2700 RPM Idle 1988 XJ 4.0

[Olympic]

Hi all. Have not been here in a while. Just been lurking, but now i need your help please.
I have 1988 Jeep Pioneer Olympic 4.0 Standard.
A few weeks ago it would sometimes would Idle at about 2700 RPM.
Sometimes i could turn it off and start it up again, and it would be fine, sometimes not.
I did some digging around and did all the stuff that did not cost a lot of money (e.g. check for vacuum leaks, replaced any questionable lines. Cleaned the stepper sensor, and tps).
I thought that did the trick since last Sunday all seemed to be well. Sometimes over the last couple of days it would Idle high when I started it, but would always go back down after a min, if not right away.
Then today when I went to go to lunch, back to 2700 RPM both there and back, then again all the way home tonight.
I dropped by a few auto parts stores thinking maybe I could get a OBD reading as i believe it is one of the sensors going bad. They were clueless.
So when I got home tonight I did more digging and turns out in 1988, this model not only does not store codes, but does not even have a service engine light?
Seems there are about 5 sensors that could cause this, all ranging from about $60 to $125.
Being electrical parts they will not take it back if I choose the wrong one.
I am leaning toward either the CTS on the side of the block (coolant temp sensor) or the idle speed stepper motor.
Your input would be much appreciated.
Thanks

[prerunner1982]

Does it have a throttle position sensor?

[Olympic]

Does it have a throttle position sensor?
Yes Sir, I am sorry. When I said TPS, I was referring to the Throttle Position Sensor. All these abbreviations confuse the heck out of me, CTS, TPS, OBD, PCM, blah blah blah :bang:

[prerunner1982]

Yes Sir, I am sorry. When I said TPS, I was referring to the Throttle Position Sensor.

No apologies needed. I know what a TPS is, I just failed to see that you mentioned it. I wouldn't totally rule it out though.... even though you have cleaned it, it may still be failing.

I am sorry that I don't have any real answer for you. When I first saw the title of your thread I thought vacuum leak as that is typically the problem, but you already covered that. TPS was my next thought. If I had seen that you already mentioned it I probably wouldn't have posted.

I am sure someone else will chime in with more/better answers.

[Olympic]

No apologies needed. I know what a TPS is, I just failed to see that you mentioned it. I wouldn't totally rule it out though.... even though you have cleaned it, it may still be failing.

I am sorry that I don't have any real answer for you. When I first saw the title of your thread I thought vacuum leak as that is typically the problem, but you already covered that. TPS was my next thought. If I had seen that you already mentioned it I probably wouldn't have posted.

I am sure someone else will chime in with more/better answers.
Cool. Your input is much appreciated non the less. Thanks.
I read some where here as I was searching the database for answers before I posted. They said "I just throw $100 bills at it till I find the problem" Then somebody else replied with "I do the same thing?" heheh I am not rich like you guys? :o

[knothead96xj]

the TPS was my first guess too. i had an issue with my jeeps idle years ago. a couple different mechanics couldnt figure it out. one buddy hooked it up to his scanner and showed that the O2 sensor was on the edge of being bad. just good enough to not throw a code but bad enough to cause the idle problems. new O2 and the problem was gone. it had drove me crazy enough to almost sell the jeep. dont know if this will help you, but thought i would throw it out...

[Olympic]

the TPS was my first guess too. i had an issue with my jeeps idle years ago. a couple different mechanics couldnt figure it out. one buddy hooked it up to his scanner and showed that the O2 sensor was on the edge of being bad. just good enough to not throw a code but bad enough to cause the idle problems. new O2 and the problem was gone. it had drove me crazy enough to almost sell the jeep. dont know if this will help you, but thought i would throw it out...
This is exactly the type of input I am looking for. Sort of help me sort it out in my head before I start throwing $100 bills at it. lol
Since I can not get a scanner reading on a Renix OBD1, or even a flashing service engine light reading for that matter, it's a crap shoot, unless you are good with a ohm and volt meter (which I am not).
But, I would think if it was a O2 sensor, I would think it would Idle poorly at all times? Idle is not a problem. When it is Idleing at 6oo RPM is Idles as smooth as it does at 2700 RPM?

[knothead96xj]

mine idled smooth, but would go up over 2500 at times. i would stop at stop signs with high idle, tap the gas like you would to un choke an old carb, then it would be fine for a while. some times it lasted for days. the inconsistency drove me up the wall. this went on for about a year. (yeah, i know:)

[Mudderoy]

Hi all. Have not been here in a while. Just been lurking, but now i need your help please.
I have 1988 Jeep Pioneer Olympic 4.0 Standard.
A few weeks ago it would sometimes would Idle at about 2700 RPM.
Sometimes i could turn it off and start it up again, and it would be fine, sometimes not.
I did some digging around and did all the stuff that did not cost a lot of money (e.g. check for vacuum leaks, replaced any questionable lines. Cleaned the stepper sensor, and tps).
I thought that did the trick since last Sunday all seemed to be well. Sometimes over the last couple of days it would Idle high when I started it, but would always go back down after a min, if not right away.
Then today when I went to go to lunch, back to 2700 RPM both there and back, then again all the way home tonight.
I dropped by a few auto parts stores thinking maybe I could get a OBD reading as i believe it is one of the sensors going bad. They were clueless.
So when I got home tonight I did more digging and turns out in 1988, this model not only does not store codes, but does not even have a service engine light?
Seems there are about 5 sensors that could cause this, all ranging from about $60 to $125.
Being electrical parts they will not take it back if I choose the wrong one.
I am leaning toward either the CTS on the side of the block (coolant temp sensor) or the idle speed stepper motor.
Your input would be much appreciated.
Thanks

I have a 1998. My idle was about 1700 rpm on ocassion. After some research I replaced the TPS, fixed it. A couple of years later it started doing the same thing. I couldn't believe the new TPS died so quickly. I replaced it. It didn't fix it. I researched some more and found there was a condition where the wiring harness would rub on the fuel rail at the back of the engine. It was wear through a wire in the harness shorting it out and make it idle high. Yeah I know. I moved the harness up and it went back to normal idle.

I used a couple of tie wraps to pull it up and keep it off the fuel rail, problem solved.

[LizardRunner]

I have to agree with everyone else, the Throttle Position Sensor is most likely the culprit here. Like tony said, check the harness first, you might just have a shorting wire there and it's a simple fix.

[xj4life2]

Early models like yours can be monitored and test via the computer but it takes a very expensive one to do it (modis by snap on or a dribble tester jeep only). If it were me I would replace the IAC motor , its the one in the front of the throttle body. 99% of the time they are responcible for high idle conditions. I always recomend to disconnect the battery when you do this and after replacement it will Idle high for a few seconds as it re learns. The IAC can also be removed and cleaned , do the TB at the same time. There is a very small hole behind the IAC that can become carboned up. you can use a q tip with some B12 chemtool on it if you want. Again it will rev up after this operation but should go back to normal within seconds if all is well. I have a link posted on this form that covers testing of all the renix era sensors.You may want to keep this handy if you plan on keeping that XJ.
Here's the link: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm
Good luck !!!

[Olympic]

Well, as of right now, It looks like all those that guessed the TPS were correct.
$132.05 with tax from O'Reilly Auto Parts and just few minutes of work all seems well for now.
I read some where here to disconnect both battery terminals and touch them together to clear the computer, I went ahead and did that, and cleaned the terminals while I was at it, Then changed the sensor and put it all back together (I had already tried the other cleaning of the Throttle Body last Sunday).
Started it, drove it around the block, turned it off, and re-started it a few times, and all seems well again.
We will see over the next week or so. This is not the 1st time I thought I had it figured out. hahah BUT looking good at the moment.
Thanks so much for all your help and input.

[Mudderoy]

Well, as of right now, It looks like all those that guessed the TPS were correct.
$132.05 with tax from O'Reilly Auto Parts and just few minutes of work all seems well for now.
I read some where here to disconnect both battery terminals and touch them together to clear the computer, I went ahead and did that, and cleaned the terminals while I was at it, Then changed the sensor and put it all back together (I had already tried the other cleaning of the Throttle Body last Sunday).
Started it, drove it around the block, turned it off, and re-started it a few times, and all seems well again.
We will see over the next week or so. This is not the 1st time I thought I had it figured out. hahah BUT looking good at the moment.
Thanks so much for all your help and input.

I've read, and done, taking the positive battery cable off and then toughing the CABLE to a ground point on the engine. I usually use a long screwdriver. I hold it on that ground for 30 seconds to give the caps in the ECM time to drain to 0 volts.