Why does my 1987 560SL idle at 1800 rpm even after driving the car for 10 miles?

5

Asked by Dec 05, 2015 at 03:19 PM about the 1987 Mercedes-Benz 560-Class 560SL Convertible

Question type: Maintenance & Repair

Upon a cold or warm start, the car idles at 1800rpms. I removed the air
cleaner and the large damper inside the intake remains fully closed. as i push
it lower the idle lowers however it appears vacuum controlled and when i
release it, the damper closes and the idle goes back to 1800 rpms.

1 Answer

69,405

Sounds like a controlled vacuum "leak". That damper is supposed to be closed when the engine is cold to preheat the air going to the throttle body. As the engine warms up the damper is supposed to gradually open to maintain a certain temperature of the air coming into the engine. This speeds up warm up time, controls hydrocarbon emissions, and prevents icing of the throttle plates in cool damp weather. It sounds like the system is set up so there's a vacuum leak when the engine is cold and the damper is closed. This would create a normal fast idle condition to prevent stalling until the engine is warmed up. These are usually pretty simple systems to diagnose. You have the vacuum motor that positions the damper and the temperature sensor inside the air cleaner assembly. There's usually a vacuum hose going from the intake manifold to the temperature sensor, and another vacuum hose going from the temperature sensor to the vacuum motor. HTH. -Jim

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