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Rough idel after start, goes away if I give it some gas

ScienceguyXJ

ScienceguyXJ

NAXJA Forum User
Location
Colorado Springs CO
My engine has been idling rough for 2 or 3 months right after start up but if I give it some gas and raise the rpms up to about 1200+ for a sec or two it goes away. It's getting worse as the weather is getting colder. The CEL has not come on yet. Just replaced the spark plugs a few months back. I was thinking maybe it's the idle air control but my mechanic buddy suggested it may be a coil going bad. Has anyone else ever experienced this? Thanks!
 
Ever clean your throttle body and iAC? That's considered normal maintenance.

Courtesy of TJWalker:
--------------------------------------------------------------------
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


 
This is your 2001, right? Could be the temp sensor not telling the PCM to richen up the fuel mixture. Check it with an ohmmeter or just replace it.

As far as the coil, you have the coil rail. That should continue to miss as it warms up.
 
Yeah I've cleaned the TB before never specifically the IAC though.

Yup it's a 2001.

It's spontaneously stopped doing it so I'll just have to wait and see if it comes back.

Thanks guys!
 
It's still barely idling rough after start up but not as bad as it was. Last three nights after work it's been really cold, around 0 degrees, and right after start up for less than 5 sec it almost sounds like it's dieseling or not firing on all cylinders... the exhaust is really loud but it stops after less than 5 sec then idles fine.... I've had 5 4.0s and never heard anything like this before, however this one is a big stroker...

Guess I'll start tomorrow by cleaning the TB and iac and probably just change out the temp sensor. If anyone has any other ideas it would be greatly appreciated.

I run mobil 1 syn and 50/50 coolant fyi
 
ScienceguyXJ said:
It's still barely idling rough after start up but not as bad as it was. Last three nights after work it's been really cold, around 0 degrees, and right after start up for less than 5 sec it almost sounds like it's dieseling or not firing on all cylinders... the exhaust is really loud but it stops after less than 5 sec then idles fine.... I've had 5 4.0s and never heard anything like this before, however this one is a big stroker...

Guess I'll start tomorrow by cleaning the TB and iac and probably just change out the temp sensor. If anyone has any other ideas it would be greatly appreciated.

I run mobil 1 syn and 50/50 coolant fyi
Click to expand...

Which temp sensor are you thinking of changing?
 
Not sure, was going to research the issue some more first today. Which one were you talking about winterbeater?
 
Personally I would run a can of BG44K through the fuel. If you can't find that brand, run some Chevron Techron fuel injector cleaner.
 
I run STP FI cleaner typically but it's been a bit since the last time. The injectors are 26# ford injectors and are only about 3 years old.
 
The STP cleaner has some issues as far as I am concerned.
 
I'll look for some of that 44k.

Finally got around to doing something the other night. Cleaned the IAC and it seems to be better now. Gonna sit back and watch it for now.
 
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