i own a 98 4.0 with a 5 speed ax15. it has the common fuel pump problem as most others and it just started acting up today. the truck usually idles around 750-800 if i'm reading my tachometer right. today when i would pull up to stop lights/signs and coasting in nuetral it would jump to about 11-1200 rpm and stay in that range or it would drop back down to 6-700 rpm briefly and then back up to 1200 rpm. my guess is a vacuum leak but i havent been able to track one down yet. just wondering if there are any other things that could lead to this problem. never heard of the fuel pump causing that but could that be issue?? or a sensor or something along those lines? as usual, thanks in advance for any advice.
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sporadic idle
- Thread starter MNxj98
- Start date
birchlakeXJ
NAXJA Forum User
- Location
- Melrose, MN
Absolutely continue your search for a vacuum leak. Another thing to consider and comes highly recommended is to clean your throttle body and your idle air control.
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
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
joe_peters
NAXJA Forum User
- Location
- Grants Pass, OryGun
All things "normal" at operating temperature (after 20 minutes of operation) the PCM uses the IAC to maintain the correct idle speed.
One of the major inputs to the PCM is the TPS. If the TPS (non-adjustable 91+) is out of specs the PCM is going to have some weird ideas on where the idle speed should be, so it will be sending extend/retract signals to the IAC in error.
Yes, clean the throttle body, IAC, and the IAC passageway with a throttle body safe cleaner (the cans will be marked as such). While you have things open, make sure the throttle plate is closing properly, and that the TB mounting bolts are tight.
Test the TPS, replace if out of spec.
Normally with an intake leak the problem effects the idle speed fairly consistently, but with modern engine management the system will try and "make things right" by enrichment of the air/fuel mixture, adjusting the timing, and moving the IAC, so things can get a bit strange.
Do the basics as described and post back your results.
One of the major inputs to the PCM is the TPS. If the TPS (non-adjustable 91+) is out of specs the PCM is going to have some weird ideas on where the idle speed should be, so it will be sending extend/retract signals to the IAC in error.
Yes, clean the throttle body, IAC, and the IAC passageway with a throttle body safe cleaner (the cans will be marked as such). While you have things open, make sure the throttle plate is closing properly, and that the TB mounting bolts are tight.
Test the TPS, replace if out of spec.
Normally with an intake leak the problem effects the idle speed fairly consistently, but with modern engine management the system will try and "make things right" by enrichment of the air/fuel mixture, adjusting the timing, and moving the IAC, so things can get a bit strange.
Do the basics as described and post back your results.
