I have a 99, 4.0, manual, cold air intake, a VERY free flowing cat and a magnaflow on it. My problem is a lingering high idle. The idle only comes back down to 750 rpm's when I have come to a complete stop. The idle stays around 1100- 1250 rpmmost of the time. Sometimes it will drop back to 750 with no problems but most of the time it is high. Also my MPG is all over the place. I drive the same- not too much of a lead foot- fill up at the same places but i can pull 20 mpg on a tank and then go to 14-15 mpg. I have searched for a while but can't really come up with something to nail it down with. Also- when I go up really long steep hills it sounds like it down shifts like an automatic, even though its a stick. Thanks for your input.
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Idle issues
- Thread starter mizzutgr
- Start date
- Location
- Rainy side of Washington
Have you looked for vacuum leaks, cracks in the exhaust upstream of the O2 sensor, and manifold gasket leaks? My bet is on one of those.
joe_peters
NAXJA Forum User
- Location
- Grants Pass, OryGun
Check to see if something is binding and keeping the throttle plate from fully closing each time.
Have you pulled the IAC and cleaned it and the throttle body?
Check the reference voltage of the TPS, and check the output voltage, which should be 17 percent of reference voltage for the MANUAL trans.
Last, you could have a "lazy" IAC.
Have you pulled the IAC and cleaned it and the throttle body?
Check the reference voltage of the TPS, and check the output voltage, which should be 17 percent of reference voltage for the MANUAL trans.
Last, you could have a "lazy" IAC.
FiveNdime
NAXJA Forum User
- Location
- Washington
I cleaned my IAC valve and my XJ ran a ton smoother and got rid of my weird idle issue as well.
birchlakeXJ
NAXJA Forum User
- Location
- Melrose, MN
Check for a vacuum leak and consider cleaning the idle air control. Here's more on that...
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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
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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
