Pulsing idle

2001 Cherokee. (just bought it 2 days ago) 149k 4.0 Aw4 231

Just bought this jeep and there is a couple of problems with it.
First, It has a weird knock. The sound goes away when giving it any throttle. Sounds like its coming from the top end. I am thinking it could be rockers, lifters or possibly a bent pushrod. Its just really starting to annoy me.

Next, It has a weird pulsing idle. I don't quite know how to explain this. But this morning when i went out to start it it didn't want to stay started. The guy i bought it from was thinking it could be the IAC but i am thinking TPS.

Any tips guys? TIA

1. Pull the IAC, clean it and the throttle body with a throttle-body approved cleaner, toothbrush (if you don't have an old one, use the GF/wifes), and cotton swabs. Reassemble.

2. Test the TPS--it is not adjustable on yours.

Post back results.

My vote is a sticky IAC.

Mine is noisy on occasion, but has been that way for 100k miles.

KH

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
　

Thanks for the replys guys... but I am thinking it may be a bigger issue. As in a cracked head.

Crimzon316 said:

Thanks for the replys guys... but I am thinking it may be a bigger issue. As in a cracked head.

Click to expand...

*Are you seeing coolant level drop in the reservoir?
*Any "milkshake" goo seen on oil filler cap or dipstick?
*Any increase in level on oil dipstick?

If you suspect a cracked head, you can pull the valve cover and see if you see the crack(s) between cylinders 3-4 is where the casting flaw is.

Or cooling system pressure test (engine running), compression test, block test (sniffing coolant for presence of hydrocarbons) and cylinder leakdown tests can help you isolate this.

Ill be pulling the valve cover after work today. See if I can't see a crack.