Why Does The Engine Run On With Ignition Switched Off?


3 Answers

Greg Allen Profile
Greg Allen answered
Engine run on is a condition where the engine of a car continues to run for a few moments after the ignition has been turned off, usually spluttering to a halt. It can also be known as dieseling due to the similarity of how a diesel engine operates;  firing without the aid of a spark plug. Run on is not as common as it once was as it only occurs in carburetted engines, throughout most of the Western world  cars produced since the early 1990s are fuel injected.

The main cause of this is not that the compression ratio is high enough to cause an ignition, but because combustion is caused due to a hot spot somewhere in a cylinder. There are a number of potential causes of this problem. A build up of carbon can build up in the engine over time, these burning ashes when hot act like a spark plug and keep excess fuel ignited. This often happens when the fuel mix of a car is running very rich and producing surplus carbon. A carburettor that is not closed properly can also cause run on as the mixture of oxygen and left over fuel can combust easily after the ignition is off. If the idling speed of the engine is set too high the momentum after the engine has been switched off may cause it to continue running for a short period of time. Engines that run too hot are also conducive to burning excess fuel after running.

Carburettor cleaners and carbon removers can be used to clean up engines which are running on due to excess dirt; a carburettor may need replacing in some cases. If the engine is running too hot it might be an option to replace the thermostat and the internal fan. Idling speed can be reduced to lower the momentum of the motor after it has been turned off.
Chris Locke Profile
Chris Locke answered
Idle speed is to high.

Answer Question