Idle speed is to high
 

I have a 1980 280E Manual 4 speed. After warming up, this car idles consistantly at 2000 RPM. How should I begin to diagnose this challenge and/or how do I adjust this idle speed?


Dave H...

That one is pretty old. I know that my idle was fixed once I replaced the O2 sensor in my 91 300E. I'm guessing that the idle may not be computer controlled so something may be sticking. In a lot of the later model cars, the idle air control valve gets gunked up and stuck causing idle issues. Also, try and find the water temp sensor (engine coolant sensor) on the engine and any air temp sensors (on the air intake) that could be old and sending wrong signals to cpu. Resistance increases with age which can cause a high idle. Other than that, old gunked up throttle linkages, springs, etc. Wish I could help but someone that owns your model will know a lot more I'm sure.

Thanks for all of your input.

All of the throttle linkage appears to be good. The vacuum diaghram on the distributor hasd two inputs one is for an advance and the other is for a retard.

At this point we have determined that the retard vacuum is not functioning correctly we just do not know why as of yet.


Dave H...

100% mechanical issue.
Idle during warm-up is controlled by THERMOSTATIC additional air valve.
There are many hoses under the intake to distribute the air to the intake.

Idle is adjusted by large plastic screw.