Shift to Park and RPMs shoot to 2000

What would cause my RPMs to shoot to 2000 when shifting from drive to Park? They stay at a steady 2000. The check engine light is not on. It may be something related to the throttle body, I just was monkeying around with it and my engine ran perfectly fine before. Could it be the IAC motor malfunctioning?

Don't know for sure. You may try the OEM tech section here. They know all kinds of stuff about these things.

You could try adjusting the cable that runs from the throttle body to the trans. (DO THIS WITH THE ENGINE OFF) On mine (and probaby yours)- it's located on the throttle body and attaches to the arm that moves with the accelerator. There is a grey button on the end of the cable where it mounts to the throttle body. You hold in that button and pull the cable housing back. Then you slowly open the throttle all the way. It will click into place where it needs to be. Make sure you get the throttle all the way open- it will self-adjust and click into place.
Give that a shot, it only takes a second and has solved many people's shifting problems.

what year is your jeep? check for vacuum leaks

it's a 92

my 90 used to do the same thing, when i put it in park or nuetral it would rev real high, turns out i had two vacuum leaks. plugged them back in and it brought the idle right back down to normal. check that first cuz its real easy, then take it from there

where were your leaks at specifically?

take a propane torch, open it up and wave it (un-lit) around, slowly, the engine bay and pay attention to the idle... if it spikes look in that area.

otherwise just trace the vacuum lines and look for leaks, again, propane, un-lit, spike in idle.

A vacuum gauge costs $20ish at autozone or the other places, well worth having in the toolbox at that price. Engine vacuum should be 15-20 hg inches at warm idle and if it's much lower then you have a leak somewhere.

As for your immediate problem 2000 RPM is high for a vacuum leak so I would suspect the IAC is gummed up. It controls air flow into the throttle body when idling. It will even manage to compensate for a small vacuum leak by closing down the orifice a little extra. But if it gets gummed up it can't move and its unable to do anything.

Can you clean an IAC or should it just be replaced?

The Idle Air Control (IAC) is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

"Gently" wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality
　

I am pretty sure that I may have damaged my IAC I tried cleaning it when I was monkeying around with my throttle body and I got pretty aggressive with it to try to get some of the grime off. It is funny that this would not throw a check engine light code though. I will replace tonight. What is funny is this problem gets worse when the Jeep runs and gets up to a normal running temperature. I also did find an air leak (crack) in the vacuum line from the intake manifold to the fuel pressure regulator.