I have a 98 Mitsubishi Montero and the engine continues to turn over after the ignition is turned off. Turning the key off does cut off fuel because you can hear the difference and turning the key back on does bring the engine back up to full idle. The only way I could shut it down was to disconnect the battery and any attempt to reconnect the battery results in sparks and the engine turning over. Can anyone suggest possible causes (I suspect starter motor) and tests. Cheers

Are you experiencing post-ignition, caused by not using high enough octane gas?

Are you experiencing post-ignition, caused by not using high enough octane gas? I don't believe it to be a fuel mixture problem as it sounds as though the starter is turning the engine over with or without fuel whenever power is applied.

What this sounds like is the starter is seized and still engaged to the flywheel... you said if you disconnect the battery, then reconnect it, the engine turns over? Did I understand you?
If that's the case, the starter is the likely culprit and would need replaced.

Sounds like the over-running clutch is stuck in the engaged position inside the starter motor. You need to just replace the starter motor.

Many thanks to txgreasemonkey and CaptainRich for your assistance. Will let you know how it goes.

Cheers from downunder