getting out of hand

Hi everyone. I have an issue with my '88 xj 4.0 thats starting to make me mad. If you start it cold everything is ok. But if you drive it then park it and come back in just a few mins to restart it it idles up to 3000 rpm and it is starting to stay up there until you put a load on the motor. I dont think its the tps because if it was sticking it would stick in its last position before shutting it off which would be idle. What do you guys think?

Jon

It's likely a sticky IAC, my experience with a wet TPS was that it idled up to 3000 at a cold start.
Clean the IAC and let a few drops of oil run behind the piston by gravity. Only takes a half hour and is free.

i need to do this. my 91 sometimes starts up rev high.

8Mud said:

It's likely a sticky IAC, my experience with a wet TPS was that it idled up to 3000 at a cold start.
Clean the IAC and let a few drops of oil run behind the piston by gravity. Only takes a half hour and is free.

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Thank you.

Stroketech said:

Hi everyone. I have an issue with my '88 xj 4.0 thats starting to make me mad. If you start it cold everything is ok. But if you drive it then park it and come back in just a few mins to restart it it idles up to 3000 rpm and it is starting to stay up there until you put a load on the motor. I dont think its the tps because if it was sticking it would stick in its last position before shutting it off which would be idle. What do you guys think?

Jon

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Back in the day my 87 waggy did this every now and then. Pop the hood and wiggle the TPS connection. (flat connector) Worked everytime. I did finilly replace the connector and havent had a prob since. Just a thought. Give it a try next time.
atriot:

Could be something in the TPS, but check the easy (cheap) stuff first. IAC, throttle plate spring, frayed throttle cable, carpet/floor mats stuck in the pedal and the mounting bolts for the TB.
I don't remember a sticky IAC ever getting as high as 3000, around 2000 was as high as I've seen. A wet TPS sensor will idle up that high and I imagine there are other types of TPS faults that will run the idle up that high.
The IAC sticking is known to often be temperature related, either cold or hot.

If I had a vote on this I would vote for the IAC being a little sticky. Clean it up and you should be good to go.

Next time it happens, shut the engine off, disconnect the TPS and restart. If it start and idles normally, you know that the problem is not the IAC, or the thottle body, etc. Just don't try to drive with it disconnected.

8Mud said:

Could be something in the TPS, but check the easy (cheap) stuff first. IAC, throttle plate spring, frayed throttle cable, carpet/floor mats stuck in the pedal and the mounting bolts for the TB.
I don't remember a sticky IAC ever getting as high as 3000, around 2000 was as high as I've seen. A wet TPS sensor will idle up that high and I imagine there are other types of TPS faults that will run the idle up that high.
The IAC sticking is known to often be temperature related, either cold or hot.

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Carpet and floor mats, You TOO???? I liked to never figured that one out on mine. It also casues low power at WOT!!!!! (Floor mat stuck under the gas pedal that is)

Loose dirty ground connections near the oil dipstick will do this too. The cold/hot cycle seems to change the ground resistance on the LOOSE DIRTY, TPS ground and causes the high idle. Of course you should check and clean all the Renix grounds, two on the engine bloock two on the firewall, and one at the battery post. Oh, and once your done with that you better recalibrate the TPS as the bad ground once it is fixed will shift the idle making you look for a non-existant bad part. Lastly, start and stop the engine 3 times to retrain the ECU as to the new, adjusted, correct idle voltage for the adjusted TPS, as it does store recent sensor min/max values.

If the TPS will not repeatably stop at 0.73 volts at the idle stop after fixing the grounds and adjusting it, the TPS is probably bad.