My 1998 XJ is really confusing me.
For the past several months (if not longer), it will randomly flare up to a much higher RPM than normal when first started. It almost seems like it revs higher the longer it has been sitting. It never lasts more than a couple of seconds before the IAC adjusts and it drops down to where it should be.

Normally it would never flare above maybe 1,200 RPM when first started, but these days it will sometimes go as high as 2,000 RPM as soon as it fires, which is high enough I can momentarily hear the lifters rattle before oil circulates.

Once it has been run that day, it won't do it again. It will fire straight up to maybe 1,000 RPM before settling down.


Here's some background on the Jeep, with some other details that may or may not be related:

• It's a completely stock 1998 with the 4.0 and AW4, currently on just over 200,000 miles. It's in good tune and runs very well.
• Over the last few years I've had some trouble with the torque converter lockup operating intermittently. The brake light switch was causing some issues, then I eventually found the lock up solenoid in the transmission was also failing when hot.
• I replaced that solenoid, which fixed the TC problem 100% for about 6 months. Recently the TC began to fail to lockup again, plus there were a couple of times where the hot idle speed randomly jumped to over 1,000 RPM and stayed there until I blipped the throttle.
• I decided to replace the TPS, as that was the cause last time I had a high hot idle. I bought a new NTK one, and now the Jeep idles noticeably smoother. I haven't driven it long enough to tell if it's fixed the TC issue.

The first few cold starts with this TPS were perfect, but the last two cold starts have seen the RPM flare worse than before. This morning it started right up and flared to 2,000 RPM, and stayed there for about three seconds before dropping down.

Any idea what could be going on here? I've thought about a vacuum leak that gets compensated by the IAC, but I can't imaging that would cause different symptoms when the car has been sat for longer.

 

Have you tried cleaning the IAC?

 

Yes! The IAC is perfectly clean. Seemingly it works fine too, as the RPMs drop right to where they should be as soon as it realizes something is up.

 

I'm back with more symptoms. It looks to me like it's definitely the TPS acting up - I got home from a one hour (each way) drive on Friday and my idle was all over the place. Saturday morning it was nearly undrivable - bad enough that I turned back home and grabbed a different car.
I took a quick video of the RPM at idle in Park:

It's pretty clear that it's a bad TPS, but my question now is, is it possible that bad wiring could be causing this? The TPS I replaced was only a couple of years old, and this one has been on probably a week before acting like this.

 

I read what you say about a possible vacuum leak, but that was my first thought. The IAC was my other thought; but I see you've covered that. IMHO can't hurt to double-check potential vacuum leaks.

 

Just figured out my TPS issue. The new ones can be bad. Test it using Cruiser54's method. Made my idle terrible, and poor throttle response.

 

I'd missed the last replies on this - other issues with my XJ were distracting me!

I got a new TPS - it's a Crown one stamped with "CTS", which I think is the company that made the original Mopar ones. It idled high all the time for a couple of drives, but has settled down and now behaves perfectly. The trouble is that the high RPM startup has not gone away, which makes me think I've got this wrong.
I was definitely having TPS issues, but it looks like these issues were not linked!

One symptom I find odd is that I can avoid the high-RPM startup by very briefly turning the engine over, quickly enough that it doesn't start. If I then start up like normal, it doesn't do the high RPM flare at all.

 

sounds like the iac motor needs to be replaced

 

Have you tried resetting your ECM after replacing the TPS?

 

Yes, computer has been reset multiple times throughout this process.

 

I want to bring this back up again as I'm still struggling with it. Since I last looked into this, I've replaced the worn bearings for the throttle shaft in the throttle body. Unfortunately the problem is not better, and I'd say it's actually starting to get a little worse.

It's still the same - when I start the Jeep up, it flares to around 2,000 rpm for a couple of seconds, before the computer realizes what's going on and uses the IAC to pull the idle back down. If I *very briefly* turn the engine over (not enough to start) before starting it properly, it does not flare so high.
This is worse when it's been sat for a few days, although it's starting to happen sooner after it was last driven.

This has me wondering if I have a vacuum leak - big enough to cause the flare on start up, but small enough that the IAC can compensate for it once it realizes it's there.

My question now is - how do I find a vacuum leak? I'm aware of smoke tests, but don't have the equipment. I also have a handheld vacuum pump, but it's not powerful enough to test things like the brake booster.