Lift Pump Bypass for VP44 Trucks?

[petersonj]

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 for an idling engine 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 after 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.

Any questions or comments are welcome.

- John

 

[Ozymandias]

You see that perfectly right.
The CP3 pump works exactly the same.

 

[BarryG]

Way back when (circa 2000) there was a lot of testing by members on this website to try and deal with the problematic lift pumps taking out the vp44s or perhaps better to say the seemingly high failure rate of the vp. Which there is no need to get into that now but one of the lingering questions even after all the testing then and up through even today is what kind of pressure does the vp need to keep it happy. While there have been revisions to the vp and the lift pump over the years since their introduction the general design is exactly the same. Dodge Ram truck is one of the few out of many uses for the engine with the vp44 that even uses a lift pump. The remainder of them just use the vp's low pressure pump to draw the fuel. One of the really big differences between the Dodge application and the others is the distance the vp has to suck the fuel. IIRC Dodge is the one who spec'd the lift pump not Cummins.

Many feel that the vp needs at least 10psi and preferably more like 14-15psi. One of the big arguments for higher pressure was to keep the vp cool as it is cooled by fuel. If you ran some flow tests you probably found that approximately 85+% of the fuel was returned to the tank. I have always felt that as long as it had some pressure (3-5psi) it would be ok. I certainly had my fair share of trouble with the VP44. In fact I went through 3 of them about 50k, 100k and 150k miles more or less. Each one seemed to last about 50ish k miles. The first two were taken out by or at least feel they were taken out by failed stock lift pumps. the last one is still working just fine at 375k miles. When I put the last one in I redid the fuel pump with a RASP (engine driven type pump via a belt) after I did some subsequent modifications I ended up converting to the 12 valve style mechanical lift pump driven by a lobe on the cam (I upgraded the cam in the truck and it had a fuel pump lobe on it where as the stock cam did not) which also cleaned up the fuel plumbing substantially from the rasp. After that I never gave my lift pump another thought as those pumps are pretty bulletproof.

Good on you for forging your own road.

 

[BigPapa]

Dodge Ram truck is one of the few out of many uses for the engine with the vp44 that even uses a lift pump. The remainder of them just use the vp's low pressure pump to draw the fuel. One of the really big differences between the Dodge application and the others is the distance the vp has to suck the fuel. IIRC Dodge is the one who spec'd the lift pump not Cummins.

Are you maybe confusing the Common Rails with the VP? All our buses with VP pumps had a lift pump. I don't think I've seen an application with a VP that didn't have a lift pump.

Our CR buses don't have a lift pump.

 

[petersonj]

If you ran some flow tests you probably found that approximately 85+% of the fuel was returned to the tank.

@BarryG, thank you for taking the time to reply. Also, I read some of your earlier posts (a few years ago) regarding a lift pump low pressure problem that was very difficult for you to diagnose. Just curious, what did you have to do to resolve that problem?

I did run several flow tests - with the lift pump running and without the lift pump running (bypassed). When compared, the results were always the same - 19 gph at idle and higher amounts at higher rpm and the volume returned was well over 70% of the volume entering the VP44 (70% is mentioned in the 2002 FSM). The return flow volume was the same at a specific engine rpm whether the lift pump was operating or not operating (bypassed).

I have always felt that as long as it had some pressure (3-5psi) it would be ok.

I completely agree. Fortunately, I have had only one failure of the VP44 and no lift pump failures. The VP was replaced and in-tank lift pump was installed - both covered by warranty. That lift pump produced 6 psi at idle and 3 psi at wide open throttle. I ran that lift pump for 164,000 miles before replacing it with a frame mounted lift pump. Obviously, the low fuel pressure supply did not affect the VP44 operation.

A bit of history on my truck – the first VP44 injection pump on my truck failed (code 0216) and was replaced under warranty . The engine block-mounted lift pump was also replaced under warranty with an in-tank lift pump in August of 2005 at 87,000 miles.

The old block-mounted lift pump was still providing 12 psi at idle and 7 psi at wide open throttle when it was replaced. The new in-tank lift pump provided 6 psi at idle and 3 psi at wide open throttle. I ran that lift pump for 164,000 miles. In April of 2016 at 251,000 miles on the truck, I removed the in-tank lift pump and installed a used FASS DRP-02 frame mounted lift pump. The in-tank lift pump was still working fine – I just wanted the lift pump to be easily accessible if it failed on a road trip. The FASS lift pump provides 12 psi at idle and 6 psi at wide open throttle.

Currently, there is approximately 109,000 miles logged on the used FASS DRP-02 lift pump. The current replaced VP44 has logged over 270,000 trouble free miles and continues to perform well.

As you see from the vehicle history, the replaced VP44 has never seen over 12 psi, and that is only at engine idle.

Again, @BarryG , thank you for your comments.

- John

 

[BarryG]

Perhaps I should say outside of the transportation on road industry. Gen sets, off road equipment ie tractors, loaders unsure about the marine uses. I think the use of the lift pump is more related to distance the fuel has to be pulled in the application more than the vp really needing it. IIRC the vp internal low pressure pump is only 3 or 4 psi. One thing I cant remember is if the vp low pressure pump feeds the vp regardless of the pressure behind it (which I am thinking is the case vs a bypass of the low pressure pump if another pump is present like the lift pump that provides higher pressure). I may be all wrong about it all, it has been a while since I worried about a VP and even longer since all those threads from way back when were testing them trying to figure out how to keep them from toasting. Seems there were two main reasons for them toasting, the computer boards would get hot and melt the solder on them or the plunger or whatever it was called would get galled up and couldn't hold pressure. Both of which could be attributed to lack of fuel pressure (but not necessarily), not enough lube for the plunger (I know plunger is not the right word) area or not enough fuel to keep it cool enough. Of course back then, bombing them at least on this forum was more the norm then. Couldn't be we were exceeding some of the design capabilities of the vp, though a properly modified vp equipped truck could lay down some fairly serious horsepower even by todays standards.

 

[BarryG]

My low pressure issue at least compared to the normal pressure I was used to seeing turned out to be a worn oring and or scarred piston in my jetcan bypass valve. It couldn't seat fully/properly due to the oring being bad and the valve piston was a bit roughed up. Once I finally figured that out a phone call, a couple days shipping and 15 minutes of repair time to rebuild the jet can cleared it up. As I mentioned previously I ended up putting a cam in the truck and used a 12 valve mechanical lift pump which put out way too much pressure for the VP so the jetcan bypass valve was the answer to bleed off the excess pressure and send it back to the tank. That mechanical pump could peg my 30psi pressure gauge and the vp spec I think is like max 20ish psi though I know some ran higher than that with at least no immediate ill effects. I had my bypass set at about 15psi and with that pump even wot it rarely dipped below it. When I was using the RASP pump the faster you went the higher the pressure it was odd watching your fuel psi rise as you pushed the go pedal down.

 

[Extreme1]

A question: if the flow is the same with and without a functioning lift pump, is the lift pump needed?

The reason I ask is because a friend's '01 2500 is having fuel supply issues. Since my truck is still down and I've used his several times, I plan on doing the work to fix it for him. The lift pump was replaced quite a while ago and was relocated to the frame. The system loses prime when it sits very long and it "runs out of fuel" with about 3/8 of a tank left. Since I need to get into the tank and probably replace some hoses, can we eliminate the lift pump? I recall that someone did years ago but I couldn't find the thread.

 

[Ozymandias]

A question: if the flow is the same with and without a functioning lift pump, is the lift pump needed?

The reason I ask is because a friend's '01 2500 is having fuel supply issues. Since my truck is still down and I've used his several times, I plan on doing the work to fix it for him. The lift pump was replaced quite a while ago and was relocated to the frame. The system loses prime when it sits very long and it "runs out of fuel" with about 3/8 of a tank left. Since I need to get into the tank and probably replace some hoses, can we eliminate the lift pump? I recall that someone did years ago but I couldn't find the thread.

LP is absolutely necessary as the VP will fail without it within a couple thousand miles.

 

[petersonj]

A question: if the flow is the same with and without a functioning lift pump, is the lift pump needed?

It is important to maintain a positive head pressure at the inlet of the VP44 injection pump. There are many industrial applications where a fuel tank is placed above the inlet of the VP44. In these applications, a positive pressure to the VP44 will always be present, so a lift pump is not needed.

However, in a vehicle application such as a truck, there would never be a positive head pressure at the inlet of the VP44. In fact, there would be varying degrees of negative pressure that would be dictated by the condition of the fuel filter, the suction strainer in the fuel tank, the pitch of the vehicle, and the level of fuel in the fuel tank. Also, should any air be introduced into the fuel system, a hard start or a no start condition could occur. Should this happen, there would be no way to bleed air from the fuel system without a lift pump. So, as you can imagine, a lift pump is very necessary and very useful.

The primary duty of the VP44 internal vane pump is to supply a flow of fuel for the internal operating components (timing piston, fill chamber for high pressure radial piston pump, accumulator, etc) and to supply a volume of return fuel flow to the fuel tank via the overflow valve. I don't believe it was ever intended for the internal vane pump to draw fuel from a distant fuel tank that did not provide a positive pressure to the inlet of the VP44 pump.

- John

 

[Mopar-muscle]

I figured this one out years ago when I was doing something to the lift pump .. I have CRS sometimes ... and forgot the plug the harness back into the pump. I started the truck up and drove to the local gas station to fuel up and didn't realize I forgot to plug in the pump till I got back home, about a 4 mile round trip. It's not something I would do but a failed lift pump shouldn't leave your stranded I wouldn't think , and this is why you should have a fuel pressure gauge on the input of the VP.

 

[jeepit]

My lift pump went out some day while snow plowing. I say some day because in winter I only use it to plow my driveway. After winter I was driving about 25 miles or so and happened to glance at the FP guage finally noticed I had Zero pressure and pump was not operating. Ended up to be a fuse blew.