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Idles rough

648 views 7 replies 4 participants last post by  Xjlover24 
#1 ·
Ok so my 88 xj 4.0 idles really rough it sat for 10 years I drained the gas out put a new fuel pump and filter in it put fresh premium gas in with some techron additives but still idles rough it will bounch from 750 to 1000 rpm at idle have no idea what's happening I've dine some research about vacumm leaks and can't find one anywhere?? Any ideas?
 
#2 ·
1. I'd start with the below procedure of cleaning the idle air control.

2. Test and adjust your throttle position sensor as well if no joy with #1

3. After sitting for 10 years, it could use fresh plugs, plug wires, distributor cap, rotor, and check air filter.

4. Cruiser54 has a bunch of good tips for your vintage XJ.
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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
 
#4 ·
Rough idle is one of those things that can have 10-15 contributors and will continue until they're all addressed. The 4.0 also idles a little rough compared to a lot of engines, especially V or opposed flat configurations, no matter what you do.

Tune up parts are probably step 1, followed making sure your air and fuel systems are functioning cleanly, followed by making sure all of your electrical systems have good grounds for good sensor feedback and low rpm voltage, the vacuum system and finally sensors and such that may not be operating ideally, even if they keep the engine running. Make sure your IAC is clean and working in addition to the the intake cleaning instructions above when you clean out the air system. The exact priority list will vary from vehicle to vehicle and what seems to be in good shape.

Engine and tranny mounts are also a good check. A lot of normal vibration will seem much worse when the mounts are shot and transferring more of it to the body than they should be. Rubber mounts will absorb and negate a lot more driveline vibes than aftermarket polys, though the polys typically have more chem resistance in a leaky vehicle and allow less large movement under torque.
 
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