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dvann 05-07-2011 09:15 PM

Desperate for High Idle Solution
03 Rubicon I6 and manual. I have an intermittent high idle 1100-1500 RPM and wanted get some advice on it. I have searched a thousand times and replaced all the TB sensors, some even twice. I have cleaned the electrical connectors. I removed my TB and cleaned really well and even removed the IAC housing and cleaned it out. I have checked for vacuum leaks till I'm blue in the face. I have replaced spark plugs. What happens is this, it does not idle high when you first start it, aside from the normal cold high idle as it should. IT will just start idling high after the initial 2 minutes and it is intermittent. What I have noticed is this, you can turn the ignition off and re-start it and it usually clears up! I also have disconnected the battery for 24hrs and I thought that doing so had fixed the problem, but it returned. I checked the upper manifold bolts and they were tight, but was not able to get to the bottom ones. The one thing that I have not replaced is the O2 sensors. I was thinking of replacing the two upstream O2 sensors. I am curious if this could be some sort of electrical problem or maybe can an O2 sensor cause an intermittent high idle of say 1100-1500 RPM. Also, the engine runs smooth as can be, no rough idle at all. If it were a vacuum leak it would happen all the time, and 99% of the time I can turn it off and restart it and it goes away. Any ideas would be much appreciated.



Black98 05-07-2011 10:11 PM

my first guess would be a small hard to see leak in one of the hoses attached to the intake manifold but the 02 sensors in the headers/exhaust manifold seem to also be a good chance if resetting the computer actually affects the high idle. GL

Jerry Bransford 05-08-2011 09:42 AM

I wouldn't expect an 02 sensor to be the cause of that problem. Has the intake manifold ever been off the engine?

And confirming you replaced the TPS (throttle position sensor) and that you specifically cleaned out the oriface in the throttle body that the IAC's plunger fits into to meter air into the throttle body during idle conditions.

dvann 05-08-2011 09:53 AM

I don't believe the manifold has ever been off. I checked every vacuum hose on the manifold. What gets me it really seems like something electrical. I can consistently turn the ignition off and re-start it when I want to make it go away, granted it will eventually come back. I did unplug the O2 sensors and let it run rough for a while like that. It never idled high during that test, but with intermittent problems it's difficult to know without just replacing the sensor. Those O2 sensors are not cheap. I was qouted something like $90 each! I sure thought I had resolved the issue after I disconnected the battery and left off for 24 hrs. What a let down that was when it came back. I don't know what else to try at this point. :facepalm:

Oh, and yes I did clean the TB out very well and the IAC housing too. I even replaced the IAC motor twice and cleaned out the connectors with electronic spray and used dielectric grease when I put them back on.

rrich 05-08-2011 10:29 AM

Do the brakes feel right? Does pushing on the brake pedal make any difference?

When it's acting up - is the dash voltmeter different than normal?

When it acts up, what happens when you turn on the headlights? Any change?

These could be clues as to where to start looking.

dvann 05-09-2011 07:19 PM

The brakes are working fine. The problem has surfaced before when the lights were on. I just wonder if a bad O2 sensor can cause this intermittent high idle. I have sprayed so much carb/choke cleaner on it looking for a vacuum leak that I think I have given the thing a permanent smell under the hood. I have not noticed any other electrical type problem on it either. One thing I did do was that the IAC sensor I used was a Borg Warner and not the factory unit. I just wonder if that has anything to do with it. I replaced it twice. I also replaced the other sensors using standard auto parts variety not factory. I think unless anyone else can chime in with a recommendation I am going to spring for the two upstream O2 sensors.

Shelby427 05-09-2011 08:29 PM

OK, some more things to check. #1 is the IAC properly installed? If the O ring isn't seated properly it can cock the IAC and the pin will bind.

Umm the carb cleaner you used. Was it O2 sensor safe? Wrong cleaner can damage the O2 sensors.

Sticking a/c boost. When the a/c kicks on it sends a signal to boost the idle.

Defective coolant temp sensor, can trick the system and make the idle go up.

As you stated, poor electrical connections too can be a culprit. Like at the bulk head connection.

Also are you 100% sure you got the correct replacement parts. There is a difference between auto and manual trans vehicles on things like the TPS. Also there are different manufacturers of them. These each can have different settings.

And just because it's a new part, doesn't mean it's working correctly. You may need to get into checking the ranges of the TPS and such to be sure they are doing what they are supposed to be doing.

Are there any TSB's on it? Check with your Jeep dealer.

dvann 05-11-2011 07:36 PM

I have not replaced the coolant temp sensor on the thermostat housing yet. So that may be a culprit. As for the A/C boost it does this without the A/C being on, so would that eliminate this perhaps. I did replace the IAT sensor. I used TB cleaner so I would hope that it would be safe for the O2 sensors. Could the O2 sensors cause an intermittent high idle? It never idles rough and always runs smooth. I made very sure that the o-ring on the IAC motor was straight, I installed it with the TB off. I actually replaced this sensor twice because I was convinced that the IAC was bad.

Shelby427 05-11-2011 08:25 PM

All the sensors will have voltage ranges they should operate at. You'll need a factory manual to get the specs. Problem is going to try and get readings when it's acting up.
Otherwise if it is a sensor and it's not acting up, the sensor will check good.

As for the O2 sensors, maybe. A lean fuel mixture will raise idle speed. Ever notice your lawn mower when it starts to run out of gas? It will speed up right before it coughs and dies. A rich mixture tends to slightly drop rpm's.

Also if you use the defroster, the a/c is on and running.

rrich 05-12-2011 01:55 AM

"""""The problem has surfaced before when the lights were on. """""
The engine controls - IAC, TPS, MAP, etc. use a ground reference on the engine, the PCM gets it's ground from the body. Body is connected to the engine, then the battery.

Lights on - body and engine ground can be at different potential if the ground connection is not good. With a 4.0 that connection is on the right side of the engine, low down. A small braided strap. The bolt on the block gets loose. I've seen at least 3 do that - usually with charging problems. Clean and tighten the bolt/connection.

Next time it acts up try this - use a battery jumper cable to jump between a good engine ground and a good body ground. If it stops acting up -- BINGO!

Shelby427 05-12-2011 06:18 AM

Nice fix rrich, hope in this posters problem that's all it is. As stated a poor ground can do some really weird things.

dvann 05-12-2011 10:01 AM

I'm gonna try that just as soon as I get the Jeep back. That ground issue sounds like that could be it. I really think from all I have seen it has to be an electrical problem of some sort. I will post my results as soon as possible. Thanks again.


dvann 05-20-2011 10:48 AM

Well guys I think I have found it - finally! I checked the grounds first and they all appeared to be good. I disconnected the battery and then I took off all three connectors from the PCM and sprayed them out with electronics cleaner, let them dry and then re-attached firmly. Re-attached the battery. I have not had the high idle ever since. I have driven it several times now and not once has the idle hung at 1100 RPM since I cleaned the connectors. I hope this helps others. I can't tell you how many man hours I spent working on this....

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