can a bad battery kill an engine at idle?
 

my S2 has been dying at stop lights. I've read lots of threads about O2 sensor, other stuff but I have since found out my battery barely holds a charge. Could this be the culprit?

also, anybody know anything about the new Optima car batteries - yellow top vs red top?

definatly!the alt doesn't kick in untill around25000 rpms, if that is your only problem or not, i don't know

if you're around 12 volts regulated voltage the fuel pump is always a draw. if'n you're running a/c you add more to it. at operating temps you should see just under 14 v at idle, if not, probably a bad alternator, voltage regulator, grounds and maybe even the battery is bad or right on the edge.
a high rate discharge test on the battery shouldn't drop much below 10 volts.

If the car starts without a jump I would doubt that the battery is the problem. I have a Diehard in my car I dont know about the Optima.

If the alt does not kick in until 2500 rpm, how come I read 13.9+ volts at high idle after starting? I think you have alternators and generators confused.....

At a high idle your alternator should be able to keep up with the power demands.

a generator makes dc voltage. an alternator make ac voltage and the diodes rectify it into something the battery likes, dc voltage.

the alternator kicks in when its lite goes out, well below 2500 rpm.

when jumping with another battery amperage is additive and you get amps from both, it gets the car started and/but not an indication that the car that had to be jumped has a good battery because it's now running.

if it had a good battery, why was the jump needed?

Where did you get that from? Maybe a very very worn one...

944s2cab, you could kill the engine from idle with a strong enough load on the alternator, but I don't think just a dead battery would do it. I also don't think this is your problem, if the car will start on it's own (which means the battery is holding a charge). If you want I can look at your car, but I won't be back in town until mid october (on extended vacation!). :)
Ahmet

Actually, does that read 25,000rpm? Hmm, even w/the multiplication factor, an alternator on our engines may never spin that fast...
Ahmet

I would say get your self a new battery and a new set of brushes for the alt just to be safe. Even with a battery that can't start the car a good alt should be able to keep the car running as long as there isn't to many accessories running, lights, blower, stereo etc

whats a kight? if you mean knight, and you have one in your car I WANNA SEE! is it elton john?! hehe

no just had a typo on light. lol Elton John wouldn't be my choice in knights anyways heh

i know, im just messin with ya... i dont know enough to be useful yet so i try to entertain ppl!

Thanks for your help guys, and to Ahmet for offering to look at it. As it turns out the battery was bad - Advanced Auto put it on their in-house tester and it showed the battery was at 55 CCA (cold cranking amps) and, tested it on its ability to take a charge. A good battery should be up around several hundred CCA. No good on either count. I bought the Optima. Car hasn't cut out since. Ahmet, I'm going to the local Octoberfest Porsche event this Sat. If you aren't there, have a great vacation!

I've had just this problem myself, where the battery was bad and the car would stall at lights. Certainly its worth getting a new battery and trying it.

Mark

944s2cab, I plan to be in upstate NY this saturday, so no way I can make it to that meet. I am planning to be back in Cary in time for the Charlotte 944 meet however (11th of october).

Out of curiosity, what accessories did you have running when the car died? I think your alternator/brushes may be getting a little old also, if everything worked fine I doubt the engine would die, I know mine didn't.
Ahmet

Ahmet, good question as there may be more to this problem. I had recently replaced my AC compressor and finally enjoying cold air. It was after that I had begun having shut down problems. Specifically, the engine would idle as normal at stops and suddenly drop, idle rough then die. It may have coincided with the compressor kicking in but a little too coincidental to be kicking in at every other stop. Other than the A/C running, maybe a standard stereo but nothing else.

Well, I have spoken too soo. On the way home today, the stalling problem returned. Mark, it may not be battery so check before you buy for that reason. In my case, the battery was clearly bad so I had to buy one anyway. I'm going to start a new thread on this stalling problem.