Isuzu npr failure to turn off when key off?

Answer 1

I'm assuming this is a diesel.

These engines have mechanical injection pumps and will continue to run indefinitely without "ignition being turned on" -- unlike gasoline engines which go off when power to the spark system is cut off.

There is usually a grounding system connected to 'key on' which shuts off fuel to the injection pump when the key is turned fully off. I do not have wiring diagrams at hand, or immediate access to my 1999 NPR to check precisely how that is done on this truck, but I would check back to determine which component of the shutoff is dead or disabled.

If there was a pre-existing issue with the truck not STARTING, particularly if there is some kind of anti-theft system acting on the fuel shutoff, and someone fixed that by disconnecting the shutoff, you may have an easier time determining where the problem is.

You can always shut off the diesel engine by interrupting the flow of intake air -- I do NOT recommend you do this with your hand! -- and this should not cause any subsequent problems with restarting the engine again.