My friend has a '91 Ford F250 with a diesel. It has 250K miles on it. If the gas tank(s) are more than half full, the fuel starts blowing out of the filler tubes. No one seems to know what's wrong with it. Has anyone out there experienced this or do you have an idea what might be wrong? Any help would be greatly appreciated.

Sandman

The fuel tank vent system is plugged. Heat from the exhaust pipes and fuel return is expanding the fuel in the tank. When that happens, it forces the fuel out the inlet. Figure out which hose from the tank is the vent line and start tracing the flow to find the blockage. This could also cause engine stalling and lift pump failure when the tank ends up with a vacuum as the fuel is being drawn out.

There was an issue with the selector valve on the older gas engine trucks that was allowing the return fuel to go into the wrong tank and it would blow out the filler neck. Never saw it on the older diesels though, but that could be a possibility.