1989 cherokee- 4.0L inline 6

Sometimes when i start my jeep i get extreme high idles (climbs to over 2000-3500) and im am concerned. what could cause this. also when i am still (running but not moving) my jeep gets a jolt. maybe a skip or knock? please help.

 

Clean your throttle body. Clean your Idle Air Control valve (a sensor attached to the throttle body. Also, test your Throttle Position Sensor to make sure you are getting the correct output. If not, replace it.

 

There could also be a vacuum hose that's come loose. Listen for a hissing noise when it's running. If you hear one look around for a small hose blowing air and pug it back in. Problem could be sad simple as that.

 

Good advice above. Check for a vacuum leak and clean the idle air control and throttle body. More on that below.
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) 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