High idle issue on 1990 420 mercedes.

006 Idle Speed Problems Idle Speed Wrong Do you have a professional type scan tool ? If not ,take it to a qualified repair shop .
007 Idle Speed Problems Scan Data for Idle

Check the brake booster vacuum hose for a leak. This could cause the hard brake pedal and the idling issues both.

Scan for trouble codes at local auto parts store. Possible Idle Air Control motor or throttle position sensor, or a big vacuum leak. Idle Air Control Valve Replacement

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Sounds like a vacuum line got knock off somewhere causing a vacuum leak and the high idle.

sounds like a bad vacuum leak in the braking system..be looking at this also..

Have you checked your OVP relay? They can be troublesome for sure. It's located behind the battery under a plastic cover. It'll be the tall relay with the aluminum housing, has a fuse on top, check the fuse first. If the fuse is good then pull the relay and give it a shake, it's not supposed to rattle, good sign it's bad if it does. Sometimes moisture can get back there and cause corrosion to the relay pins and connector. If it pulls out hard then I'd suspect to see some corrosion, most all mercedes electrical connectors are silver plated and turn black when oxidized. If so clean it up by lightly scrubbing with a scotch pad. I've had problems with them before and had luck repairing them myself by removing aluminum shell and reheating all the sauter on pcb carefully. If the printed circuit board looks burned in any way there's a possibility that the electronic components may be toast. May be easier to buy a new ovp relay but they can be pricey. The OVP relay is one of a few possible culprits to your issue, best case scenario imo. Worse case your cylinder compression is too low. Could be a cold start valve issue too. Hope this helps.

Sounds like a vacuum leak. This will cause the idle speed to be high. Additionally most cruise controls are operated by vacuum pressure. Look around the intake manifold for disconnected rubber hoses. May have popped off from dry rot or stretched out etc. Find the vacuum leak and fix it, both problems should go away.

The idle speed is computer controlled. First look for a possible vacuum leak. If there's no vacuum leak, the problem could be the TPS (throttle position sensor), the IAC motor (idle air control) or the PCM (powertrain control module). You will most likely need a scanner to find the cause if it's not a vacuum leak.

Typical idle for this motor is right around the amount you have described. Check to see if any check engine lights are on, or if the ABS light is on. A typical issue with this motor is the OVP relay that when not operating properly causes the engine to rev much too high (~1500-2000). Also do your revs drop significantly when you put the car into gear? If not then there really isnt an issue. When the motor is idling too high and you stick it into gear the car will typically lurch really hard.