Dieseling or engine run-on is a condition that can occur in spark plug, gasoline powered internal combustion engines, whereby the engine keeps running for a short period after being turned off, due to fuel igniting without a spark.
Dieseling is so-named because it is similar in appearance to how diesel engines operate: by firing without a spark. The ignition source in a diesel is the heat generated by the compression of the air in the cylinder, rather than a spark as in gasoline engines. The dieseling phenomenon occurs not because the compression ratio is sufficient to cause auto-ignition of the fuel, but a hot spot inside the cylinder starts combustion. An automobile engine that is dieseling will typically sputter, then gradually stop.
Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for igniting unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, rough metal regions within the piston chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated spark plug can retain heat and cause the same problem.
A carburetor that does not completely close can contribute to running once the engine is off, since the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.
Incorrect timing.
An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to burn.
An idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turn over and burn more fuel and lock itself into a cycle of continuous running.
Retarded ignition timing can cause this because when the initial is retarded you must open the throttle more to maintain a certain idle sped. Opening the throttle blades can cause run on. Advancing your initial will allow you to close the throttle more at idle and reduce the chances of run on. Also, a cam being advanced can cause extremely high cylinder pressures because the intake valves will close sooner. High cylinder pressures can also cause run on.
Dieseling is not nearly as common as it once was, because it most commonly occurs in engines equipped with carburetors. The vast majority of vehicles manufactured after 1990 are fuel-injected: The injectors and high-pressure fuel pump immediately cease supplying fuel to the cylinders when the ignition is switched off. If the injector is damaged or malfunctioning, a small amount of fuel can enter the chamber and be ignited, causing a sputter or two after the engine is switched off.
