distributor cap and rotor, new plugs. Haven't replaced the plug wires yet, but there has not been any codes thrown - had this tested and no history of defective ignition. Air filter is new. Haven't done a compression test yet, could be an exhaust valve possibly but just doesn't act like a valve issue. Any ideas? thanks.

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Unread 11-28-2007, 12:33 PM   #2
88XJSport
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Why not just change the wires? I had a bad wire, and I was having a miss as well. Changed my wires and everything was fixed. I cant say for sure your problem will go away, but i'd rule it out if i were you.
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Unread 11-28-2007, 12:40 PM   #3
1990JEEPXJ
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and maybe even go a little farther and just do an overall general tune up so you can start ruling things out.
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Unread 11-28-2007, 12:55 PM   #4
88XJSport
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Quote:
Originally Posted by 1990JEEPXJ
and maybe even go a little farther and just do an overall general tune up so you can start ruling things out.
YES. Including bench cleaning your throttle body, and cleaning the IAC pintle.
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Unread 11-28-2007, 01:21 PM   #5
SteveTheFolkie
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Your O2 sensor may be on it's last legs - it's used to determine (along with other variables) the air/fuel ratio - it's possible that it hasn't deteriorated enough to pass a code -
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Unread 11-28-2007, 03:34 PM   #6
tjfun
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Maybe I should clarify, the miss is only evident at idle and the transition from just off idle. I can't feel any miss or hesitation while at speed, and acceleration while driving is smooth, only from idle to slight acceleration. If it was wires etc. wouldn't the miss be more constant and during acceleration at any speed? I thought about just replacing the O2 sensors, but at 60-75 bucks each don't want to through $ away. This has been going on since I got the vehicle, a little over a year now. With gas prices I finally am getting around to looking at all efficiencies.
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Unread 11-28-2007, 04:29 PM   #7
BrunoS
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welcome to the rough idle club.

I suffer the exact problem. A warmed up, slight miss at idle, a stumble, can't be traced.

After a lot of work, came up with this from the forum:

In a well running 4.0, the long-reach spark plug specified is too high of heat range and causes the plug to get too hot at idle, which is corrected by higher air/fuel flow when you first step on gas. But at idle, the plug starts to overheat. Other than that , the motor runs great, and I get around 300-350 miles per tank of gas.

Anyhow, the fix is changing out the plugs to a short reach plug, such as an autolite 3924 or equivalent. I've bought them but have not installed them. I'm currently on the "champion truck plugs" of the long reach variety and they have not cured the rough idle. The 3924's are old school, normal plugs, designed in the 70's during the last of the performance era. The newer long reach plugs reach too far into the 4.0 head and run too hot in a good and well maintained and driven 4.0. Proof? I have perfect idle when I first start, until the motor warms up. The plugs are perfectly clean, almost grey-white, no vaccum leaks, great power...just a rough warm idle stumble.

One mech on this forum noted that his shop never cleans and reinstalls misfiring plugs and uses the short reach plugs in the 4.0's to cure this problem. I've held both up and they are exaclty the same...except the "standard" plugs don't reach so far in. There is additional huge info on spark plugs on a Dodge truck forum as well, with about the same info on long reach vs short reach plugs and also about plug heat range...that is, go one step cooler for each performance mod. these are guys that drag race and dino their motors after working on them.
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Unread 11-28-2007, 04:41 PM   #8
tjfun
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BrunoS
Thanks for the info, what I was looking for and does make sense. I noticed the same, when cold the engine seems to idle better, especially now that the ambient temp is averaging around 0F lately, 'Ole Jeep is running smoother and the transition time is much shorter. I will pick up a set of those plugs tonight and try em out, will let you know. Thanks.
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Unread 11-29-2007, 08:30 AM   #9
tjfun
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I installed the 3924 plugs last nite, drove around a little and a few miles this a.m. Same deal, idle is a little worse now, more noticable miss. I'd say that the acceleration stumble is about the same. I read somewhere about a TSB on an injector insulator kit to keep an injector from getting too hot and causing a miss, but I think it's for a 2000+ year range. Also seem to remember someone commenting that at idle the O2 sensors do come into play? This stumble is acting like a fuel/air ratio issue, but again it is only at idle and momentarily off idle on acceleration. At speed there are no issues. Think I'll put the stock plugs back in, and maybe check out a high output ignition system. Any suggestions/recommendations on ignition systems?
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Unread 11-29-2007, 07:56 PM   #10
BrunoS
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you could try insulating plug #3 with some heat wrap; your problem could be a worn out TPS or oxy sensor. once warmed up and in closed loop operation, the TPS and oxy sensor signals are used by the ecu to control ignition and injector pulse width along with the idle air motor.

here's a quick check: disconnect the TPS and drive until fully warmed up. yes, the CEL will turn on, but it should run fine, if it smooths out your idle then your TPS is an issue. as for oxy sensors, usually the first one in the exhaust manifold wears out first, and if you have 120k miles with original oxy sensor then it's beyond it's service life.

your rough idle is probably a heat soak issue with either a plug or injector, imo. since the 3924 plugs did not do anything for you, i'm a little shocked since that's my planned fix (have them but not installed them) but then again i've had a rough idle all along with my XJ.

you could use a volt ohm meter to check out the tps for wear, but probably cheaper to spend 40$ on a new one , again, if you have a lot of miles with original parts it's an issue.

here's a forgotten item: if you can get to it, a fuel filter change can do wonders for idle smoothness and acceleration power. on my 96, it was a $12 part and 10 minutes labor. felt like i dropped a new motor in!

anyways, try one thing at a time and don't go nuts. if it runs well so far, then be happy. if you try to fix it all at once, you'll never know what the main problem is.

i will try to heat wrap the #3 injector in a few days, and after that, i've got an oil change with the 3924 plugs scheduled.

for you, try the fuel filter, then heat wrap, then TPS, then oxy sensor if all else is running well with the motor.

how did the old plugs look? clean, gray, or brown, with ash? if it's the heat wrap solution, i would suspect that all the plugs look like they're running a bit hot.
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Newly Painted! Emerald Green '96 XJ 4x4 4.0 HO 5-spd
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Drilled out Stock Airbox / Gauge Cluster Upgrade
Magnaflow Muffler / High-flo Cat / MSD Coil
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