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Weird Idle

L

low_life

NAXJA Forum User
Location
Anderson,South Carolina
Over the past day my 89 xj 4.0 has been idling weird. When I left work yesterday and started it up it reved to like almost 3000rpm. I put it in gear and it stayed idled at around 1400 in gear. Normally in gear its around 700.
By the time I got to my house it was back to normal, but it kinda erratic, almost like a misfire. Today its doing the same thing but not quiet as bad, its still a lot higher than normal and not as smooth. Any ideas? Also, I have only owned this xj for a few months but every since I have had it the 2-3 shift has been very firm, almost neck snapping. But with the idle acting weird it not as bad, it feels almost normal.

Thanks for any ones input.
Scott
 
Two or three things can cause what you describe. An erratic vacuum leak, a bad throttle position sensor TPS, or a idle air controler IAC.
 
My '92 XJ's idle was very strange when I first bought it and got it running again, it was a faulty TPS which was a very cheap fix at a mechanic.
 
Its usually a combination of the TPS and the IAC that causes the high idle-
there was a recall on the TPS`s of that year(I believe)- check with alldata, then your jeep dealer-.

If the TPS *ISNT* faulty- what happens is the IAC will stick just a hair open, the TPS wont kick back down...its usually just plain carbon poison. Every time I change the oil, I pull the IAC to clean the pintle, and check the TPS on my ohms meter for a clean sweep.
On my last 89XJ, I had two dead spots on my TPS- one was a hair under WOT, the other about 1/3 throttle-neither place I usually ran, so I left it.
My 89 MJ has one dead spot almost 3/4 through the sweep....If Im that deep into the throttle, I got problems. :)
 
Thanks for all the info. I looked at the throttle body tonight, and it looked pretty gunked up. I imagine the iac is probably the same. Going give it all a thorough cleaning tomorrow after work and see what happens.
thanks again everyone.
Scott
 
Pull the throttle body and clean it on a workbench. It will clean up much better than trying to clean it in place. Use a cleaner that is 'sensor and plastic safe' when cleaning the sensors. It might also be time to park at a parts store and start replacing vac lines one by one with all new hoses. We did that with our YJ and it solved a wealth of problems involving cracked and split hoses. I messed up and bought a bunch of hoses and then had to make repeated runs for the plastic L, T and Y connectors not to mention I had a bunch of hose sizes that I did not need left over.
 
I have the exact same problem. I changed the Idle Air Control Valve and the vaccum lines on top of the engine and still no luck.

So I guess my next question is - what is the TPS and where is it located and how do I test it?

XJ '89 In-Line 6 Automatic 4x4

Many thanks guys! Hope to fix this problem that's been haunting me for three years off and on.

John
 
I solved this problem on mine; the manifold bolts around #6 were just a tad loose, enough to cause a vacuum leak. It idles like it should now.
 
This can also be the warning signs of a failing cps... Mine did that in the early stages of failure. Is it ever hard to start? If so CPS might be going...
 
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