My 94' F-250 4x4 with the 351 won't start. I cranked it up to let it warm up in the cold weather. It ran for about 3 minutes and then shut down. It did not starve for fuel, just shut off like I turned the ignition off.

The engine will turn over but will not start. I have tried starting with both tanks but neither make a difference.

What should I start looking for?

 

Most likely you lost spark. You could make sure by cranking the engine over and pulling the coil wire or a spark plug wire and checking for spark. See if the engine will start after about 3 hours, that will give a clue as to which component is bad.