Hello,

I have an idle situation that I need help with. I have a 1990 300se/M103 with 190,000 miles. Between the time that the car is first started and reaching operating temperature, something happens that causes a change in vacuum and a rough idle. Here is what happens. I start the car and warm it up for a few minutes OR start cold and go. All is normal and the economy gauge is to the left in the range it should be. I drive a short distance to the next stop and the economy gauge SOMETIMES will jump up to the middle of the range and the car will idle rough consistent with a loss of vacuum. Revving does not change the condition. Then I continue driving, and for the rest of the stops and starts over a 14 mile commute all is normal.

Other than this annoyance, the car runs beautifully. Everything that is prone to failure has been replaced. New head gasket, ignition parts, OVP, EHA, fuel pump, filter, injectors, O rings, and seals. I also cleaned the idle control valve. I have had the system checked for vacuum leaks by a Bosch certified shop and there are none. At the same time I had them set the fuel distributor pressure since I had to install a new EHA. About the only thing I have not replaced is the airflow potentiometer. I checked the black and white vacuum check valves for proper function and I am assuming correct functioning of the vacuum switches both fender wells. So, has anyone experienced such an intermittent rough idle/vacuum loss situation and what might be the solution? It seems like it is triggered by something that does not occur again once the car is fully hot. Except for this little problem, the car runs like a turbine. Any thoughts?

Thank you,

Lloyd