I have an 01 cummins and seems to be leaking fuel out of the fuel overflow fitting at the rear of the head. Does anyone know what would happen if this hole was just to be plugged?

 

I believe that is the fuel return line if there is a line connected to it.. if so I wouldn't do it the unburned fuel needs somewhere to go.. Not sure if it has the same effect on our trucks but when I was in diesel school our insructors told us that Drivers in their big rigs would clamp the return line to get more power... long story short they blew holes in the side of their block because the engine would take off..Lol

 

its a very bad idea to block that off. it will cause a major build up of fuel pressure in the head and blow out seals in the injector and in the connector tubes. if not just that worse like a blown engine.

 

if you plugged that hole, the truck would not drive. That is a fuel return line to leak access fuel out of the head that is not being burned. If you plugged it, fuel would pool on the pistons, cause white smoke to burn and would shut off and not start. Those OFV suck and the springs on the inside bend or break and begin to leak. U can either take the bolt out and replace it, or you fix the one you have. Or u can completly take that line off and run -an fittings to pushlok to completly get rid of the 1/4" line restriction. Return line head bolt size is metric 12x1.5