Engine/Transmission (1998.5 - 2002) Fuel Return Line

[RCrosby]

Hey Guys, Looking At Changing My Fuel Sytem. I Been Looking At All Forums On Lp Problems. Had To Change One Lp Under Warranty. Has Anyone Ever Removed Check Valve On Return. with Stock Lp To See What The Psi Would Be. It Seems At Idle 14psi. Fuel Is Returning To Tank. At 11-12psi Running Speed. Fuel Is Not Moving.

 

[Texas Diesel]

https://www.turbodieselregister.com/forums/showthread.php?t=11672



Think of it this way: RE: post #27 at (WOT) 0 PSI he had 30GPH, at (idle) he had 11PSI and 18GPH.



The VP-44 has a variable speed (engine RPM) pump (vane type I think) in SERIES with the stock lift pump. The sneaky problem is we are measuring PRESSURE at the point between the two pumps. In other words, you have pressure at idle when the flow is lowest (18GPH) and no presure when the flow is greatest (30GPH WOT). The lift pump FEEDS the low pressure pump in the VP-44, just because you measure no PSI does NOT mean there is no flow, instead the low pressure pump in the VP-44 is using ALL the flow from the LP.



Make any sense :-laf Clear as mud huh? I have a diagram penciled down that shows fuel flow, perhaps I should scan it.



What makes a VP-44 die when the LP fails is that the stalled lift pump becomes like a closed valve and the VP-44 must suck fuel through the LP. Cavitation results which will kill the VP-44 quickly.

 

[rweis]

And if you feed the VP44 with 14 - 17 psi at all times? VP44 bypass valve should be open bypassing fuel back to the tank when VP input fp is > 14 psi. Right?



Bob Weis

 

[Texas Diesel]

Bob, that would be correct in my opinion.



The question I have is what route the bypassed fuel takes. Does it go through the VP-44 (cooling it?) or around it? This would be interesting to determine.