Engine/Transmission (1998.5 - 2002) LP questions

[keimmmo]

Just wondering... does the lift pump run all the time when the motor is running? If so, how does it regulate the maximum pressure?



Steve

 

[B.G. Smith]

Lift pump

Yes, it runs all the time when the engine is running, the pressure is regulated/maintained by the voltage being regulated to the pump. bg

 

[15w40]

The lift pump voltage is controlled by the ECM (Engine Control Module) mounted on the driver’s side of the engine. It will reduce the lift pump output pressure when starting by quickly turning on and off the 12 volts to the lift pump. This causes the lift pump motor to rotate slower and will reduce the fuel pressure to the injection pump to about 7 psi. After the engine starts, the ECM will apply a constant 12 volts to the lift pump and cause it to run at full speed for as long as the engine is running. The lift pump pressure, running at full speed, is limited to about 15 psi by a built in regulator valve that will re-circulate fuel from the pump output to the pump input. This re-circulated fuel passes through the motor assembly and is used to lube the motor bearings and cool the electric motor. As the pressure gets close to 15 psi, a spring in the regulator valve will compress and allow a ball valve to “spill back” the excessive pressure. In my opinion, it will also cause the fuel to pick up particles of graphite from the motor brushes and metallic particles from the motor commutator and bearings. Hopefully the filter will catch all the junk generated from the pump.

 

[keimmmo]

Thanks. I thought it ran full time at full voltage when the motor was running. I realized from previous reading that there is reduced voltage when starting to reduce the pressure.



I have installed a pusher pump using a second factory LP. I do not see any problems running it full time if the original pump also does... unless someone knows something from experience that I don't.



The second pump sure solved my low fuel pressure problems! I run a 1 1/2 quart spin on primary fuel filter and the factory filter as my secondary. (Dirty fuel in Mexico. ) I had trouble maintaining over 5psi at cruse and quickly hit zero if I pushed any at all. Now I idle at 26psi. Cruise at around 20psi. Never below 14psi at sustained full throttle. (New filters. ) That ought to lube and cool the VP-44!



I am aware of possible hot hard starting problems with too much pressure. The way things are hooked up I can keep the starting pressure low, so I do not anticipate a problem there. Does any one else see a problem with these fuel pressures?



Thanks again. There is no school like personal experience! You have saved me plenty of expense and problems with my trucks!



Steve

 

[Ncostello]

I'd be leery that the original pump is dead and the pusher is going to be pushing against a big obstruction... the impeller of the second (OEM) pump.



But if your taking pressure readings up near the VP and its staying high... then maybe you'll be alright.



Perhaps the spillover valve is weak in the stock pump and this pusher is somehow "fixing" that problem. It could be the motor is OK and the pusher is forcing the valve closed by giving you high positive inlet pressure from the backside of the valve. FWIW.



I'd also say your at the high end of the spectrum when it comes to safe VP44 inlet pressures. Just from what I've read it seems when your up in the 20's it becomes questionable.

 

[keimmmo]

I think the original LP pump is still fine. Although it had trouble maintaining much pressure even at a moderate cruise, it made 18 psi with engine off and 12 psi at idle. I am measuring the pressure at the schrader valve on the injection pump. The problem seemed to be more a lack of volumn, which I contribute to the pre-filter I run and the worthless pusher pump I was using. I needed something to push the fuel through the first filter to get it to the original pump with enough volumn to maintain good pressure. I have more pressure than I expected, but at least I can now maintain plenty of pressure!



I have just replaced the injection pump, so I feel it is in good shape. I installed the pressure gauge with the new pump. I think I know why the old injection pump died! I don't want to hurt this pump! I know my fuel pressure is on the high side now. But, will it hurt anything?



Steve

 

[Brand-X]

If you are concerned about to much fuel pressure at start up then I would run the ground that goes to you new pusher pump through a Hobbs pressure switch tapped into your oil galley use maybe 25-30 psi then it would most likely be running just before the pusher pump turns on .



George