I know what you say is good advice, but I personaly don't believe thats the problem. The engine light problem would have started intermitently. It did not. When it ran what I would call fair, I tried to drain filter while running and It started to rap right off. I will try to supply external fuel, but as far as getting codes checked. I can't even drive the truck. Chuck.
THIS TRUCK BEING DIESEL HAS NO O2 SENSORS BUT JUDGING BY YOU DESCRIPTION YOU EITHER HAVE BAD UNDER VALVE COVER CONNECTORS OR BAD INJECTORS
HERE ARE A FEW TESTS TO DO AND LET ME KNOW WHAT YOU FIND