617.952 idle speed question - PeachParts Mercedes-Benz Forum

xdjio · #1 01-31-2008, 01:16 AM

Hi guys,

First off - a preemptive thanks to anyone who can answer this simpleton of a question (I hope). Lately (in the last 2 months) I've notice my idle speed immediately after starting seems too low.

The case used to be that I'd start the car (it lives in a warm underground garage) and the idle would be 750 rpm almost exactly (whether the engine was cold or not). Nowadays, the engine starts up and it's 500. It doesn't quite /sound/ like it idles at 500 to my ears, and certainly the idle does not feel any rougher than when it used to read 750, but 500 is what my tach is telling me.

If I go on to drive a few blocks, the idle "usually" will seem to reset itself and sit at 750. Sometimes 600, 650 but 99% of the time, after a reasonable 15 minutes of driving, the idle behaves itself and sits at 750 or thereabouts.

Should I try to do anything about this? I doubt it's having any immediate negative impact on my motor... but, perhaps I am wrong. Could this be a linkage issue? maybe there is a screw I can adjust manually? Seems like a good weekend project, but some ideas on where to start would be most appreciated. Seems like some other 300DT owners on this forum have had similar symptoms...

pawoSD · #2 01-31-2008, 01:29 AM

The tach's on these cars are not all that accurate. The degree of accuracy is about +/- 200 rpm. Its 99.9% likely the engine is still idling at the same speed as before....the tach is probably just "slow to react" sometimes....and they do seem to "reset" themselves after you've begun driving. Ours does it too. I'd not worry about it....just old electronics acting up.

xdjio · #3 01-31-2008, 01:33 AM

That was my other thought - maybe it's just the tach being 24 years old.

That said, how is the tach actually coupled in to the motor - correct me if I am wrong but I have this idea in my head that it's a magnetic coupling to the transmission bell housing. I wonder if there is any value in cleaning / reseting that?

What is more likely is I will continue to live with it and all will be well

ARINUTS · #4 01-31-2008, 01:52 AM

OK, Besides the tach being off, is there a way to adjust the idle?
I'd like to raise my idle about 200 rpm or so (for the winter months)

toomany MBZ · #5 01-31-2008, 05:36 AM

xdjio, I don't know where the sensor is on an '84, and cannot trouble shoot. Bound to be an electrical connection somewhere that may be the culprit.

AIRNUTS, in the pic it says to detach connecting rod 2, I did not do that, otherwise I can attest, this is how the idle is adjusted. You'll need a deep socket to loosen the lock nut in order to be able to turn the screw.