JayKinFL

1991 420SEL. I think my idle is too high. can one of you tell me how to adjust the idle? what should my idle be at (in park with the a/c on). i know i am supposed to adjust it through the idle control, but i don't know how to do it - or where it is located. also, has anyone ever heard of a "slow" speedometer? when i am doing 60, it says 45, when i am doing 40, it says 20, etc. there's not a real math theory behind the actual speed and the speedometer speed. when the car is at rest, the needle is just where it's supposed to be (dead center of the first 'mark' on the speedometer). thanks!

nsmith17044

You cannot adjust the idle speed manually. It is controlled by the idle controller module (black box on the firewall) and should be around 800-850 rpm. However vacuum system leaks can cause a bad idle. I would check all of the vacuum lines, most likely something is cracked. Also, the idle air valve (small cylindrical object to the left of the distributor above the water pump) may have some cracked rubber hose fittings. One way to find a vacuum leak is to take a propane torch, NOT LIT, and squeeze the trigger sticking the tip around the vacuum lines. If the engine speed surges you probably found the leak. I have seen cracks in the idle air tubing that cause this type of problem.

Regarding the speedo problem, has the instruments ever been apart? If someone pulled it apart and didn't put the indicator back on right it will read a constant value slower or faster. A bad speed sensor on the transmission might cause this type of problem but I haven't ever had one of those go bad.

Good luck!