Any suggestions to keep my car from "dieseling"?

Dear Car Talk

Dear Car Talk | May 01, 1995

Dear Tom and Ray:

My car engine does not always turn off immediately. It "diesels." My cousin suggests turning the key off while the car is in Drive or Reverse. Should I do it?

TOM: You can do it, Bill. But you might as well try fixing the problem first.

RAY: 99 percent of the time, dieseling is caused by an idle speed that's set too high. The engine is turning so fast that it simply has too much angular momentum, and can't stop right away. So when you turn off the key, the engine keeps turning

TOM: And because it's still turning, fuel gets sucked in from the carburetor. And since the engine is still hot, some of that fuel gets combusted. So the engine continues to run (albeit poorly) for a few seconds after you shut off the ignition.

RAY: And since heat plays a crucial role in dieseling, anything that makes the engine run too hot is bound to make your problem worse. So in addition to having your mechanic check the idle speed, you should also have him check the timing, the cooling system, or anything else that could lead to overheating.

TOM: If your mechanic is unable to solve the problem--or unable to solve it cheaply, then you can resort to putting it in Drive or Reverse. That simply "loads up" the engine, and drags down the idle speed. That's why it often stops the dieseling.

RAY: But since it may be able to fix it permanently with a turn of the idle screw, you might as well have your mechanic take a look at it before you resort to such a "heap maneuver."

TOM: That's some cockamamie starting, stopping, or driving procedure known only to the owner of the car that positively identifies the car from this point forward as a matter how much you protest.

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