1979 300TD. On the dashboard,left side there is a black lever that when turned to the right slows down the idle/engine. When all th way to the left engine idles higher and runs smoother. What is its function?

It is to adjust the idle speed of the engine. with cold engine you need it to run a bit faster to keep running.. when its warm you can slow it down again.

I would also use it with A/C on to raise the idle slightly. Not a good idea to turn it unless the throttle is depressed slightly. Where it connects beside the valve cover it can become stripped unless throttle depressed.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

or it can break the cable for the idle adjustment knob as was the case on mine. Fixed though and only really use it when it gets cold out to warm it up, but not to fun trying to drive with it turned up, since my car is manual.... makes for some tricky clutch work

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1983 Euro 240D 277K
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Before I knew what it was I did not touch it. Sure makes starting "Thor" a lot easier. Jerry.