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89 XJ Randomly Revs Engine Upon Starting

Moriarty581

NAXJA Forum User
Location
Maine
I have an 89 XJ with the 242 4.0 5spd with somewhere around 200,000 miles on it. My mother tends to drive it more then I do as I work in Montana now and my jeep is in Indiana. From time to time when you start it up (hot or cold) the engine rev's to 2500-3000 and stays. i tryed tapping the gas pettle but when you do that it just goes up and stays. when you let out the clutch the rpms stay constant. generaly the idal stays hi for less then a minunte and then returns to normal. The strange thing to me is that it dosent do it all the time or in any pattern that i can see. within the past year i have replaced the MAP sensor,EGR valve, O2 Sensor, 6 Fuel injectors, Fuel Filter, Distributer cap and rotor, Plugs/Wires. It never had done this prior to last spring. The only thing that my mother does that is diffrent from me is occasionaly pumps the gas pettle (shes old and used to carberators) but it does it when that isnt done too. I am stumped so any help would be great.
 
Pull the brake booster vac hose and let it suck up a bunch of Sea-Foam while the engine is running... Search, this is a very common problem.

helped me! Good luck.
 
I've seen this happen to me several times. I never was able to really track it down all the way, but it is vacuum related. My guess is that it is EGR related on my 87 stroker.
 
The problem lies primarily with the electronics. Make sure that the elecrical connector is clean and tight for the Idle Air Control (AIC) on the front of the throttle body. Also seperate the large electrical connector above the vacuum booster, clean the contacts and reconnect. That should take care of it. Also another thing when the idle sticks high, turn the ignition on and off three times. When you turn it on wait for the system to fully cycle. I wait til the radio comes on, then repeat.
HTH
 
Having spent the last 5+ years with five different 87-90 4.0 XJ/MJs...This (intermittent hi-idle @ startup) has popped up and gone away on every one. If I could isolate it and repair it, I'd be shocked & amazed.

I have tried (with no success) diagnosing individual elec. components with a DMM, cleaning the IAC and throttlebody with spray and seafoam (I do this every 2-3 oil changes as a 'habit'...) Replacing components, swearing, begging, crying, throwing tantrums to no avail... "usually" just shutting it off and restarting it "fixes" it.

On my 88, it had a spell where it wouldn't idle lower than 12-1500 despite anything I tried... I tweaked the lever connected to the TB butterfly (the throttle-stop?) with a handy pair of vise-grips and it's been mostly happy for over a year.

That same 88 Limited, before we bought it, wifey brought it home for us to test-drive and inspect. She let the youngest daughter move it to give it a bath (it was covered with pine pollen) She fired it up - and it went into it's demonic 3000 rom mode. Not wise enough to know better, Kiddo throws it in reverse - with the drivers door wide open- and it took off backwards. A large tree and the open door had a fight and the tree won. Luckily it didn't rip the door completely off the pillar, but it did tear it up beyond repair. What sux is that if it hadn't become wounded, we probably wouldn't have bought it.

So I :dunno: what to say... it's an 87-90 Renix thing :)
 
I dissconnected the Idle Air Control motor about 3 years ago on my 89 because of the same high idle and i have never had a problem with it since.Mine was so bad that it would do it while i was driving AND on start up.Intermittently of course.
I usually plug it in twice a year (summer,winter) so that when i turn on the A/C the engine wont bog at idle.And then i unplug it again.
 
The power cycle bit is for the newer non Renix computers.
 
thanks guys, I will try the suggestions that you have. It helps to know that I am not the only one plauged with this problem.
 
Check for vacuum leaks, I found a small hose off by the pwr. steering pump, helped a little, then I tested TPS, found it way out "wack", adjusted to spec......end of problem.
 
There was a tsb (technical service bulletin) on this issue. It is due to a faulty bonding inside the tps. I think it was changed for free in the warranty period.

I have the same problem on my 89. I revs up to 2000 rpm for 4-5 seconds.

Torfinn
 
dizzymac said:
Check for vacuum leaks, I found a small hose off by the pwr. steering pump, helped a little, then I tested TPS, found it way out "wack", adjusted to spec......end of problem.

SO where can you find specs for the TPS and how do you adjust it?
 
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