Cummins isx

Is everything else is OFF??? Dash, AC blower?? If so problem is the fuel shut down solenoid if not look for electrical.

 

What is the year of your truck ?
On ISX engine 1998 up to 2009 the engine ECU is controlling the fuel shut off solonoid (not the key switch), a sticking shut off solonoid valve can do this.Or electrical problem.
On the LH of the engine (1998-2009) you have 5 solonoid,the fuel shut off solonoid is located in the middle, check the voltage with a multimeter on the center solonoid stud, the voltage should turn to zero when you turn the ignition key to Off position, if the engine continue to run with voltage at zero on the solonoid you have probabbly a defective valve disq seat (or rust inside).
Let me know please.
Ben
Is this happen after a repair ?

 

is there something called a fuel pump relay or soloniod shutoff?? Maybe that is the problem. Otherwise you need to choke the fuel like with your bare hands to kill it. Lol...

Oh. Benrab said it. Go with what he is saying..

Lol..

 

Mine did this when the cab grounds were going bad.

Simple way to check is with jumper cables.

Key the truck off. Clip a lead to the (-) terminal on battery and the other end to a ground lug on front of cab passthrough area on drivers side.

If it shuts off, your grounds are bad.