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need help with a rough or low idling xj....

milehigh

NAXJA Forum User
Location
Denver, CO
its a 2000 with 126k 4.0 of course. Problem is it was hard to start this morning, as in had to use the pedal to keep it running for a minute. Then it idled on its own. However on my way to work I noticed that it would idle rough because it was idling down low 300-400, but didnt die. When I got to work to mess with it, I rev'd it up and let off the gas and same symptom, act like it was gonna die then idle at 300-400. Aside from that it was running fine. I did have to clean the battery terminals yesterday, had a no start stituation. I cleaned them with electrical contact cleaner, and a wire brush. Does this sound related? Or is it something else like TPS or CPS going south? Thanks
 
sound like your IAC is bad, seem like it's stuck partially closed so the engine barely get enough air to runs.

my suggest is clean the hole and the IAC and try again.
 
I had a bad battery cable for a while in my 96 creating the same simptoms. everytime I started I had to keep the RPMs at 1500 and slowly throttle down so the computer could relearn the idle.
make sure you have a good connection at the battery and the cables are in good shape.
 
For 1997-2001 (and maybe 1996) OBD-II when you (1) let the battery rundown by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) or when the battery goes bad and needs replacing, the engine computer (ECU) re-boots and the idle settings are deleted. Your symptoms are difficulty starting and the engine will have low idle and may stall unless you keep you foot on the gas pedal. The ECU will relearn the idle settings after a short period of time. If the symptoms reoccur, you need to have you battery load tested. Even with low voltage, the engine will start but the ECU can re-boot multiple times and a new battery may be required.

Loose or damaged battery cables or ground wires may also cause or contribute to the problem.

Cleaning the IAC and using fuel injector cleaner should be part of your routine preventative maintenance.
 
thanks guys, i havent messed much with newer cars so when stuff like this happens im a bit stimied as where to even look. and chilton manuals are very limited in help. I really should find me a fsm, I wasnt even aware of cleaning the IAC on reagular basis. Ill try all the suggestions, hopefully just dirty IAC or bad connection.

I am aware of the fact that the computer resests after that kinda thing, but have never had that issue in the past with resetting the computer and having rough running conditions.
 
If you are so inclined....

The 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.

CLEANING THE IAC

1. Remove the IAC with a torx 15 driver (2 bolts)
2. Gently wiggle out the IAC from the throttle body
3. Clean the IAC with throttle body cleaner (not carburetor cleaner ) Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle as it is fragile and you can damage it pretty easily
4. Also clean where the IAC seats in the throttle body with the same throttle body cleaner
5. Consider cleaning the entire throttle body itself as long as you’re at it
6. Reinstall IAC and check idle quality
 
think I may have fixed it guys, I cleaned the IAC and throttle body. Seems to run fine now, but well see in the morning thats the real test. Thanks for the detailed explanation, I actually just now read that. I meant not knew where to look when I have a problem as in all the sensors and electronic crap as to what really controls what. Carbs are easier to fix and cheap to tune, but fuel injection is great for tuning on the fly and driveability.

Im glad you posted that, as I do have one question about your procedure. You mentioned using throttle body cleaner and not carb cleaner. Why is that?
 
well bad news started it this morning and the problem is still there. hard to start, had to give it gas and then it would start and wouldnt stay idling on its own, but clears up after a minute or two. runs and starts fine after its warmed up. I tried it once I got to work and it started up just fine and idled normal. The other thing Ive noticed when I start it cold in the morning is that it cranks kinda weak like the battery is weak. Again once its warmed up and the problem disappears it cranks just fine. Could the IAC still be alittle dirty? Or heading south for the winter? Or is there something else amiss?
 
Carb cleaner works but is definitely more harsh on the parts which can be sensitive to that solvent. Throttle body cleaner is a safer bet. Let us know if the symptoms return.
 
For 1997-2001 OBD-II when the battery goes bad and needs replacing, the engine computer (ECU) re-boots and the idle settings are deleted. Your symptoms are difficulty starting and the engine will have low idle and may stall unless you keep you foot on the gas pedal.


Have the battery load tested at a repair shop or auto parts store.
 
did that tonight, and the battery as you guessed it was bad. Doesnt take much to start a xj, with my galaxie it is either dead or it isnt. Reread your post this morning after my brain was actually working, made perfect sense. Still glad I checked my IAC it was crusty. Im taking the battery in tomorrow think its still under warrenty.
 
got the battery replaced, 6 years old but still under warrenty! but it definetly fixed the problem. Thanks for the help. Im still learning on these new cars, that kinda thing is never an issue on the old school stuff.
 
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