rough idle - intermittant - PeachParts Mercedes-Benz Forum

pdxman · #1 11-05-2003, 09:00 AM

I have an '84 300D with 155K. Since the cooler weather has arrived, I have noticed a 'rough idle' when the car has not warmed up. the 'rough idle' I am describing is not in the manner that the car runs rough though, it seems more like a hard stress is being applied to it. It seems to be related to when the heater is on and the climate control cycles on or off. I know this because when it went into one of these phases (right when I heard the compressor kick on) the car started to do its thing and as soon as I turned the heat completely off it immediately stopped. It also seems to be a little more pronounced when in drive with the brake applied vs. in park at idle.

My questions; This is the first car I've had with the climate control (which P.O. replaced once since new). With the heat on, should the compressor still cycle on/off? I hear the occassional 'clicks' when running heat.

Could this be a compressor failure where it would be seizing or something and the engine has to work really hard when compressor is engaged?

The car does some shaking when this happens but I also noticed that the engine mounts look mushy and probably due for replacement.

Thanks anyone.

diesel don · #2 11-05-2003, 11:19 AM

This is real NORMAL stuff.

1. You can live with it or try finding some smoother options:

a. Rack Dampner Pin. On my '84D, the was the culprit. Replaced it and things smoothed out. You can also just back it out a turn or so for colder weather. I just did this on my 2nd '84 and it worked.
b. Motor mounts are easier to replace than they first appear. Sometimes this helps. But with your problem coming and going, I would do these last.
c. I might also bump the idle up somewhat.

Don

lrg · #3 11-05-2003, 12:40 PM

What you describe sounds like the normal cycling of the AC compressor when the engine is still not fully warmed up. You may not have noticed it so much before because the weather was warmer and the engine got warmed up relatively faster. You'll notice it even more when the car is stopped and in gear since that is an additional drag on the engine which effectively lowers idle RPM. The compressor will run as long as the center button (or either of the two buttons to the left of it) is pressed. This system is designed to use the AC to dehumidify the air before it gets heated so on those settings the compressor will run regardless of the temp you set. If you turn the system off (far right button) or on economy (second button from the right) then the AC compressor is not engaged at all and you should find your car idles more normally. Personally I'd just ignore it but if it really bothers you either don't use the AC modes until the engine is warm or like diesel don suggested turn up the idle speed a tad.

pdxman · #4 11-05-2003, 10:01 PM

I'm satisfied enough just to hear that is normal operation. Thanks for the scoop on how this climate control works. My last vehicle was a 240D with a bare bones system.