I posted about the limp mode issue and I tried out this "Just answer" pay for answer service. The RV technician advised filling the Fuel filter with transfluid to clean out the lifting pump. He agreed with many that the fuel filter is most likely to blame but they always put transfluid in the fuel filter to clean the pump.
Does the Duramax route fuel up to the fuel filter THEN the pump or is the pump in the tank?
Oh so your talking about the high pressure pump? I wouldn't want to run anything foreign through it. Clearances in the pump are too tight for anything thick in viscosity to get through without possible damage. The injectors are easy to carbon up with something like oil. HP pumps and injectors are just too costly to replace and too few shops can accurately diagnose problems with them.
I would just replaced the fuel filter before trying anything more invasive.
The Duramax has a CP3 High Pressure Pump that sucks fuel from fuel tank through fuel filter. A leak can cause air to get into the system. You will not see any fuel leak. Because fuel is being sucked from tank.
Have you checked primer pump on filter to see if it needs to be pumped to refill the filter. Also the bleeder screw have been known to have bad O'rings.
I would never run anything through a CP3 High Pressure Pump other than fuel.