I will probably be challenged here, but the following is my perception of how the VP44 fuel system works. Some of this comes from many hours of research over the years reading many solid documentations of return flow testing of the VP44 fuel system. The rest comes from my own experiences and VP44 testing with my truck, and my background in fluid power.
I was thinking that this carter pump should provide plenty of pressure,
Pumps do not provide pressure. They provide flow. When flow meets resistance, then pressure is generated. When a lift pump output volume (flow) exceeds the input volume (flow) of the VP44's internal fixed displacement vane pump, then the pressure will rise and continue rise until the operating pressure of the pressure regulating valve inside the lift pump has been reached and allows the excess flow to be diverted back to the suction side of the said lift pump.
In a nutshell. The fixed displacement vane pump inside the VP44 provides the flow (pressure regulated from 116 - 319 psi) to all internal operating components, flow to the injectors, and flow to the 14 psi overflow valve to return lots of fuel back to the fuel tank. The lift pump has nothing to do with this. The primary purpose of the lift pump is to provide more flow than the VP44 injection pump can consume under any operating condition. That's all. Nothing else.
From the 2002 FSM -
The fuel volume of the transfer pump will always provide more fuel than the fuel injection pump requires. This will be why you always see a positive pressure on your fuel pressure gauge.
From Bosch -
The first thing the incoming fuel sees in the VP44 is a vane-type fuel pump (called by Bosch the ‘Internal Fuel Pump’) which raises the pressure to “8 – 22 bar (116-319 PSI) depending on the application and RPM” (direct quote from VP44 Service Manual and Bosch Fuel Injection Pump Manual).
@Joe Mc mentions, "feed pressures less than 10# are not good for the life of the VP".
2002 FSM states, "(6)
Start engine and record fuel pressure. Pressure should be a minimum of 69 kPa (10 psi) at idle speed.
What the FSM does not state and also what Bosch kept quiet about for years was that the lift pump pressure was not nearly as critical as was believed at the time. Early on, the real problems were inside the VP44 injection pump. Things like: rotors that did not get deburred, plastic diaphragms (instead of steel) for dampening pulses, plastic parts in timing piston assemblies, poor solder connections in the PSG, etc. Bosch quietly fixed those issues and the Bosch certified remanufactured VP44 injection pumps came out with the improvements automatically.
My own personal experience supports this. A bit of history – the first VP44 injection pump on my truck failed (code 0216) and was replaced under warranty along with the engine block-mounted lift pump which was also replaced under warranty with an in-tank lift pump in August of 2005 at 87,000 miles. At first, I was disappointed that I received a remanufactured VP44 injection pump, but as I learned more about VP44 failures, I was very happy that I received a remanufactured pump.
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. Some would say that low fuel pressure like this would trash the VP44. Not the case at all. 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 106,000 miles logged on the used FASS DRP-02 lift pump. The current replaced VP44 has logged over 273,000 miles. I am not concerned about low fuel pressure.
So, the thing is my pressure may be more a result of my VP44 (overflow/return spring) or whatever it is that regulates the pressure, being the reason I do not see a higher pressure than this???
It has nothing to do with the VP44 injection pump. It has all to do with rated flow and pressure setting on the lift pump. Plus there are other factors that muddle with the expectation of a particular lift pump. Being that it is an electric pump, wiring size and good connections are key to having proper voltage at the lift pump. Voltage drop reduces flow performance. As pressure rises, the lift pump speed slows resulting in lower flow output. Usually when these pumps are advertised, a flow rate will be given, but a flow rate at a specific pressure is not given. So, in the end it will be difficult to know exactly what you are getting in terms of flow. But, it really doesn't matter anyway - as long as there is a positive pressure at the inlet of the VP44, the VP44 will be fine.
I would not worry over your fuel pump pressure. But, you can if you want to.
- John