Idle trouble after disconnecting battery

[3xX]

Hi, I have a 1990 525i. Just recently, I have been working on the stereo system. My dad and I have been disconnecting the battery, for safety reasons when working on it. We always disconnect the red, or positive cable from the battery.

My problem is, everytime I disconnect the battery, then reconnect it after I am done working on it, and start my car up, it idles funny and the engine dies at times. The car will idle at very low RPM's, and even die at times when I put in the clutch or am in neutral and slowing to a stop. This will go on for a while, 2 or 3 days, then it goes away. I have brought my car to a BMW mechanic, and they have run a full diagnostic test on it, and found nothing wrong. The only thing they did not test, was the "brain" of the car, the CDM, or something like that.

I wanted to know if anyone knows what the problem might be, and or how to solve it. I would appreciate any help. Thanks

[GreenDragon]

Not sure really, but a few ideas:

Is it disconnected for long? If so it may simply be the ECU re-adjusting (air/fuel mix, etc). Have you tried starting it, turning all electrical components off (radio, amp, lights, plug in vibrator, jet propulsion system) and just letting it run for a while after re-connecting the battery?

The battery may need replacing. It could possibly be the battery is recharging and doesn't have enough juice, or your alternator may need replacing. Have you installed an amp and all that bumpin' stuff? You may also be drawing too much power after the disconnect during recharg. Consider getting a capacitor (one rated for your wattage, don't ask me because I haven't the slightest idea) so your system isn't drawing too much power from teh battery.

Hope this helps some, good luck and welcome to the Forum!

[3xX]

I have tried shutting off everything that is electrical. And it doesn't help any. As for the stereo. I haven't installed any aftermarket components yet. Everything I have in the car is stock, I was trying to figure out if a certain deck will fit my car, thats the only tinkering we were doing. Other than that, it seems that the problem persists unless I just drive it out for 2 or 3 days. Any other ideas? I will ask the mechanic about this, as we just narrowed the problem down to knowing that this only occurs when we disconnect and reconnect the battery.

[Furious]

try holding down the "trip reset button for like 10 seconds" it will try and reset the DME. (i think) it will flash a bunch of numbers on the odometer and then all the gauges will go over to the max (ie. MPH needle and RPM needle will go all the way over to the far right then back) try that.

[3xX]

Very strange, but when I got home today from school. I decided to just start my car and let is sit and run at idle for around 20 minutes. After that, I shut my car off, and started it back up, and that seemed to solve the problem. I am going to the mechanic anyway to pick up a part, and I will ask them what the issue was or could of been. Thanks for all your help.