Sometimes when you turn the key off the engine keeps running - sometimes for a long time - sometimes backwards? What causes this phenomenon? How can I prevent it?

The most common cause of that is the failure of the anti dieseling mechanism, sometimes an "anti dieseling solenoid", found on most late model carbureted cars. What is happening is that the throttle is remaining partially open when the engine is shut down, which gives the hot engine sufficient fuel to run without a spark from the spark plugs. Most hot engines have sufficient carbon build-up that remains glowing red hot and acts as an igniter for the fuel. The solution is to make sure that the throttle closes completely when you turn off the ignition switch. Check the throttle stop and make sure that the fast idle on the choke or the "bottom stop" isn't what is stopping the throttle from closing. It must be the anti-dieseling mechanism and that mechanism must be functional.

Some motors (Olds for example in 85) used an actual servo motor for this function. The motor drives a worm gear which advances or retracts the idle speed control rod depending on what the computer tells it to do. When it is in the "closed throttle" position and the key is killed it retracts completely to allow the throttle plate to close completely thus preventing the "dieseling" that so many cars are experiencing.

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