diesel
For a spark-ignition internal combustion engine to continue running after the electrical current to the spark plugs has been turned off. This occurs when there's enough heat in the combustion chamber to ignite the air and fuel mixture without a spark, the same way that heat and pressure cause ignition in a diesel engine.
diesel: engine keeps running after you kill the ignition
To diesel is when a spark-ignition gasoline engine continues to run and fire after you have switched off the ignition. The engine turns over on its own, firing cylinders without input from the spark plugs, until the remaining heat in the combustion chamber drops below the ignition point of the fuel mixture or fuel starvation stops the process. The phenomenon is named after Rudolf Diesel's compression-ignition principle, though it is unwanted in gasoline engines and the reverse of normal diesel engine operation.
Dieseling occurs because the combustion chamber has retained enough residual heat from recent firing cycles to ignite fresh fuel and air without external spark. Carbon deposits on the cylinder head, piston crown, and valves raise the local temperature even further and act as glow points. This is most common in older or poorly maintained engines with high compression ratios, or in engines running on low-octane fuel that burns hotter and deposits carbon more readily. Modern engines with better cooling systems and fuel quality rarely exhibit the problem.
Why it matters in the workshop
Dieseling is usually harmless but indicates underlying issues. Heavy carbon buildup, a faulty heat riser valve, or an out-of-spec ignition timing advance can all promote it. If an engine diesels badly, rough idling, stalling, or hard starting may follow once the engine cools. Some technicians use it as a diagnostic sign that an engine is overheated or running too lean. In rare cases, continuous dieseling can damage the starter motor if the engine cranks violently as fuel floods in and combusts.
Mechanics used to see dieseling frequently on carbureted engines from the 1960s and 1970s, especially in hot climates or at high altitude. The cure was simple: use higher-octane fuel, clean the combustion chamber, check the ignition advance curve, and verify the cooling system was working. Fuel additives that raise octane or suppress carbon were marketed for this purpose. Electronic fuel injection and precise ignition control in modern engines have made dieseling almost extinct in regular service.