Idle Jumps UP and DOWN when clutch is in

[abemcclellan]

when i have my headlights on in my 325is, i'll push in the clutch when crusing at any speed, and the idle automatically jumps up and down from 500-900rpms until i come to a complete stop. do i need to tighten my alternator belt? or maybe it could be my voltage regulator? i need help with my car. -Thanks

[Autotechnica]

It wouldn't be related to your alternator. Pushing down on the clutch doesn't affect power output. Why would you push down on the clutch and let it drop to idle speeds when your cruising? Doesn't really make sense to me. But think about it, your flywheel is spinning so fast, all of a sudden you just cut off power to the flywheel and it starts to lose momentum, of course your idle will bog. That's one of the reasons why downshifting is important. When you slow to a stop, if you're not use to downshifting, just put it in neutral and let off the clutch, there's no reason to hold it down the entire time. Once you let off the clutch in neutral and come to a stop, the RPM's should not bog at idle.

Bry

[chrisNX2000]

heh i may not know too much of this but i experience this with my nissan, with us nissan people it's usually due to out egr system, it's dirty. so our idle get pretty stupid sometimes. basically it's emessions related.

[abemcclellan]

well i do downshift when i drive. but i just got this car and i'm just checkin it out. i've had 2 other bimmers beside this one so i'm not unfamiliar with them. but it only happens when my headlights are on. thats why i'm trying to figure out if this is an electrical problem or what. but if i push the clutch in or take it out of gear while crusing with my lights on, then my idle jumps up and down. then when i come to a complete stop, THEN the idle stops jumping. this never happens during the day when i don't have my lights on. and when they're jumping up and down, my lights dim bc my car about dies. -Thanks guys

[Gator]

What you describe sounds like a problem with the battery, voltage regulator, and alternator circuit. With the lights on there is more or a load on the electrical system and the battery and alternator can't supply enough power. Your lights dim as a result and the system voltage drops. As a result of the voltage drop your idle stabilizer etc doesn't function properly and you get rpm swings. With problems like this my experience is to always check the battery first as it's the easiest to deal with and can lead to misleading diagnosis of other parts.

[abemcclellan]

alright thanks. i'll prolly have a dealership check it out.