84 300td inconsistent idle - PeachParts Mercedes-Benz Forum

ez31011 · #1 11-13-2007, 09:57 AM

I'll try to keep this brief:
Car used to idle around 700-750 and then drop to around 600 when the a/c compressor was engaged. However a few months back the rpm dropped significantly lower with the compressor engaged sometimes causing the car to stall.
So to temporarily alleviate this I've turned up the idle speed. It now idles around 900.

Some more info: if the car is in park it idles at about 900 rpm and if the compressor engages the rpm barely drops, then when I put the car in gear the rpm will usually (not always) drop to around 550, and when I put the car back in park the rpm remains at 550 until I "goose" the throttle and it returns to 900 rpm.

And to keep me guessing this doesn't happen all the time, sometimes I'll come to a light (with compressor engaged) and the rpm will be at 900 and other times it will inexplicably drop.

Things I know: it's not the compressor turning on and off
it's "under the hood" (symptoms remain when I take the fuel lines and tank out of the loop)

I'm not really concerned about this, since the car is running fine, it's just idling a bit high at the moment. I thought maybe someone would have some thoughts on this...

tobybul · #2 11-13-2007, 10:24 AM

Welcome to the club.... although my idle does not change even with the ac on.... but idle does vary sometimes especially when its cold. I have noticed an improvement since I started using synthetic oil. Maybe better lubrication plus synthetic cleans all surfaces it is exposed to - in time.

I had it at 900 rpm due to low idle but now back to 750 and seems to have stabilized.

ez31011 · #3 11-13-2007, 12:39 PM

doubt it's the cold, since this started in september when it was still plenty warm here (dc). Interesting thought about the oil, I am going to be changing it in the next couple weeks and was planning to use AMS so I'll see if that has any impact.