Below is part of an article from a VP44 seller.


Hard starts, as they relate to VP44 injection pump problems, are caused by lack of fuel or over pressurization by the lift pump. If a lift pump is going bad and failing to supply the proper amount of fuel to the injection pump, then the truck will hard start due to lack of fuel. However, if a new lift pump has been installed and is pushing over 15 PSI to the VP44 injection you can also get hard starts. The VP 44 is a very sensitive pump and needs approx. 12 PSI to run properly. Once you start getting about 15 PSI to the injection you actually begin to over pressurize the pump and cause it to act as if you had flooded the engine.




Does this mean one doesn't need all that fuel pressure like pumps from fass, AirDog etc.? The pump itself converts low pressure to high pressure...Here is what a new Carter in-tank lift pump puts out:
The module we sell P76473M is a high flow design.


At it has a maximum static pressure of 17psi


It flows 37 gph @ 16 psi


It flows 55 gph @ 15 psi


It flows 80+ gph at 11 psi


Other pumps are said to supply 16,17 and sometimes higher fuel pressure at idle but this article states that is too high and will damage the VP44 pump.

 

25+ psi starts getting you in the danger zone of damaging a vp44. What the article is talking about is that higher pressure can make it hard to start which is true if the pump is wired to run full blast as soon as the key is turned on. But if you are using the factory LP wires to run an aftermarket LP when you turn the key on the LP only turns on for half a second to prime the system. It only runs constant after the engine is running. The reason behind getting a LP that runs at a higher pressure (14+psi) is because the fuel return valve doesn't start to open till the vp44 has 13psi of fuel. The two biggest reasons a vp44 fails is it gets too hot and lack of lubrication. Fuel acts as both lube and coolant for a vp44 so we want fresh fuel constantly circulating through it. I have ran 20psi for years without any problems.

 

The lift pump pressure is only one parameter pertaining to fuel system

operation/diagnostics, and as long as you have positive pressure to the VP-44 at

adequate flow, your engine should run fine. "Adequate" means normal performance,

starting and smoke levels. Any issues with the engine running outside of the

below listed pressures being in their respective proper ranges should be addressed.

The numbers in the manual are:



Idle speed:

700-1000 rpm

Maximum fuel inlet restriction to lift pump:

6 in Hg

Maximum Fuel Pressure at Fuel Filter Outlet (engine cranking): 4 - 8 psi

Minimum Fuel Pressure at Fuel Filter Inlet (engine running): 8 - 17

psi

Fuel Drain Line Maximum Restriction:

10 psi

Fuel Inlet Maximum Temperature:

165 deg. F

Engine Minimum Cranking Speed:

150 rpm



Excessive smoke can be a symptom of improper injection timing, which MAY be

caused by insufficient fuel to the VP-44. Variable pressure within the pump

rotates the cam ring that affects/effects injection timing changes; insufficient

fuel delivery to the pump will affect the pumps ability to establish the correct

timing per operational condition. Normally this is a blue-ish white smoke.

Black smoke is indicative of over-fueling (relative to the amount of air).



The fuel flow test (005-011) states to run the engine at high-idle (running on

governor with no load) speed; the outlet side of the filter should read 10 psi

minimum. To be thorough, one needs to check at high-idle as well as idle and

measure at the correct places. Under high-idle conditions the pressure drop

across the filter is to be 5 psi maximum.



The manual further states, not to operate the fuel system with suction

restriction of more than 3 psi but Bosch prefers that there is no negative pressure on

the VP-44. We are not aware of any durability validation tests at "outside of

acceptable operation" parameters or that negative pressure might cause VP-44

failures due to the lack of sufficient fuel for pump lubrication. Unlike the

P7100, the VP-44 is lubricated by the fuel which circulates through it. Bosch may

be able to advise the effects of low lift pump pressure on the VP-44 injection

pump.



The output of the fuel lift pump is checked via test (005-045-012) by measuring

the output pressure, using a pressure gauge at the fuel filter head tap. For

long injection pump life, the Bosch VP-44 pump requires clean No. 2 diesel fuel,

with no water or foreign debris. Owners should be very careful during fuel

filter maintenance or replacing the lift pump, to ensure that no debris gets into

the injection pump side of the filter.



Always use the lift pump to fill the filter. This can be done by bumping the

starter with the key switch but not starting the engine. The lift pump will run

for 25 seconds. Turn key off and repeat two more times then filter should be

full.



We thank you for your interest in Cummins products. Please let us know if you

need assistance in locating the nearest Cummins-authorized Dealer or Distributor

Service Provider. For assistance in locating a Service Provider, feel free to

use Cummins North America Dealer Locator, which can be found on Cummins website:



http://wsl.cummins.com/ServiceLocato...servicelocator



Please let us know if you have other questions and if away from your computer

or have a time-critical request that needs more urgent attention, feel free to

call us toll-free (from North America) at 1-800-DIESELS (343-7357).



Email direct: powermaster@cummins. com



Regards,

--

Powermaster

Customer Assistance Center

Cummins, Inc.

Columbus, Indiana, USA

 

That's correct you want enough pressure to keep cooler fuel running through the VP. I run mine at 17-19 psi. Depending on if my foot is in it or rolling down a hill. The pump will run at 10 psi but it will be full of hot fuel and IMHO will reduce pump life. Also running 2 stroke in the fuel will help with pump life as well.