Thatguy

I seem to be having what I consider an odd issue..... After the car has been running a while if I get stuck at a long light or waiting in line (like at the bank drive through) The care will idle rough for a moment then be fine or sometimes it just cuts outs.

I have a 99 xj and thank you for any help.

cruiser54

Have you ever cleaned your throttle body and IAC?

Any engine codes flashing on the dash?

Thatguy

no lights come on at all. I put a bottle of carb cleaner in it but have not gone digging into anything yet. I have to wait till i have a day off.

cruiser54

Clean your throttle body and IAC first.

tjwalker

I agree with cruiser54 here. Your throttle body and idle air control should be cleaned. More on that below.

Also a good time to review tuneup hardware status. Fresh Champion copper plugs gapped to .035, new plug wires, distributor cap and rotor (brass contacts). A solid secondary ignition is very important.
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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

Thatguy

I am going to start work on it on friday but wanted to give an update that it cut off with no warning while driving at about 45 on the way home from work. I put it in neutral started it back up and drove home fine from that point on......

tjwalker

That symptom could be your "crankshaft position sensor" starting to fail. It is a very common failure on the 4.0 engine. In fact, I have a new one for my 99 sitting about a foot away from this computer. If you do end up buying one of these, buy it DIRECTLY from Jeep. Save yourself the heartache that many experience when they purchase an aftermarket crank sensor. They are NOT all the same!