Ford: 1991 Ford dually F-350, fuel pressure guage, psi readings

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I need to know why when I turn the key off it won't go off I just have to let it die could you give me some possibilities. It is a standard and has 4 gears and overdrive if you need to know that. Thank you.

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is this vehicle gas or diesel?

diesel
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let me do some research on this .

Ok that will be great!

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well  this may be a little hard but lets try If there is any way you can do a fuel pump test that would be the best test now you may be able to rent a fuel pressure guage from auto zone ,napa ect. the kit will come with the psi readings for your vehicle.see what the psi is when it is running and then watch the guage and see if the psi drops more then likely it will your injector pump may be bad and when you shut off the truck the pump is stoll leaking fuel and supply fuel to the eng. and that whats keeps it running.