1990 XJ high idle (no time to search, sorry.)

My 1990 XJ with 4.0 AW4 was running fine when I parked at the gas station. I went in and bought a 6 pack and came out. When I started it it idled at approx 3200 rpm. It never did come down, I had to drive home like this. How do I isolate either a bad TPS or IACV. I am an aircraft electrician with a pretty good DMM here at the house, I just need some specs. Thanks.

In addition, It idled fine when I put it in park to shut it off. I'm fairly certain it is not vacuum related. This just came out of the blue with no poking or prodding in the engine bay. Thanks!

i chased that same monkey in my renix era 4.0
after 2-3 tps's and an IAC, i unplugged the IAC and called it a day.... IT NEVER GAVE ME A PROBLEM AGAIN!!!!
i realize that there is some thing to fix or replace, but after cleaning c101 and adding a ground to the TPS, i was tired of it.
i tried swapping relays, and tested each relay in the series of 4-5 of them next to eachother. Good luck.
BTW, there is a write up around here that explains all of the how to step to test the iac and the tps.

I pulled the IAC, wiped the carbon off of it and stuck it back in. Now its fine but I am sure it will come back at the absolute worst possible time...

I have the opposite problem, low idle. I would pull the TB off and clean it good and it wouldn't hurt to pull the entire intake and clean it.

the IAC is a bipolar linear step motor, so I really don't know how to test it except checking continuity/opens etc. See a schematic for which pins should show continuity and which should be open. The TPS is pretty much just a variable resistor with one end grounded, the other end connected to +5V, and the center wiper sent back to the ECU + TCU though. I'd have to bet on an intermittent TPS or a jammed IAC like you already checked - x2 on xjtrailrider's suggestion of cleaning the throttle body, the chamber that the IACV plunger seats on can get just as covered in deposits as the plunger itself. I'd tend to think that a gunked up IACV would cause a low idle though, unless there is something really big stuck in there that is holding the valve open.

My bet's on the TPS. Maybe the MAT/MAP/whatever else related an XJ has though?

my 89 did the same thing(5-speed), idled up to 3200rpms. i corrected the problem by reseting/adjusting the tps to spec and cleaning the tps wiring harness. i believe the problem had more to do with moisture in the tps.

also check that the throttle plate isnt stuck open.

After I re-installed the IAC, the motor was idling ok. I went to the TPS and manually pressed the lever that follows the throttle plate in order to simulate an erratic TPS. The motor blipped a tiny bit but that is all. There was no change in idle speed. I think after work I am going to inspect the ground for the TPS. For some reason, now the idle stays at about 1300 at all times. Still high, imo.

Should be more like 700.

I can't believe no one asked the obvious question here, WHAT KIND OF BEER DID YOU BUY?
seriously, my jeep gets pretty pissed if I go cheap on the beer man....

My jeep did the same thing once turned out to be the TPS.

Curtis_H said:

My jeep did the same thing once turned out to be the TPS.

Click to expand...

x2