Ideally, a car's engine should stop running as soon as the driver switches off the ignition switch. There should be no more raw fuel entering the chambers and the spark plugs should no longer ignite it. Things don't always work this smoothly in an internal combustion engine, however, especially when other sources of ignition and raw fuel still exist. Whenever a car's engine continues to run after the ignition has been shut off, mechanics refer to it as dieseling.
Dieseling occurs whenever fuel is ignited by another heat source other than a spark plug. It could last a few seconds or few minutes, depending on the amount of fuel remaining in the combustion chamber and how long it takes for the ignition source to cool down.
Dieseling refers to the process used in diesel engines to ignite the fuel and keep the motor running. Instead of a series of explosions generated by timed spark plugs, in a diesel engine the fuel is ignited by high pressure in the piston chamber, sometimes aided by a single "glow plug" that helps warm the engine block. Unlike traditional internal combustion engines, a diesel engine may have to idle in order to keep the ignition process running properly. In a traditional gas-powered engine, however, such a practice of keeping the engine running is not strictly necessary. If a gas-powered engine begins dieseling, there is usually something wrong somewhere.
One possible cause of dieseling is the use of cheaper grade fuel and oil. Whenever an engine burns lower quality fuel, carbon deposits often develop on the valves and pistons moving rapidly within the combustion chambers. If a carbon deposit causes the piston to rub against metal, a hot spot could easily develop. When the ignition is switched off, the spark plugs no longer deliver a spark hot enough to ignite the remaining fuel in the combustion chamber. A red-hot carbon deposit, however, could still set off an explosion if it comes in contact with unburned fuel. This ignition of raw fuel is often the reason a car starts dieseling. It will only stop when the remaining fuel is gone or the carbon deposit has cooled down.
Another possible reason for dieseling could be a faulty link between the carburetor and the engine block. Many older cars have a fuel choking system which is supposed to shut off all delivery of gas to the engine. Sometimes this choking system fails to work properly, allowing gas to flow into the combustion chamber. If that gasoline comes in contact with a hot piece of metal or carbon, it will explode and cause dieseling. More modern engines have a fuel injection system designed to stop introducing fuel into the combustion chamber after the ignition switch is turned off, so dieseling is not as common but still possible if fuel remains in the chamber.
One of the best ways to reduce the chances of dieseling is to keep your car properly supplied with oil and engine coolant. When car engines overheat, hot spots can develop and cause dieseling. You may have to have your pistons and valves examined for the development of sharp edges which could become overheated. Switching to a higher grade fuel or using carbon-removing additives could also help prevent future dieseling.
Some experts suggest that dieseling could be triggered by inferior or improper spark plugs, so you may also ask a mechanic to make sure you have the proper size spark plugs for your car's engine. Many dieseling problems can be addressed during a regular engine tune-up, so tell your mechanic about any irregularities you may have noticed after shutting off the ignition.