Why does my engine "run on"?

[Kevin Wood]

Why does my engine continue to run after the ignition has been switched off?

Kevin

[Kevin Wood]

There are two basic causes for "running on" :

1) No spark:

The heat in the combustion chamber is sufficient to ignite the air - fuel mixture so the engine will fire without a spark once the ignition has been switched off.

2) Weak spark:

A wiring anomaly causes a weak spark to still be generated when the ignition is off. The engine continues to run, either indefinitely or for a few revolutions, or until the brake pedal is pressed or other electrical load is activated.

Causes:

1)

Possibly too hot a spark plug causing the tip to be sufficiently hot to ignite the air fuel mixture.

Engine running lean leading to an abnormally hot combustion chamber - or an easily ignited mixture.

Retarded ignition leading to a hot combustion chamber.

Engine overheating.

Engine has a build-up of carbon in the combuston chamber which "glows", causing ignition.

Not normally a problem with injected engines since the fuel supply is cut off by the ignition switch.

Solutions:

Cooler plug.

Richen idle mixture.

Correct ignition timing.

Rectify overheating problem.

Rectify carbon build-up.

Some carbs incorporate a solenoid valve to cut the fuel when the ignition is cut (not the DCOE).

2)

A current path supplying the ignition circuit when the ignition is switched off:

A motor on the ignition circuit running as a generator as it spools down (does it only happen when the cooling fan or the heater blower are running at the time of cutting the ignition?)

Current feeding back from the alternator warning light into the ignition circuit (does the ignition warning light come on when it's running on?).

Carbon build-up in the ignition switch itself.

Solutions:

Ensure large motors are fed through a relay from the battery via an appropriate fuse not directly from the ignition circuit.

Ensure there is enough constant running load on the ignition circuit to prevent any small leakage current from operating the ignition coil - run the fuel pump from this circuit, for example.

Place a diode in the line from the alternator "IND" terminal to the warning light to prevent reverse current flow.

Ensure the engine management is fed from the ignition line directly. If it's fed via a relay from the battery only a very small current is required to hold the relay "pulled-in".

Clean up the ignition switch contacts.

[ljsanders]

How long is your definition of "running on"? 1 second, 5 seconds?

Or should it be instant off as soon as the ignition is turned off?

[Kevin Wood]

It depends what's causing it. If it's the cooling fan, there's only a certain amount of energy it can supply, so it runs on for as long as the fan is stil spinning. A few seconds. Normally if it's hot engine internals on a carb engine it'll only be for a few seconds and may be related to how hot the engine was when shut down It'll normally run very roughly, often not on all cylinders.

The alternator one could keep the engine running indefinately or until an electrical circuit (e.g. brake light) tries to draw some more current from the ignition supply. It will probably run pretty smoothly unless there's only just enough energy to fire the coil in which case it might misfire a bit.

Kevin