'99 XJ Dead Battery, won't idle...

[Scott Williams]

My son left the interior lights on last night, and the Jeep wouldn't even turn over this morning... I finally got it jumped this afternoon, by giving it gas while I cranked it... but now it won't idle. It will run if I keep my foot on the gas, but as soon as I let off, it dies. It doesn't even try to idle.
I'm going to get the battery checked and replaced, but that seems like an odd problem (that I've never seen) after a dead battery...???
Any ideas?

 

[JerryV]

Try letting up on the gas really slow after it is running. I think that the computer needs to find idle rpms again.

-jerry

 

[RichP]

It is not a real good idea to jump these new vehicles. The better solution is to put the jumpers on the dead battery and allow the running vehicle to charge the battery back up, usually takes about 20 min to accomplish this from a dead one, then take the jumpers off.
If the battery is now able to start the engine you might want to disconnect it again, let it sit for a half hour or so then reconnect it. The computer has to relearn the settings.

 

[Scott Williams]

What does jumping do that is not good for it???

 

[seanR]

My 2000 did the same thing, I left the CB on for two weeks, disconect the neg cable and let it sit for a 1/2 hour, hook it back up, every thing will be just fine.

 

[RichP]

The new cars and ignition systems are very sensitive to differentials in voltage. Your ground may not be the same as 'the other' ground. You can destroy BOTH vehicles systems if your system spiked when you start it or his spikes when you hook it up. There is a safe way to do it but I don't unless the person I'm jumping is willing to make good on a new CPU. The other thing is that with the electric fuel pump you really need to have good voltage in the system. 12v motors don't like running on low voltage and what they lack in voltage they will make up with current. It is just much safer to hook up to the dead vehicle, charge that battery for 10 or 15 minutes [geeze, go get a cup of coffee or something] then disconnect and allow them to start on their own.
I carry a cigarette lighter jumper that you plug into the cig ligher or aux plug and it will charge a battery enough to get the other vehicle started provided the battery is not totally trashed. The other thing is that normal batteries do not like being drained dead, thats why they make red, yellow and blue tops

 

[theslacker]

If the battery is really flat, (discharged) and you tried to charge it with the alternator, you can end up blowing the diodes and will have to change the alternator, To check, make sure the battery is fully charged, (12.6v or 1.275 specific gravity) then put a voltmeter on the terminals at idle. If it's under 14.0v find a shop.

 

[dmillion]

Modern cars, trucks, and Jeeps run off the battery. That is, they get the electricity for the spark from the battery, not directly from the alternator. The alternator just charges the battery, so that the battery can continue delivering spark. This is why yours will barely run after being jump started. The battery voltage is so low that it cannot deliver a good spark.

The thing to do is to get a battery charger and leave it hooked up overnight so that the battery gets back up to a good charge. The alternative is to jump start it again, but leave the "jumping" car hooked up long enough to get some charge into the dead battery, and not just start it and disconnect them right away.

 

[Matthew Currie]

Modern cars, trucks, and Jeeps run off the battery. That is, they get the electricity for the spark from the battery, not directly from the alternator. The alternator just charges the battery, so that the battery can continue delivering spark. This is why yours will barely run after being jump started. The battery voltage is so low that it cannot deliver a good spark.

The thing to do is to get a battery charger and leave it hooked up overnight so that the battery gets back up to a good charge. The alternative is to jump start it again, but leave the "jumping" car hooked up long enough to get some charge into the dead battery, and not just start it and disconnect them right away.

In some cases, I've been told the voltage problem is the opposite of what is intuitive. When the battery won't take a charge properly (often the case when stone dead), it doesn't put any load on the alternator, as well as not buffering it, and the engine electronics get a signal that is too high and "dirty.". Some systems are more senstive than others, and it seems that Chryslers are among them. But the solution is the same. Get the battery fully charged or replace it.