Oil and Excess Pressure in fuel

[tree98]

First off you should send a sample of the fuel out to get tested so you know exactly WHAT is in the fuel instead of guessing.

What do you mean when you say the "fuel tank builds excess heat and pressure " how hot is it getting? Why would the fuel tank be building pressure? It's vented, it's impossible to build pressure unless the vent is plugged, in which case I think it would actually pull a vacuum.have you checked the vent tube?

Is this thing demandated? If so who did it? The only thing I can think of that would cause excessively hot fuel would be if your cylinder head was running much hotter than it should (how high is your coolant temp getting?) or you have an obstructed fuel return circuit.

I really hope you put a new genuine cummins head and injectors on it and not some after-market crap.