When I turn my jeep on it will fall on its face as soon as I let go of the key. However if I turn the key and give it gas (hold around 2000RPM) for about a minute, I'll let off the gas and it will idle right around normal. It will drive fine after. This phenomenon seems to only happen when the jeep hasn't been driven in a while (~10 hours), or maybe just when the engine is cold. Any idea what may be causing this?

2001 Cherokee Sport
Automatic
167xxx miles
No engine mods

 

Gunked up and stuck IAC would be my first guess based on this.

 

Alright I'll give that a shot. Thanks

 

Courtesy of TJWalker:
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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

 

I had the same issue with my '99. IAC cleanup took care of it.

 

I would think so too, however its weird that its idling fine afterwards (still using the IAC then)....and it occurs when engine cold/not driven for few hours. You'd suspect an IAC gunked up enough to cause symptoms, to be not so situation dependant. Just a thought.

 

Yeah, but TB and IAC cleaning are considered normal maintenance anymore. Best to eliminate it as a possibility.

 

Thanks everyone! I'll clean it out tomorrow, sounds easy enough.

As for idle after it starts, it's not normal, but it doesn't fall on it's face. Sometimes at a light it'll get down to ~500.

 

Exactly where mine idled at before the clean up. Had to give it some throttle until it warmed up.