Lift Pump Bypass for VP44 Trucks?

[Tractorman]

Lift Pump Bypass for VP44 Trucks?

 

The Modification – This subject matter might be a bit controversial, but I believe I have found a good solution regarding driving a VP44 equipped truck when a lift pump has failed while on the road.  After I did an experiment for checking fuel return volume from the VP44 fuel injection pump on my truck, I realized that I came upon what I believe to be a simple solution to one of the biggest fears for VP44 truck owners - a lift pump failure while on a road trip.  For the experiment I routed a fuel line with a one-way check valve around the lift pump so fuel would still flow when the lift pump was non-operational (simulating an electric lift pump failure).  The reason I say “fuel would still flow” is because the VP44’s internal vane pump will draw the fuel into the VP44 injection pump.

 

This lift pump bypass circuit includes a one-way check valve to flow fuel in a direction from the fuel tank to the stock filter housing.  This lift pump bypass circuit on my truck is now a permanent circuit.  Below is a diagram of the circuit.

 

 

 

 

The Experiment - I arrived at a conclusion after experimenting with fuel return flow volume with or without an operational lift pump.  The results of the experiment showed that fuel return volume did not change for either lift pump condition.  Each test from the experiment showed averaged return fuel flows at approximately 19 gallons per hour, whether the lift pump was operating or not operating. 

 

The conclusion - My conclusion is that the lift pump only provides a positive pressure to the VP44 injection pump.  It is the VP44 internal vane pump that takes over from here and supplies the fuel flow and operating pressure for all of the VP44’s internal components, including injector flow and fuel return flow for cooling.  It is not just this experiment alone that helps me draw this conclusion.  After much research over many years regarding the VP44 injection pump operation, and reading many posts from people who have done VP44 return flow volume testing with a flow meter, I am confident in my belief that the lift pump’s only job is to supply a positive pressure to the inlet of the VP44.  The lift pump accomplishes this by providing more flow that the VP44’s internal vane pump can consume at any time.  The VP44’s internal vane pump takes over from this point and provides flow and pressure to cover the needs for VP44 pump operation and cooling, which includes fuel flowing over the 14 psi overflow valve.

 

The Test Drive - I test drove my truck for over 25 miles without the lift pump operating. My low pressure fuel indicator lamp remained on during the test drive. The truck drove fine.  I shut the engine off and restarted it several different times.  I applied wide open throttle several times.  There were no performance issues during the test drive.  If the lift pump failed on my truck on a road trip, I would not hesitate to continue driving the truck with this lift pump bypass circuit in operation.  

 

This simple fuel supply modification will allow a truck to continue to be driven with a lift pump failure.  The VP44 internal vane pump will continue to supply fuel to the VP44 injection pump via the lift pump bypass circuit and will also continue to supply ample return fuel flow to the fuel tank for cooling the VP44.

 

This modification could be a practical solution for those who drive trucks with a stock engine mounted or frame mounted lift pump.  An in-tank lift pump bypass circuit would be more difficult to build, but would be doable.

 

Lift Pump Bypass Operation - During normal lift pump operation, the one-way check valve will not allow fuel to return to the suction side of the lift pump.  A fuel pressure gauge at the inlet of the VP44 pump will always register a positive pressure. 

With a lift pump failure, the fuel will pass through the free flow direction of the one-way check valve directing fuel around the lift pump into the VP44 pump inlet via the stock fuel filter housing.  A fuel pressure gauge at the inlet of the VP44 pump will register zero at this time.  There will actually be a small vacuum generated as the internal vane pump will be drawing fuel from the tank through the suction screen and fuel filter.  What is important to note is that the same volume of fuel will be flowing through the VP44 injection pump and returning to the fuel tank as when the lift pump was operating.

 

I am not recommending or suggesting that anyone perform this modification to their truck.  However; personally, I am very confident that this modification will cause no harm to the VP44 injection pump should a lift pump failure occur.  In fact, I believe the bypass circuit offers more protection to the VP44 pump than a circuit without the bypass feature simply because a failed lift pump could block or severely restrict the fuel supply circuit.

 

I am posting this information on this particular site because, in my opinion, this site provides the best and most accurate information when it comes to second generation Dodge Cummins trucks, especially VP44 trucks.  I say this with one exception – and that exception is:  I don’t think this site has provided the best information on how the lift pump operates in conjunction with the internal vane pump inside the VP44 injection pump and how the 14 psi overflow inside of the VP44 injection pump interacts with the internal vane pump.

 

I do think that members of this site go to a lot of effort to provide very useful information to solve problems for owners of Cummins equipped Dodge trucks and I am thankful for that.

 

Any questions or comments will be appreciated.

 

- John

 

 

 

 

 

Edited October 11, 2021 by Tractorman
proof reading

[W-T]

To provide any fuel flow in regard to a failed primary pump is a good thing. I believe this prudent method could be a real asset in a failure on the road situation. I like it John and eliminates the alternative of just sitting on the side of the road. BTW, I've followed your observations of volumetric flow and it has been quite informative.

 

Cheers

W-T 

[Tractorman]

Thank you @W-T!  That was the best compliment I could have ever received!

 

- John

[Mopar1973Man]

The check valve inside is a harden steel ball bearing on a spring. The seat of where its at in the pump is just cast aluminum. After a period of time that steel ball check valve will beat the seat to death till its in constant bypass producing no pressure. I've found at least 3 pumps in my years where the steel ball was found jammed in the vanes of the pump.

 

Kind of like the old P-pump check valve the banjo was made of softer steel and the steel check ball will beat the seat out if till most of the fuel is running out the return.