Dieselling is an automotive term for when a four stroke otto cycle engine continues to run (albeit roughly) even after the ignition has been turned off. This is also known as run-on. Dieselling only affects carburettor fed cars, as EFI systems automatically stop feeding fuel to the engine when the ignition is switched off.
Dieselling is usually caused by a combination of low
octane fuel, high running temperatures, and high
compression ratios in an engine. The effect is created by fuel in the
engine cylinders being compressed enough (and hot enough) to self
ignite without the aid of the
spark plugs.
The Custodian says Another potential cause of dieselling is carbon buildup in the cylinders; carbon deposits will glow white hot after the engine shuts down, acting as unintentional glow-plugs and igniting fuel. This was why older cars were decarbonized. :-)
This effect is referred to as dieselling because this is the way that diesel engines are DESIGNED to run. In a diesel engine, the diesel fuel is compressed until it is able to self-ignite (sometimes with the aid of glow plugs).
Petrol engines are not supposed to run this way, and an engine that tends to 'run on' after it's been turned off will eventually suffer damage for the same reason as an engine with detonation (also known as ping or knock) problems will. Damage caused by knock/dieselling can include excessive valve and piston ring wear, and in extreme cases the pistons and/or crankshaft can be badly damaged. This is because the premature ignition tends to apply force to the piston before it has reached its apex and is ready to come down again.
If your engine tends to diesel on shutdown, there are several ways to combat the problem. The first thing to investigate is your cooling system. If no major problems can be found there, it could be your engine requires a higher octane petrol to run healthily. New spark plugs may also help in certain circumstances. If this still doesn't solve the problem, and the car doesn't appear to have problems with 'knock' when normally driving, an anti-dieselling valve can be fitted to stop the flow of fuel into the carburettor once the ignition is turned off. This will always fix the problem, but could mask other problems with the engine. Alternatively, if you're feeling stingy and would rather ruin your drivetrain than your engine, just intentionally stall the engine to stop it.