A condition where a spark-ignition engine keeps running, sputtering, or knocking after the key is turned off. This happens when hot spots in the combustion chamber, such as glowing carbon deposits or overheated spark plug electrodes, ignite the residual fuel-air mixture without an electrical spark. The engine effectively mimics the compression-ignition cycle of an actual diesel engine.
I shut the truck off but it kept trying to diesel for another five seconds.
This issue is mostly seen in older carbureted engines, as fuel injection systems cut off the fuel supply immediately when the ignition is switched off.
Also heard as
- run on
- dieseling
Related terms
Preignition
The uncontrolled ignition of the air-fuel mixture in a cylinder before the spark plug fires.
Knock
An abnormal and damaging combustion event where the air-fuel mixture self-ignites prematurely.
Detonation
An uncontrolled secondary explosion in the combustion chamber that causes severe engine damage.