Engine/Transmission (1998.5 - 2002) Lift pump again - humor me please

[Charliep]

My 98. 5 consumes 5 gallons per hour at 70mph pulling a 7500lb Airstream. The manual says 70% of the fuel delivered to the injector pump is returned to the tank. This means the lift pump must supply approx 16gph. Make sense?

Now for you engineer types, how much fuel will an ailing lift pump supply at 2,4,6 and 8 PSI given the capacity of our fuel lines and fittings? At what point, or pressure, does low fuel supply pressure become critical?

 

[Steve St.Laurent]

That makes sense at cruise pulling your trailer on flat ground. Now make it a large incline with a head wind and you're trying to accelerate and now you're talking a lot more than that.

 

[15w40]

An ailing lift pump? It will depend on why it is ailing. There are a couple of different ways these pumps show problems. One is the electric motor shows wear on the comutator, the part that feeds power to the rotor. This problem will cause the motor to run at a lower speed and flow a lower volume of fuel. Another common failure is with the internal pressure regulating “spillover” valve. The valve is supposed to open at about 15 psi but as they wear out the pressure will fall. As the pressure falls the volume falls also. So to answer your question, many lift pumps begin to fail at about 50K miles. If you are still on the original 1998 pump that came with your truck, it may be time for a new one. The newer ones have been improved and “hopefully” will last a lot longer.