Engine/Transmission (1998.5 - 2002) Question about VP44 overflow valve????

[Kyle]

The service manual says the overflow valve on the VP44 opens at 14 lbs. Does that mean that anytime I read less than 14 lbs of pressure that there's no fuel going back to the tank?

 

[DavidC]

Never recieved an answer

Kyle,



I have asked this question twice and never received a reply. I assume like you that under 14 psi there is no return to the tank. I do not know how precise this 14 psi setting is or if there is a tolerance. Sometimes at idle my gauge shows 15 psi and may be returning fuel, anything off idle though is below 14. I would guess that the only real way to find out might be to install a clear hose.



Dave

 

[JerryWallace]

Kyle



You will always have some fuel going back to the tank.

Do a search for (VP 44 Theory)

I posted a picture that shows a cross section of the pressure valve that your talking about, 2nd page

Look just below the valve. You can see a small return port that will always let some fuel return to the tank.



Jerry

 

[Kyle]

I don't get it

I see a picture of a valve that's pushed open (presumably due to 14 lbs of pressure). Where's the path for fuel to escape when the valve's closed?



I read the test procedures last night in the service manual. It says (and I'm paraphrasing) that if you put less than 14 lbs pressure on the inlet to the valve, it should not leak air. That sounds pretty closed to me. What am I missing?

 

[JerryWallace]

Look closer



there are two exits.

one is controlled by the check ball the other is not. {free flow}



Jerry

 

[JerryWallace]

A closer detail



jerry

 

[Motorhead]

Thanks for the pics Jerry. I always wondered the same thing as to wether there was fuel flow thru the pump at lower pressures. Chris

 

[Royal]

KYLE--- Your electric lift pump has little or nothing to do with fuel return,except to bleed air out of the system after filter change etc. The lift pump can then force fuel and air thru the small orifce under the overflow valve and back to the tank. The VP pump has a 5 blade vane type positive displacement pump that pressurizes the high press. plungers,provides pressure to operate the hydraulic advance system,and provides a internal housing pressure in the VP assembly. The over flow valve maintains the housing pressure at 14 psig. This return aids the cooling and lubrication of the internal components and also vents out any air bubbles that get in the pump. The vane pump pressure is a function of pump/engine speed,and the pressure is regulated by the internal pressure control valve. It is a spring loaded slide/piston valve that is adjustable during calibration. It simply returns excess pressure to the inlet side of the vane pump. The vane pump pressure is approx. 150 psig . Estimate, as I am not priveledged to test specifications.

 

[Kyle]

Oh! -- "Nevermind"

Thanks, NXS -- Now I see it (Duhh!)



I'm still confused by the test procedure for checking to see if the overflow valve is stuck open.



Here's the question I'm really trying to answer. How can we tell if / when the VP44 is "starving" -- before it's too late?



It would be difficult to measure the volume returned to the tank. Plus that would vary greatly depending on RPM, HP output, lift pump condition, etc.



How about if we measured the return fuel temperature? Or the temp difference between the fuel going in and the fuel coming out? How about measuring the temp of the fuel inside the VP44? Is there a "reservoir" in such a location that you could drill and tap the pump housing for a temp sender? Would any of these give a clue?



If so then the obvious question would be what temps are tolerable and what's "too hot"?

 

[Gary - K7GLD]

Interesting thread - and a close inspection of the supplied drawing seems to suggest the free-flowing orifice is pretty much limited to merely bleeding relatively SMALL amounts of air thru the VP44, and would be of LITTLE value in providing sufficient internal VP44 cooling in extended operation where fuel flow is LESS than 14 psi?



Or am I missing something, and overestimating the "value" of internal cooling of the VP44 from fuel bypass flow?

 

[juicedcummins]

I just stuck a new overflow valve in from cummins a couple of months ago and it isn't the same as the old ones.

It does not have the small bleed hole any more. {Why?}



And yes cummins says it's the right one for my ride.

 

[Roger rodbolt]

Good Explanation

Royal,

Thanks for the great explanation. Larry

 

[Royal]

GARY----At idle speed, the internal fuel pressure will be well above the 14 psig and the overflow valve will be open. At idle speed the pump is just loafing and will have plenty cooling and lube. The pressure will increase rapidly as eng. speed increases.



Juicedcummins---Bosch makes the overflow valve,and they may have made a design change so you cannot see the orifice. You could try to blow thru it when not installed to see if it vents. If it doesn't you mave have starting problem if you get air in system. You might also check with a local Bosch service shop and see what they say.

 

[andrewkoperdak]

Sticking neck out

I believe that there isn't much need to allow much flow back to the tank under load. My reasoning is that there is sufficient quantity of fuel flowing threw the VP to the injectors to cool the pump adequetly. At idle the pump isn't getting that flow so the relief provides sufficient flow at and above 14 psi to keep the pump cool. There is a lot of heat generated making the 5000 to 14500 psi necessary for pop-off in the injector, even at idle. The relief takes care of that when our trucks aren't dumping it into the engine for power.



Andy

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[juicedcummins]

Anybody out there with a pusher setup try drilling out the overflow to see if it will bring the pressures down a bit?



This could increase the cooling and shouldn't't affect the full load pressures too much. {just a thought haven't looked at a blown up view of a vp44}