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Sporadic High Idle on Start-up

CamoChero

NAXJA Forum User
Location
Dallas, Texas
I have an '88 4.0 aw4 and every now and then, when I start it up it will idle around 1750-2250 rpms. I shut it off and restart it and that takes care of it for a while. It doesn't matter about heat or cold, or day/night, full moon, planet alignment or any other situation lol. It seems totally random. Sometimes I have to shut it off and turn it back on a couple of times to get it to "fix" itself. I'm thinking it might be the IAC but not sure. I just recently purchased it, so I'm not sure about tune-ups and what not, nor any idea the last time an O2 sensor was replaced. The only thing I have been able to do so far has been some gaskets and an oil change. The high idle thing doesn't affect it while its running, i.e. it won't happen while I'm sitting at a red light or stop sign, it just happens on start up. Any ideas?
 
Definitely pull the IAC, clean the throttle body with an approved cleaner paying particular attention to the IAC port, clean the pintle of the IAC being careful, they can break. Never energize the IAC when it is not mounted, there is no built in stops and over extension will break it.

Hit your grounds--Renix are susceptible to all kinds of weird electrical issues related to grounds. Replace the POS braided strap from the head to the firewall with a 4 or 2 ga. cable, clean the metal surfaces down to bare metal, treat with an anti-corrosive agent, make the connections TIGHT. Hit the grounds on the dipstick tube--ICM, ECU, o2 sensor--critical elements! Treat the same, bare metal, anti-corrosive, TIGHT.
 
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
 
 
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