High Idle at Start

[HenryD]

I have a 1988 Cherokee 4.0L with 300,000 Miles on it. As of recently, whenever I start it up, from cold or warmed up, it will shoot up to 2000-3000. I can stop it and start it again and after a few tries It always comes back down. Sometimes it will be totally fine and not happen at all. Once it’s running right, it is totally fine for the rest of the drive.

I don’t believe it to be a vacuum leak due to the inconsistency of it.
I have replaced the IAC twice.
I have replaced the TPS very recently and adjusted it correctly (I think?), but when I unplug the TPS the issue goes away completely.
Anyone know what could be going on?

 

[BALTANAKT]

Sounds like a tps issue to me! Maybe reset it again and make sure your contacts are clean... If that doesnt work I'd try another one.

 

[HenryD]

Alright, I readjusted the tps (as per cruiser’s instructions), didn’t work

Replaced tps again, didn’t work

Tested grounds, all good ~.1 ohms

Cleaned the big renix harness connector, same issue still

I’m really stumped. I think i’ve got to be missing something!

 

[uglyrenixxj]

I had this issue many years ago when i bought the xj (89) and did the typical tps replacement and adjustment (w snap on red brick scan tool).
Im pretty sure that the guy before me sold it because he couldn't figure out the high idle a startup.
In the end, someone had smacked the throttle body linkage (likely in an attempt to loosen something in the throttle body) a few times and bent the throttle linkage. By the time I figured this out, I had swapped in a new tps AND a new iac motor. Adjusting the T-10 idle screw did nothing for me.
While I COULD have straightened the linkage, I was plenty frustrated with the whole thing and swapped in an UNbeaten throttle body, w my new tps and iac motor. Haven't had a problem w it in over 20 years.
FYI, for a few months, I got by, by unplugging the I.A.C. connector once the idle was where I wanted it. That worked fine till I had time to figure it all out

 

[Bent]

A slight leak in the manifold gasket will also cause a high idle problem. That was the case on my '87. Sometime before I bought it someone had threaded a metric bolt in the head and it wasn't holding.

 

[Saudade]

If you haven't resolved this yet, I can't adjust the TPS on my '88 to spec without having the same high idle at startup. I ended up adjusting it to a point where it idles fine. I don't recall the percentage or input/output voltages (perhaps I need to write it down for next time). It took a bit of trail and error.

Anyway, it runs fine, starts fine (no high idle) and recently passed smog.

 

[Ecomike]

Alright, I readjusted the tps (as per cruiser’s instructions), didn’t work

Replaced tps again, didn’t work

Tested grounds, all good ~.1 ohms

Cleaned the big renix harness connector, same issue still

I’m really stumped. I think i’ve got to be missing something!

It takes extra fuel and air to do that. ECU will supply the extra fuel, if the extra air is there.

Air could be a random vacuum leak, and they can be at many places in the dash vacuum lines of the HVAC system, any place under the hood, a long list. I would suspect a vacuum leak. Get the jeep running and touch and feel all the vacuum lines to see if moving one affects the RPMs...
Vacuum leaks at the small plastic tube lines are the most likely causes.