If your vehicle is showing engine continues to run, chug, or sputter after turning the key off (dieseling), here are the most likely causes and the parts you'll need to fix each one.
If the idle speed is set too high or the throttle plates do not close completely, the engine continues to pull in air and fuel after the key is turned off. This is the most common cause of dieseling in older carbureted engines.
Excessive carbon deposits inside the engine cylinders can glow red hot. These 'hot spots' act like spark plugs, igniting residual fuel and air entering the cylinders even after the ignition system is shut down.
Installing spark plugs that are too hot for your specific engine application can cause the ceramic tips to remain glowing hot after shutoff, which ignites leftover fuel in the cylinders.
In modern fuel-injected vehicles, true dieseling is rare because fuel is cut off with the key. However, if a fuel injector is stuck open, it can leak fuel into a hot combustion chamber and cause the engine to sputter after shutoff.
A failing ignition switch or a stuck relay (like a radiator fan relay backfeeding power) can continue to supply electrical power to the ignition system or fuel pump after the key is removed, keeping the engine running.