A stuck AAV can cause a high idle or a loss of signal from the controller can also cause it. The little control module is a frequent source of broken solder connections, which will cause a fast idle.

It is also possible that you have a fairly large vacuum leak somewhere. I'd start with the AAV. Pull the plug off (with the engine running obviously) and see if you have voltage present. If you have no voltage, or an unexpectedly low voltage, start looking for wiring problems, blown fuses, or that idle control module.

If you have voltage present, you very well could have a stuck AAV. If you plug it back in, feel the AAV body, it should be sort of vibrating/humming. If you feel the AAV vibrating/humming and the idle speed doesn't change when you plug it back in, it's likely stuck.

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Current stable:
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1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
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1986 500SEL Euro (Rusted to nothing at 47K!)

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