I have 96' C220 and occasionaly my idle goes wild but only when car is in drive and I have my foot on the brake pedal. It sits at 600 mostly, but for few seconds goes up and down to a grand, then 800 and so on back to 600 stable for a while. Car wants to go as this is happening but I step harder on brake pedal to control it. I wonder if this is a faulty idle motor if these cars have it and I wonder where is it located. By the way no codes which is truely wierd.

 

no vacuum leak?

 

Intake air leaks can cause this and will not set off any codes. Inspect the intake air pipe and pay particular attention to the area AFTER the MAF sensor.

You can also spray some sort of starting fluid or equvalent around the intake manifold gasket and see if the engine RPM changes.

 

Wicked thinking. I'll test that. Thanks a million.