Car stays running when off? Get your car inspected at your home or office.

We’ve all heard a car continuing to run after the ignition key has been turned off, and it usually sounds pretty bad, with all kinds of clonking and coughing noises. These days, though, you’re more likely to hear this in the movies, when sound-effects staff is trying to show the viewers what a clunker this particular actor is driving. Modern cars are seldom afflicted with this particular problem.

The phenomenon often called “dieseling” is caused by a condition known as pre-ignition, where hot spots in the engine’s combustion chamber fire the compressed fuel mixture without needing a spark. This syndrome can also be present when the car is running under load, producing a characteristic – and potentially damaging – pinging sound.

What is actually happening is the fuel charge exploding all at once instead of burning across a moving flame front away from the spark that normally initiates combustion. Also known as detonation, the sudden explosion is often caused by the use of a too-low octane-rated fuel.

If this is happening to your car, it’s probably an old one with carburetors. New models all use electronic fuel injection, which shuts down when the key is switched off. Without fuel going to the cylinders, there can be no running on. Carburetors, however, will continue to supply fuel as long as the engine is turning over.

Because this phenomenon often occurs in older cars with a higher build up of carbon deposits in the engine, you may need to use a higher octane fuel than was originally specified. If this fails to quell the running-on mischief, then a tune up and possible replacement of spark plugs with a colder heat range may be called for.

Replacing your intake side valve stem seals may sometimes correct this issue as well.