98 XJ intermittent high rpm in idle, park, and stopped

[killnine]

Hi folks,

I've been troubleshooting this issue for a couple weeks. 98 XJ sport, 4.0. Suddenly it started jumping up in rpm after it warms up. I notice it of course when it's parked or braked while driving at a stop light, stopped. Instead of 800 or 900 rpms it will be somewhere between 1800 all the way up to 2200 rpms. The strange thing is that it doesn't do it all the time. I've had it acting up, and then shut it off and restarted it and it was fine for a while. Sometimes it won't do it the whole time I'm driving, until I come to a stop at home and put it in park, and then it will decide to jump up. I also noticed that when it is acting normally the exhaust seems tame and not very hot. When it is idling high, the exhaust is very hot out the tailpipe. Hot enough I can't hold my hand behind it for long. Just a note. To be honest I don't think I've ever tried to hold my hind behind the exhaust at 2000 rpm before to know if that's normal or not.

Here are some things I've checked.
Vacuum leaks - I've verified a slight leak at the intake manifold, at the end near cylinder 6. A bit of smoke leaks out during a smoke test. I haven't found any other vacuum leaks. I'll be trying to tighten this down tonight.

Exhaust leak - I had an exhaust leak after the primary O2 sensor, and before the cat. I fixed that last night but didn't notice any major difference this morning. I haven't reset anything yet though.

TPS - This was replaced with an expensive oem brand last year, and all the old problem symptoms have never come back (ie. bad shifting, etc.).

IAC - Cleaned it out a few days ago. It idles smoothly sometimes, and sometimes just very high. It never 'hunts' for idle.

Throttle body - cleaned out a few days ago

MAP - Replaced a couple months ago. Not sure if the replacement might be the source of the problems.

IAT - Cleaned off the IAT a couple days ago. It wasn't terrible but it was gunked up.

O2 sensors - Using the torque app and a bluetooth obdii reader, I get up and down readings within the expected range on the primary sensor, and the secondary sensor pretty much is always flat down near the bottom of its range. I think this is normal for the primary, but the secondary shouldn't be flatlined like that down that low I don't think. This might have been due to the exhaust leak, and I haven't checked it again after fixing the exhaust leak.

So after all of this when the issue isn't happening, it sure does idle and run nicely. But the problem still continues.

Other strange problems lately:

Sometimes the gas gauge is slow to rise to its full measurement after starting the ignition.

Next steps:

I plan to tighten the intake down and see if I can get rid of the slight intake leak. I think it's been like that long before this issue though, probably since I replaced the stock exhaust manifold with headers.

I have new O2 sensors coming just in case. They are only a few years old but if necessary I will have them.

Any one have any other ideas? I've read quite a few threads like this and they aren't all exactly similar to this, so I'm a bit baffled. Anything I'm missing?

Thanks!
Alex

 

[Ecomike]

Bad IAT sensor, the one you cleaned, or a bad TPS, (check the wiring, grounds and connectors on both) or a variable vacuum leak. A sticking fuel injector is a rare possibility.

 

[killnine]

Replaced the IAT, mostly because it was showing close to 200+ F, and I thought that can't be right. The new one shows the same temps. I guess the incoming air gets pretty hot in there. The issue didn't return after replacement. It could have been the IAT, or maybe the computer finally decided to idle down after fixing the exhaust. We'll see after a day of driving.

I did notice when it was doing it, that there was a loud 'click', right before it would idle higher, every time. It would sometimes do this 3 or 4 times, idling higher each time, to the point where I would shut it off it would start to idle so high. As far as I could tell it was coming from the IAC, but I'm not 100% sure.

After replacing the IAT, it hasn't done it again, but the clicking is still happening, it is just not idling up. Very strange.

 

[Ecomike]

Replaced the IAT, mostly because it was showing close to 200+ F, and I thought that can't be right. The new one shows the same temps. I guess the incoming air gets pretty hot in there. The issue didn't return after replacement. It could have been the IAT, or maybe the computer finally decided to idle down after fixing the exhaust. We'll see after a day of driving.

I did notice when it was doing it, that there was a loud 'click', right before it would idle higher, every time. It would sometimes do this 3 or 4 times, idling higher each time, to the point where I would shut it off it would start to idle so high. As far as I could tell it was coming from the IAC, but I'm not 100% sure.

After replacing the IAT, it hasn't done it again, but the clicking is still happening, it is just not idling up. Very strange.

Sounds encouraging, The IAT may have tested OK at test time, but failed at other times, electrical part failures can come and go, they have gremlin cloaking devices LOL. The trick is to have a test meter watching them 24/7 that can catch them when the idle takes off into orbit.

Is the clicking the AC clutch, or electric fan cycling?

 

[killnine]

Sounds encouraging, The IAT may have tested OK at test time, but failed at other times, electrical part failures can come and go, they have gremlin cloaking devices LOL. The trick is to have a test meter watching them 24/7 that can catch them when the idle takes off into orbit.

Is the clicking the AC clutch, or electric fan cycling?

It might be. I normally associate that with a speed up of the idle just a little bit, but maybe it is somehow not working correctly. It did sound similar. I'll take a better look tonight.

 

[killnine]

The clicking is coming from the IAC. I replaced it, but it's doing the same thing. I tried to clean up as many grounds as I could. TPS seems to be getting good 5v. Ground to tps shows very low < 1 ohm. The odbii app is showing the throttle position at somewhere between 1-15% usually, which is fairly normal. Sometimes when I start it up it will be perfectly normal until it warms up to around 200 degrees, and then boom, the computer starts triggering the IAC, at steps it seems. First it will go from 700-900 rpm, then 900-1300, then it just jumps up to like 2300, and it will climb all the way to 3000 rpm, which is where I usually just shut it off. O2 sensors are still showing what appear to be fairly normal readings. When it is idling normally it is maybe at 10 degrees advanced. 15 vacuum. When it starts idling higher of course vacuum and advance climbs. The advance will usually be around 35 when I shut it off. This is getting to be dangerous, so I'm SOL at the moment until I figure this out.

 

[killnine]

Well, I feel like an idiot. I knew there was a leak in the intake manifold and my logic was that this wouldn't have caused such a high idle, and it wouldn't wait until it warmed up. Well, it does, and it did. I tightened all of the intake and exhaust manifold bolts. Most of them were loose. Runs better than it has in a long time.

 

[killnine]

I should add to this post that the take away for me here is that when your intake air temperature sensor is showing you 220 degrees or even close to that, unless your IAT is bad, you are sucking air into the intake from somewhere other than just the normal ambient air.

 

[Ecomike]

I should add to this post that the take away for me here is that when your intake air temperature sensor is showing you 220 degrees or even close to that, unless your IAT is bad, you are sucking air into the intake from somewhere other than just the normal ambient air.

Good point, I have never seen over 160F on the intake manifold temp on my Renix and thus only about 140 F on the intake air temp at say 2000 rpm.

I was wondering about that, and I have never ever heard an IAC make any noise I could hear, must have been the loose bolts-manifold clicking (expansion-contraction), which makes total sense.