My 85 300D seems to start nicely, idles great, accelerates fine, shifts through all gears acceptably. All is well until I get off the freeway and hit the first light. From there the idle lopes and shakes the car. Ran a can of diesel purge through it yesterday; drove it to work today and noticed it at the first light after 15 miles of freeway at 3k rpm. Same thing on the way home. When I got it to the driveway, i popped the hood and visually inspected the fuel lines and vacuum lines.

Disconnected the line that goes into the vacuum control valve, the one that sits aft of the Injection Pump Part Number 1230700046. The idle immediately smoothed out and just sounded relaxed and un-labored. Was thinking that this valve strictly controlled the smoothness of the shifts, but it definitely had an effect on the idle. Is this a failing or failed valve that is not releasing enough pressure?


Have not done anything else other than the purge.

I've owned this car for about a month now and have no history on it other than what I've visually discovered. It has not been neglected. The diesel purge did not produce any discolored fuel bypass; no blowby on oil cap. Have not done a compression test yet. Motor mounts appear to be original, but seem to be ok. I don't think this is motor mount suspension related.

I know I have a vacuum issue somewhere as the locks will only pull down if I immediately shut down and lock. Engine does shut down normally with the key as well as the emergency stop.

Any help or insight would be greatly appreciated.

Mike