I have a 91 f-350 that I'm having trouble starting new glo plugs was installed put didn't fix the proplem

Egr valve

it already has glo plugs to help starting
check that they are working
diesels need a fast cranking speed so make sure that the battery is in top condition
have a compression test done to check for correct compression pressures
have the injectors serviced because if they squirt instead of atomize then it make starting difficult

if it dont spin over try taking all the plugs out see if it spins free if so does water come out of any of the plug holes

clean your injectors and there could be a relay going out.

You've done a great job of trouble shooting thus far. I would certainly suspect the injector after all of that. But, I didn't see anything about you replacing the spark plug itself. I would try that first as a crack in the insulator (even hairline) will cause mis-fires. If the spark plug doesn't fix the mis-fire, it cold be caused by a bad fuel injector, bad signal (like a loose harness or bad wire) to the fuel injector, or a leak or valve issue on that cylinder.

I have the same problem few days ago, and still didn't find any solution. Can any one please help?????16

please describe the faults and what you have done to it ??

could be the alternator

you have to connect them to switched power, check the available connections with the kry on/off until you find the right power.

might be that there is a defective glow plug there. can be that one of those glow plugs has a prob with its own insulation and is causing a short circuit.