Engine/Transmission (1998.5 - 2002) Fuel pressure drops to 5 at WOT? Still ok?

[VIPERBLUELX]

I've got a 2000 Dodge Ram. Only mods are BHAF, straight pipes, and TST Powermax1. I have a brand new Carter lift pump installed last week and new fuel filter. Pressure at idle is 12psi, when I step down on it driving normal it will slowly fall to around 8-9. But if I hold it at WOT it will gradually fall down to 5psi and hold. Is this normal since all the fuel system is stock? Should I do the Vulcan big line kit and see if that helps?

 

[16919]

I would try to do some mods to get that pressure up. It really shoold not go below 10 @wot. Try the big line kit or a pusher pump or a fass.

 

[TinyInOhio]

You are on the edge of damage to the VP44. Fuel flow is what cools the IP, and you are not flowing near enough fuel. James is right, time to address the situation with a pusher, or remove LP and go with the FASS.

 

[rweis]

The Bosch calibration machine is fuel pressure range is 10 - 15 psi which would imply to me that is what the installed pump should be getting.



There is a diaphram that flexes with psi and if too low or too high the diaphram flexes too much and damages its seals and guess what quits?



Bob Weis

 

[Luke Warmwater]

Not sure I buy that. I would think that any positive pressure at the head of the VP would be fine. Flow is not going to be improved greatly whether you have 5psi or 25psi. You are exceeding the capability of the VP to accept fuel. I think that low pressure readings can indicate that the LP is becoming weak and is a sign of potential impending doom. I would not worry about it much. Mine will drops to 6psi at times WOT and has done so even after installing pusher. If you ever see it drop to 0 then the VP is drawing more fuel than can be provided and things will turn ugly.

 

[Nate]

I agree.



I just put a new pump on my buddies truck, sits at around 13psi at idle and 5-6 psi WOT... . which is an improvement over 0psi that he had before... for about a year.

 

[Hahn007]

I agree.



Ditto

 

[Joe Mc]

19 psi @ idle, never below 17 psi @ WOT on 5-5 - see link in signature "GFS-392"

 

[keimmmo]

Something else to consider concerning fuel pressure is at what altitude you live. I live at 6000 ft above sea level, but frequently get down to sea level. I will see a 2 psi difference is fuel pressure across the board between the two extreemes!!! It has been that way since I installed the guage 4 years ago!



Steve Keim

 

[rweis]

Sure I have seen reports of VP input psi from 1 psi to over 30 psi. Also the argument "as long as it is positive it is ok", also the in tank lp psi "fix" is fairly low. However everyone is guessing as to what the real psi should be.



One of the VP rebuilders (II, Bret) posted that the Bosch spec was 13. 5 psi. SouthEast Power Systems (we had a club meeting there recently) I asked the same question and they said the Bosch test stand spec was 10 psi - 15 psi to calibrate the VP44.



As far as I know those are the only directly attributable precise VP44 psi definitions I have run across in 4 years because I want to know what the heck it is as well.



II (Bret) said the problem is the diaphram seals are only designed to flex 5mm with 13. 5 psi as the "no flex psi". He feels 5mm flex allowed to be within specs so the seals are not distorted is about 3 psi. Over or under the 5mm flex of the diaphram the seals begin to distort and in time (depending on the severity of the distortion and the cycles of distortion) the seals fail which prevent the high pressure side of the VP44 from being sealed and therefore no high pressure which prevents the injection cycle from happening, and therefore "failure". I think the diaphram has been updated over the years from a steel diaphram to a stainless steel diaphram. I am not sure why (not a metalurist or dynamic engineer) but I would surmise that the newer diaphram is designed to last longer and was therefore a weak point in the past.



In BK old post of "pumps, lines, and whatnot" they found that as the input psi got over about 20 psi the return fuel at the same operating conditions became hotter, indicating more stress was happening internally at the same work level. That thread even took the input psi to 50 psi to see what would happen. That is a long thread, but worth reading. Their summary was the input psi probably should not go over about 20 psi.



There is very very little true factual data on what the design specs for fuel input should be to the VP44. All I have been able to find is stated above and therefore I run mine at 13. 5 idle to 16 WOT trying to maintain as close to design specs as possible. Volume is a seperate issue and I think that the larger fuel feed lines (AN-6 or greater and only full flow fittings) are a good thing, but have no factual data to support that assumption.



I have zero idea as to what happens with altitude only knowing that altitude does change the characteristics (physical or chemical) of most fluids.



All in all, it is a crap shoot trying to keep the VP44 running.



Bob Weis

 

[Moose Truck]

What he said

19 psi @ idle, never below 17 psi @ WOT on 5-5 - see link in signature "GFS-392"

Ditto, I did the same thing, after three Carters in two years, I Went GDP GFS-392. Idles at 16, Cruises at 16, can suck it down to 14PSI, however I only have a Vanaaken box for fueling. Moose00 Oo.

 

[VIPERBLUELX]

I think the Vulcan relocation/big line kit will be my next mod.