Engine/Transmission (1998.5 - 2002) LP failing or Bypass valve problems?

[WOT]

I am having fuel pressure problems. At first I thought my Autometer FP gauge was getting ready to go (connected at bottom of fuel housing), but confirmed pressure fluctuation with a mechanical gauge at the inlet to the VP44.



Anyway, it will occasionally drop to 9-10 psi idle (normally 15psi idle & 11-12 WOT) and about 5 WOT (sometimes). The really interesting thing about it is when I am cruising down the road (running maybe 13 psi) it will drop to 9 psi. SOMETIMES if I hammer it it will actually jump back up to 13psi! This is what I don't understand, why is it going up when I mash it? It doesn't alway do it, just sometimes. Anyone ever have the bypass valve on the outlet of the VP44 stick open and cause problems? Was thinking if it was cheap I might pick one up and try that before i replace the lift pump.



The fuel filter is about 4 months old, but since the reading is erratic I don't think is a plugged filter. I would think it would stay low all the time if it was plugged.



Thoughts?



TIA,

WOT

 

[RichardFerrell]

Clean and treat the electrical connections to your LP and fuel pressure gage. My NEW LP did exactly the same thing and I found the installer left the connections on the new PSI gauge only finger tight. Fixed that and it has been correct ever since!

 

[Gary - K7GLD]

Yes - by all means, check your electrical connections - let us know if it helps or not...

 

[WOT]

Cleaned all the connections and drove it for a day and all looked ok. Then had the APPS die . Now that it is back up and running I still have screwy fuel pressures at times. I have connected a mechanical gauge and verified the electric gauge is reading right. I just don't get how sometimes it can increase when I hammer on it. Do you think it is possible the bypass valve is sticking open sometimes? Just seems strange.



WOT

 

[rweis]

I put a Mallory 4307M after my lp to keep the bypass valve closed. So far so good. Set to 12psi bypass.



One idea might be run some fuel lubricant through it to see if the bypass valve is sticking, it might just reseat.



Bob Weis

 

[Gary - K7GLD]

Fluctuating fuel demand/flow variations from low to full flow would certainly contribute to PSI fluctuations if the bypass valve was getting flaky...

 

[WOT]

Thought about the fuel additive... . I put some Power Service additive in it @ the last fillup. Doesn't seem to be making a difference.



Jegs has the Mallory 4307M for $99. I think before I shell out $99 I want to make sure it isn't the lift pump dying a slow death.



Anyone know the part # or how much the bypass valve costs? If it's cheap I might just drop one on to see what happens.



Seems like I also have read about a spring loaded bypass of sorts on the lift pump itself, but can't remember for sure.



Another thought I had was some crap floating around in the tank and plugging up the inlet. I wouldn't think the pressure would come back up on until fuel demand reduced and the blockage washed away. That is unless hammering it also slings it to the back of the tank... Wan't there a recall about baffles that we not secured with the tank?

 

[Gary - K7GLD]

Diesel rated external bypass valves cost more than the pump - and the one in the LP is not serviceable...

 

[WOT]

Interesting thing happened... . It was very consistently dropping to 9 psi and going up to 12 or so when I have it some juice. The tank was at 1/8th so I picked up some fuel and after stopping at 5 or so places on the way home I never had it drop. Not sure if this indicates the lift pump is getting weak and has a harder time pulling from an empty tank or if I have a big chunk of algae or something floating around in there.



Also, I called Cummins and got the price of the valve that's goes into the bango of the outlet of the VP44 - $22. The more I thought about it the more I couldn't really figure out how that one works. I thought it blead off anything over 14psi, but then I got to thinking that the VP44 ALWAYS has fuel flowing through it. If that's the case then that valve must be open all the time to allow flow. Can anyone shed some light on this?



Thanks!

WOT

 

[Gary - K7GLD]

On the OUTLET of the VP-44?



Sounds wierd to me, but if you follow up, keep us posted - but its the LIFT PUMP where you need the regulator, not the VP-44...

 

[WOT]

OK... Actually broke out the service manual. Looks like the OVERFLOW VALVE that I don't understand. It's on the port just in front of the inlet with the shrader valve on it. Of course I didn't find a good description of the part it plays. Does unused fuel flow out of the VP44 somewhere else and tie back into the FUEL DRAIN MANIFOLD that's in the back, drivers side of the block?

 

[Gary - K7GLD]

Only thing at the inlet to my VP-44 is the Shrader valve and banjo bolt - no visible "valve" assembly *I* can see!



I thought the fuel bypass circuits were INSIDE the VP-44...

 

[WOT]

the "banjo" bolt to the front of the shrader is it. It will look bloated compared to the other banjo bolts. Maybe it actually connect to the outlet circuit of the vp44 and the overflow valve part of it connects to a circuit that ties to the inlet. Need a VP44 expert to chime it!

 

[Gary - K7GLD]

HMMMmm - 2 "banjo" bolts there:



#ad




Never paid any attention to the one on the left - maybe that's it?

 

[WOT]

Update

The lift pump finally went south. After a trip to the Kenworth shop (cheaper than Cummins, go figure), $134, and a 1/2 hour of work I have good fuel pressure again. If only the FASS or Preporator was a little more reasonably priced... oh well.