96 XJ High idle in park/neutral some times

Hey folks. I'm running a stock 96 XJ, 4wd, 4.0 HO, 277,000 + miles. For the last week or so the RPMs have been jumping up to 2,000 when I put it in park after driving it. It doesn't do it every time and it seems to happen at random. Doesn't matter if it's hot or cold etc. It seems to be happening more frequently as time goes on. I've also noticed that before it does this, the XJ wants to go without me pushing the gas. Tonight I had to hold the brake coming up my driveway when I usually have to give it a little gas. I knew it was going to rev up so I put it in Neutral instead of Park and it the RPM's hit 2500 and stayed there. I turn it off when it does this. I can re-start it right away and it will run normally.

I haven't had any work done to it recently, not even an oil change. Last thing I did was adjust the TV cable and I added some Lube Guard Instant Shudder Fix to the transmission fluid in an attempt to alleviate a TC issue. I wasn't having the RPM issue until weeks later.

Ideas?

I should add that there has been no CEL. I also see from similar posts I've read that an 1,100 RPM idle in Park isn't normal. I did not realize this because mine has always idled at that speed. Looks like I've had an issue for a while.

My throttle linkage just below the throttle body got hung up randomly like that once. Don't remember what was bent but I lined up the linkage so it didn't bind up anymore. I don't think a vacuum leak would rev it that high, maybe. Another might be the IAC if your year has one. Take it off and clean the carbon off and where it goes into the throttle body. Don't pull or twist IAC when cleaning with alcohol.

I got under the hood this afternoon. The linkage seemed fine. Throttle and cables move smoothly.

I cleaned the throttle body which was very dirty. I disconnected the wiring harness to the IAC and started to remove it, but decided to wait until I had a better torx tool because the screw on the bottom was kind of hard to get at.

I put everything back together and started it for the first time today. It started out at 1,000 RPM and once it warmed up it was idling at 1,600 RPM. The CEL light also came on.

I turned it off and reset the PCM. Started it again and it was idling at 750 RPM - great! I went inside to wash my hands and grab my wallet so I could drive it. When I came back out it was idling at 1,000 RPM. I drove it about three miles and pulled over. I put it in park and it revved up to 2,000 RPM again.

I guess my next step is to look at the IAC and check for vacuum leaks.

Spray some carb cleaner around where the injectors seat in the head. Could be bad injector seals.

Also why, would you add shudder fix to the transmission oil in order to solve a T-case issue? They are seperate from each other...

I would grab a TB gasket and take the TB off and clean it. If it was that dirty you need it off the clean that IAC passage good anyways. I am still leaning to IAC if the TB was very dirty as you said.

When it revved up to 2000 did you pop the hood and look at the linkage ? move it by hand ?

When its revved up I would be doing the " wiggle " test on related parts. I have had vacuum hoses come off and they don't rev your idle that high. Knocked a connection off doing my waterpump a few weeks ago and it revved idle maybe and extra 500 rpms.

Original_MudButt said:

Spray some carb cleaner around where the injectors seat in the head. Could be bad injector seals.

Also why, would you add shudder fix to the transmission oil in order to solve a T-case issue? They are seperate from each other...

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Not T-case, torque converter. It didn't fix the problem I was having which was the torque converter failing to engage at highway speeds, still having that problem. It did take care of the shudder that I didn't even realize was happening. It's smooth as silk at a stop now and it shifts better than it did.

Eric the car guy from YouTube says using propane to check for leaks is safer than carb cleaner. I will definitely check the injector seals. Thanks for the tip!

Injector seals will leak gas too .........:flame: Smell test too .

MACH90XJ said:

I would grab a TB gasket and take the TB off and clean it. If it was that dirty you need it off the clean that IAC passage good anyways. I am still leaning to IAC if the TB was very dirty as you said.

When it revved up to 2000 did you pop the hood and look at the linkage ? move it by hand ?

When its revved up I would be doing the " wiggle " test on related parts. I have had vacuum hoses come off and they don't rev your idle that high. Knocked a connection off doing my waterpump a few weeks ago and it revved idle maybe and extra 500 rpms.

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I tried moving the throttle by hand with no luck. The throttle moves into the closed position under the power of the spring with no problem. The butterfly looks right and the linkage doesn't seem to be hanging up in any way.

I'm certain the IAC is filthy as well as the passage. It took several applications of carb cleaner to get the butterfly area clean. I'll take your advice and remove the TB.

I'm also wanting to remove the IAC and plug the hole to see if it will lower the idle that way. From what I've read in other threads, that will tell you if you have a leak somewhere.

I'm thinking it could be a combination of a vacuum leak and a dirty IAC. It has idled at 1,000 the whole time I've had it which I think could be a vacuum leak. The 2000 RPM idle is a new issue which I think could be the IAC.

MACH90XJ said:

Injector seals will leak gas too .........:flame: Smell test too .

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Noted, thanks!

1000rpm is likely a vacuum leak, about where mine revs when I leave a hose off. After you do IAC just start pulling off vacuum hoses til one doesn't change engine speed and look for cracks and holes in that line.

I looked at pic of your motor, way less vacuum hoses than mine. I was gonna see if you had vacuum actuated 4wd like mine. That is a whole lot of lines that can leak.

MACH90XJ said:

1000rpm is likely a vacuum leak, about where mine revs when I leave a hose off. After you do IAC just start pulling off vacuum hoses til one doesn't change engine speed and look for cracks and holes in that line.

I looked at pic of your motor, way less vacuum hoses than mine. I was gonna see if you had vacuum actuated 4wd like mine. That is a whole lot of lines that can leak.

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I couldn't tell you if it is or not. I know a vacuum line when I see one, but I don't know where all of them are or what they all do.

One thing that makes me wonder is the wiper blades. They move pretty slow and sometimes they move faster or slower depending on the RPMs. Then again, I don't know how they should behave because I've never had a new Jeep. Seems like they should work better than they do though, which makes me think there could be a leak somewhere.

Jeep wiper system is shit. Worse vehicle I ever had for wiper system. Can you tell I am tired of screwing with my wiper system.

Wiper system has no vacuum parts, all electrical. I would say if your voltage is acting weird check and clean up all your grounds first. I have found they are pretty weak in the wiper motor department. But hey, we are driving rigs with 200k plus miles shits gonna break.

Huh. I did not know that. I've never seen electric wipers be so sluggish so I just assumed the ran off vacuum. Something else to add to the list.

Be careful with that list. It's a jeep and the list will never end. I'm sure there are people in padded cells somewhere saying, "rear main seal, rear main seal, rear main seal " over and over.

After watching a bunch of YouTube videos and reading several articles about how the IAC works, I am 98% sure the IAC is going to be at least half of my issue.

It's common sense now that I think about it. I read that the IAC gets oil deposits on it, and if you don't change your air filter that oil collects dirt from the air intake, then that dirt collects more oil and the cycle continues. All that build up eventually stops the plunger from moving back and forth properly and from making a good seal causing either too much or too little air to flow to the engine.

After learning this, I remembered that my air filter has about 21,000 miles on it. I looked at it when I cleaned the TB. It didn't look bad, but one small part of the seal was chewed up like it hadn't been properly installed. I'm sure this has been a big contribution to my current problem.

I have probably doubled my knowledge of auto mechanics since I got my Jeep back in March so all you tech savvy folks will just have to look over me. I know just enough to get myself in trouble and I appreciate the opportunity to come here and learn.

I pulled the IAC today. That lower torx bolt was a PITA, but with a ratchet, an extension, a swivel, and an extra set of hands it came out. I pulled chunks of grease out of the hole, it was pretty bad. I got everything cleaned up and back together and went to work delivering pizza. I noticed a smoother idle right away. The RPMs no longer fluctuate when the brake is pressed, the wheel is turned, etc. The throttle responds better, it runs better. I still need to completely remove the TB and clean everything better. I just didn't have time today.

My idle speed has improved. I get a 800 RPM idle at start up with the engine at operating temperature every time now. However, after I drive it and then put it in neutral or park, the RPMs still go up to 1,400 to 1,600 RPM's some times. Some times it will go to 1,100 to 1,150 which seems about right. I'm having a hard time understanding this. If I had a vacuum leak, whether it be in a line or loose manifold bolts or whatever, I wouldn't expect to be getting the 800 RPM start up idle with a hot engine. Then again, I don't know much about engines. Could there be another cause?

I made up a set of vacuum plugs for the intake manifold a long time ago. High idle, usually around 1500-2000 RPM often means a vacuum line is broken, They don't have to break all the way, they can be cracked and the RPM increase incremental.

I cut short pieces of the appropriate sized tubing around two inches long and plug the ends with a bolt. I keep a set in my toolbox.

Don't fart around, plug them all, and see what happens. If the high idle remains it is unlikely to be a vacuum line issue.

The booster can cause high idle issues. But you will often hear the vacuum leak before it becomes large enough to be an issue, It will hiss when you apply the brakes. I've only seen it once, but the check valve for the booster can fail.

The intake can cause high idle issues. Especially if the idle changes as the motor heats up or cools. I periodically check my intake/exhaust bolts anyway. They do come loose. I periodically re torque mine and about half the time find some loose.

The TPS can cause high idle, usually way high, 3000 RPM or more. I'd be suspicious if my idle went up after getting the motor wet from splash or washing. Spray cleaner can also cause TPS issues, the TPS isn't sealed well.

The throttle body bottom gasket can be a problem, reusing an old seal can be iffy. The bottom of the TB rarely sits exactly where it indented the gasket the first time when reusing the old gasket, there is a little play in the mounting bolt holes.

Less likely is a faulty MAP, usually when they fail they often richen the motor way up, depends on how faulty they are. The programming is going to try and optimize fuel air mixture and may cause idle issues.

Even the battery temperature sensor can cause idle issues, when mine was bad (in my 96) the idle went up a 100-150 RPM or more. The same can happen with a faulty engine temperature sensor, 100-150 RPM higher because the motor stays in warm-up mode.

Have you tried unhooking your battery cable and rebooting the computer?

I've had good luck spraying a little penetrating oil into the IAC inlet while the motor is running, while I work the throttle a little. Four or five squirts usually cleans everything up quick, just be sure not to lose the nozzle down the TB throat (the voice of experience speaking). The vast majority of the times I've had IAC issues, my battery or my alternator crapped out completely a week or three after the IAC issues first showed up. I don't really know why, but it has happened to many times to be a coincidence.

Ohm test your grounds. Corrosion and old age degrades the ground contacts. Same with the ground crimp mounting rings.

Use the key test to see if you have any MIL codes.

I've run out of ideas, for the moment.

fishinpolejoe said:

My idle speed has improved. I get a 800 RPM idle at start up with the engine at operating temperature every time now. However, after I drive it and then put it in neutral or park, the RPMs still go up to 1,400 to 1,600 RPM's some times. Some times it will go to 1,100 to 1,150 which seems about right. I'm having a hard time understanding this. If I had a vacuum leak, whether it be in a line or loose manifold bolts or whatever, I wouldn't expect to be getting the 800 RPM start up idle with a hot engine. Then again, I don't know much about engines. Could there be another cause?

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The bolts for the intake/exhaust manifold are a common problem for working loose as MUD8 said above. Why the idle can be effected by a heated up motor is , metal expansion/contraction. To add to the normal expansion/contraction we have aluminum and steel that exp/cont at different rates. At least thats my theory why I have to retorque those manifold bolts periodically.

You should use a torque wrench, buy an expensive one if you can afford it but my old $40 craftsman has worked for me over 20 years. Recently bought a high torque one for my axle/ball joint job. You can put a wrench on the manifold bolts and snug em down without, atleast you can see if it helps your warmed up idle before applying proper torque. Be careful not over torquing, exhaust studs/bolts are usually brittle from constant hot/cold cycles.

A leaking vacuum will always leak. 750/800 rpm is normal idle. Block those lines off like MUD8 said, will quickly eliminate hoses.

I would finish that TB/IAC cleaning job though. I think you know what to expect from half way doing something.

I had a very similar issue recently, my jeep would randomly try to idle up in gear and if I popped it into neutral it was rev to 1800-2100 rpms. I checked for vac leaks, cleaned the TB and idle valve, finally I hooked up my DRB and found that the TPS would randomly show that the throttle was cracked open when it wasn't. I replaced the TPS and I haven't had any strange idle issues since.

Just and FYI with the JTEC jeeps (96-01) if the ecu thinks the throttle is open it will automatically command the idle valve to fully open, so if you have a bad TPS it will try and idle high.