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Interesting low idle issue and resolution


NAXJA Forum User
My 2000 XJ was running fine up until about a week ago when I accidentally let the battery totally drain down by having the driver door open for perhaps three days while in my garage. I was quite surprised that it happened because its a brand new high capacity battery with lots of Ah of capacity. Anyhow, I hadn't realized that the battery was dead and tried to start the XJ. Got the clicking starter, so I threw the charger on the battery overnight.

It fired up perfectly the next morning, and all seemed good until I rolled up to my first stop light where the engine went to a 500 rpm idle and then died. I got it restarted, and for the rest of the day, it managed to barely maintain the 500 rpm idle. I had OBDII logging running, and the idle fuel trims were bouncing around from neutral to strongly negative, and ign idle timing was was sitting at 24 deg in D and 20 deg in N/P. I had assumed the that ECU would retrim ign timing and IACV, but it never did after two days of driving it around. It ran totally normal as soon as I gave it any amount of throttle. I checked TPS voltage, and it was good. It varied smoothly between 0.9 V at idle and 4.0V at WOT as provided by both a DVM at the TPS and the OBDII logging.

I finally pulled the negative terminal at the battery, popped it back on, and all was good again. Idle was back to 690 rpm, idle ign timing was about 9 deg, and fuel trims were lightly fluctuating around zero. In retrospect, something with trying to start the car with the very low battery must have whacked out the ECU memory and made it misunderstand the IACV and ignition timing around idle. Very strange.
Sounds like you found the problem, clean cables with good connections!
Wasn't cable cleanliness or connection. My battery terminals and grounds are in perfect condition. When I bought my XJ this summer, I replaced the battery, cleaned up the terminal clamps, and cleaned up all the grounds in the engine bay. The ground strap connection to the battery was clean and tight before and after this issue. The issue definitely arose from the ECU, sensors, and IACV being exposed to very low voltage.
My jeep was doing this to after a completely dead battery. However I found after taking off the IAC valve it was heavily carboned up and after cleaning it it was fine.

Also I have found that with a completely dead battery my Jeep will not maintain idle without dying without running for a while. I have had to drive it and keep the RPM's up while stopped at lights... however after about 20 minutes driving and then letting off the gas it will idle.

I always assumed it was because the battery being dead couldn't generate enough spark and the battery needed to charge but maybe it has something to do with the computer system resetting itself.
Several years later. I had exact same problem. Battery low. Jump started. Idle low 500 rpm. Then, after driving home, i pulled negaitve battery terminal off. Then replaced and problem dissappeared.