1990 Jeep Cherokee 4.0 high idle problem

[jet_aj]

I have a 1990 4.0L/Auto XJ with an annoying idle problem... I've searched all over and have tried about everything I can come up with to no avail. I am a mechanic by education but not trade so I haven't done any diagnostic stuff for about 3 years, and I am to the end of my knowledge of this particular system.

When I first start it, it idles just fine at about 500-600RPM, not rough. After about 10 minutes after running and with the engine warmed up by now the idle raises to between 1500-1800RPM. This wouldnt be such a big issue if it was a manual trans (had a 92 wrangler with similar problem but wasnt really an issue) With the auto when I come to a stop the rear wheels want to keep pushing the jeep, which makes breaking harder, and if the roads are snowy/icy it makes for some interesting other problems... tuning sideways/ect... so I have just been putting it into Neutral when I come to a stop, then shifting it back into drive when the light changes... Not bad on streets, but sucks when I want to do some wheeling.

What I've done:
Found this article and tested accordingly
http://autorepair.about.com/library/faqs/bl051i.htm

I've replaced the IAC (no way to check so I just replaced it) = No difference
Tested and replaced the TPS = no difference
Tested the MAP = GOOD condition
Tested the CTS (at sensor and ECM) = GOOD conditon
Checked ALL vacuum lines for breaks/ect. everything seems to be in good shape but I can get new vacuum harnesses for about $35 from www.rockauto.com and thought about just replacing them...

Could the O2 sensor be causing and sort of issue here? I didn't think it would affect idle just the rich/lean condition...

Any help would be great

 

[jet_aj]

solved the problem.... missing manifold bolt=vacuum leak=high idle
I hate finishing other peoples jobs (guy I bought it from replaced a head gasket and forgot to put all the bolts back in)

 

[Jared]

Glad you solved this. A very effective, but potentially dangerous way to find vacuum leaks is to spray starting fluid slowly around the engine area. When the idle raises, you just found your leak. It will suck up the starting fluid and raise the idle speed. Just be sure to have a good fire extinguisher handy!

Jared

 

[hockeyrules]

high idle problem

just wanted to say thanks! I was having the same problem and couldnt put my finger on what was wrong, did all the electrical checking as was mentioned here and couldnt find the problem, came accross this posting earlier today, went out with the wrenches and tightened down the manifold, it was the victim of a former slacker who didnt tighten the bolts from changing the head gasket, no more problem!