97 XJ Rough Idle warm, RPM's running low - JeepForum.com
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Unread 02-20-2013, 01:09 PM   #1
bgreen527
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1997 XJ Cherokee 
 
Join Date: Jun 2011
Location: Arlington, Virginia
Posts: 1
97 XJ Rough Idle warm, RPM's running low

Long time reader, first time poster. I've got a 97 Cherokee Sport 102k miles. For the past month I've gone through some ups and downs with it. First I started to have a problem with loss of power and a misfire on #2, then a week later I started to have a really rough idle when sitting in traffic after the engine was fully warmed up. Never stalled despite some crazy shaking at times. It fires up just fine warm and cold.

CEL never came on, no codes thrown. Cleaned intake manifold, transfer case, changed oil and filters, replaced spark plugs, cleaned fuel injectors, 02 sensors were fine, replaced a going-bad coolant pressure sensor. Car seemed to be running fine for two weeks. I don't drive it daily, just short trips around town and longer drives on weekends.

I thought my problem had been solved until yesterday when I had to run 3-4 errands, lots of stopping and turn on/off. Engine starts to idle rough again at stoplights, needle bouncing between 750-1000, never stalling, occasionally "adjusting" itself and idling fine!

Another slight problem I have noticed recently is that my RPMs rarely exceed 3k even when floored while moving. It gets up to speed and shifts fine. It will hit 5k easy while in Neutral though..

I'm just about at my wits end.. HELP!

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Unread 02-20-2013, 05:05 PM   #2
tjwalker
It's the crank sensor!
 
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1999 XJ Cherokee 
 
Join Date: Nov 2007
Location: Central Minnesota, MN
Posts: 8,408
1. Clean throttle body and idle air control. How to do this shown below.

2. If #1 above doesn't help, I'd replace the idle air control.

3. Probably not related, but finish your tuneup with new plug wires, distributor cap and rotor.
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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
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99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
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