Engine/Transmission (1998.5 - 2002) Minimum FP question ???

[JPLB22]

I have a question about the minimum inlet pressure for the VP44 for safe opperation.



When I try to think logically about it this is what I think. The fact that there is any prussure at the inlet means to me that there is an excess of fuel that is causing my gauge to register pressure. So if there is say, 3 psi, then that says to me that the VP is not having to do any work in pulling fuel from the tank. As in if there is positive pressure at the inlet than the VP is not overdrawing the LP.



So what difference does it make to the VP44 if there is a positive pressure of 3 psi or a positive pressure of 9 psi at WOT ??



I would think that the only time you would run in to trouble is if your pressure at inlet fell to 0 psi or in to the negative.



If I am wrong or don't know enough about what the VP needs please let me know. TIA



Jeff

 

[RustyJC]

In your analysis, remember what the excess fuel at the VP44 is used for - lubrication and cooling. Without getting off onto the pressure versus flow bunny trail, although the VP44 may have enough fuel to run the engine at 1 PSIG inlet, how much fuel relative to design do you suppose is available for lubrication and cooling before it's returned to the fuel tank?



Rusty

 

[JPLB22]

I guess my next question would be...



If you have two identical trucks and they are running at WOT, if truck A has an inlet pressure of 9 psi and truck B has an inlet pressure of 3 psi, will truck A return more fuel to the tank than truck B ??



I would think that the fact that there is a build up of pressure at the only inlet to the VP means that the VP is using all the fuel that it can handle. Yes/No ????



Thanks.



Jeff

 

[RustyJC]

Originally posted by JPLB22

If you have two identical trucks and they are running at WOT, if truck A has an inlet pressure of 9 psi and truck B has an inlet pressure of 3 psi, will truck A return more fuel to the tank than truck B ??

Yes, all else being equal. Think of the system in very simple terms. Assume the tank is vented to atmosphere. Assume no pressure losses in the piping between the VP44 inlet and the tank on the VP44 return side. Assume the amount of fuel "swallowed" by the VP44 for fuel injection purposes is the same in both cases, so we can ignore it and treat the VP44 as an orifice (flow restriction) with X PSIG on the upstream side and 0 PSIG on the downstream side.



So long as we aren't at the choke or stonewall point of the orifice (basically, sonic flow velocity), increasing differential pressure (i. e. , upstream minus downstream pressure) across the orifice will cause flow through the orifice to increase. Think of a garden hose with one end open to atmosphere and the other end hooked to a variable pressure source - a faucet. More supply pressure equals more water flow at the hose outlet, right?



Since downstream (tank) pressure is constant at 0 PSIG, increases in supply pressure produce increasing differential pressure. Therefore, a truck with X = 9 PSIG at the inlet will flow more fuel through the VP44 for cooling and fueling and then return it to the tank than will a truck with X = 3 PSIG at the inlet, again all else being equal.



Rusty

 

[JPLB22]

Thank you RustyJC, I think you have shown me the error in my thinking.



I was thinking along the line that the VP was a unit that had a maximum GPH at inlet. And if you had positive pressure at inlet you were pushing more fuel than it could pass.



Just to make sure I have this strait, the higher the psi at inlet the higher the psi in the return line. Meaning more fuel passing through for cooling and lubricating purposes. Right ??



At what minimum psi level do you decide it's time for a new LP ( under WOT of course ) ??



Thanks



Jeff

 

[15w40]

I have been thinking about what fuel pressure is needed at the VP44 for a couple of years and have followed all the discussions on the forums. I will try not to step on anyone’s toes, but here is what I believe makes sense.



Bosch engineers claim that any pressure between -2 psi to about 19 psi will keep the VP44 happy. I am not sure I would be comfortable with the negative 2 psi.



The VP44 has its own lift pump built in. This pump is driven at ½ engine speed and is a positive displacement rotary vein pump. At idle it acts as a significant restriction to the electric lift pump and at idle, the pressure is about 15 psi at the VP44 inlet. Excess fuel inside the VP44 is both returned to the inlet supply and recycled back to the tank. As engine speed increases the VP44 allows more fuel volume to flow in and the fuel pressure from the lift pump drops off. At full engine speed the fuel volume is at a maximum to the VP44 and the lowest fuel pressure reading is seen. A good lift pump and a stock engine will give a pressure reading of about 8 psi.



Now if the engine is bombed and more fuel is injected, the internal VP lift pump flows even more fuel because the internal pump pressure drops and there is less fuel being re-circulated to the input. This re-circulated fuel enters the inlet by bypassing a spring-loaded spillback valve. In this case, the stock lift pump will show a lower pressure than if the engine was not bombed.



Cooling and lubrication of the VP44 is important to keep it healthy. At maximum RPM there is a constant flow of about 33 GPH back to the tank, assuming that there is a positive pressure at the input to the VP44. Problems may occur if the lift pump cant keep up with the flow needed by the injection pump. In my humble opinion a positive pressure at the input to the VP44 is all that is needed to keep it cool and lubed.

 

[rweis]

Ok,



Now add a twist, remove sulfur from the fuel as mandated by 2006.



So how is that "dry" fuel doing in the lubricity and cooling of the VP now?, oh, and don't forget the lubricity of the lp itself.



Everyone has an opinion, here's mine:



Pleanty of fuel at WOT, and dose every tank of fuel with a lubricity and water additive and GET THE WATER OOOOOUUUUUTTTTT!!!!



So far the central florida TDR club has gone to two major outlets that rebuild high pressure pumps (both the mechanical and the electronic). Same question. Same answer.



Interestingly enough, as the fuel is getting "dryer" they have a "need" to add a second shift to the high pressure pump rebuilding line. WHY?



We looked at pump after pump after pump after ... you get the idea. Water damage, water damage, heat damage, heat damage, water damage, heat damage ...



We are probably measuring the totally wrong thing for the high pressure pumps. Maybe we should monitor return fuel line fuel temperature, and high pressure pump inlet psi.



Their answer was the same: Pleanty of well lubricated water free fuel. How much is enough?, don't blow the seals in the high pressure pump.



They had a VP with a blown seal between the low pressure and high pressure section and when the two sections "equalized" the encounter was so violent that it BROKE OFF metal actuator levers attached to the punp shaft.



For me: a 10,000 hour MTBF pre lp lp, the OEM lp at the tank, 14 - 20 (I could go with 19) psi under ALL conditions monitored in two places, double water seperators, double 5 micron filters, fuel with lubricity and water removal additive in EVERY tank, and a spare LP in the tool kit with the tools to change it on the side of the road in the middle of the night.



My "opinion"



Bob Weis

 

[klaybus]

FP

Minimum of 7 PSI when mine was replaced under Cummins 100,000 mi. warranty back in December of 02. Now they have gone to a flow rate instead of PSI so I'm not sure.

When mine falls below 7 PSI in the future I will replace it. A LP is much CHEAPER than a VP44.



Clay

 

[A Johnson]

I'm in 15w40s' corner on this one.



I think the thing to remember is lower than normal lift pump pressure means the lift pump is dying and the question is when will it quit.



I believe condeming a lift pump for low pressure is as valid of a test as the volume test. On the other hand it's not worth arguing about .



A Johnson

 

[15w40]

If the model of a VP44 is a fixed size orifice then a higher fuel pressure will cause more flow through the pump and will provide more cooling by carrying away heat faster. The problem with this model is it is not backed up by flow measurements. Several years ago a flow meter was used to measure the rate of fuel flowing back to the tank with the pressure at the input to the VP varying. It remained at a constant flow rate as the pressure was changed at the input. The flow measurements were made at WOT.



This tells me that the fuel pressure to the input side of the VP will have no affect on pump cooling since the rate of fuel flow does not increase with higher pressure.



Fuel lubricating the pump is important. How does an increase in pressure to the VP help here? Adding a fuel additive may be the only answer.

 

[RustyJC]

Originally posted by 15w40

If the model of a VP44 is a fixed size orifice then a higher fuel pressure will cause more flow through the pump and will provide more cooling by carrying away heat faster. The problem with this model is it is not backed up by flow measurements. Several years ago a flow meter was used to measure the rate of fuel flowing back to the tank with the pressure at the input to the VP varying. It remained at a constant flow rate as the pressure was changed at the input. The flow measurements were made at WOT.

Agreed - I tried to explain the fluid dynamics of the relationship between pressure and flow using a model that may well be oversimplified since I do not have flow data nor internal schematics of the VP44.



I question, however, the implication that supply pressure is not important to the VP44. If that were the case, why was VP44 supply pressure the fuel transfer pump (or lift pump) acceptance/rejection criteria used by Dodge for years? As an aside, I personally suspect their recent change to a flow test may be more economically driven than technically driven.



Personally, as we say in Texas, I ain't got no dog in this fight. I'm still on my original lift pump that puts out 16+ PSIG at idle, 15 PSIG at cruise and 12-13 PSIG at WOT. Based on reported experiences of others on this forum and the apparent (although admittedly subjective) correlation of low supply pressure with accelerated rates of VP44 failures (measured flow rates notwithstanding), I'll be changing my LP long before VP44 supply pressure drops to 1 PSIG!



Rusty

 

[78repowerwagon]

I disagree. My truck on short bursts will pull down to 5psi with everything seeming ok. I went to the dragstrip and 5psi for 13-14 seconds made it lope at the end of the track like it was running out of fuel. My take on this is with a fuel box and medium injectors the small inlet hole on the side of the vp will not let enough fuel through at 3000+rpms with only 5psi.



I am running a race lift pump and was not aware I had lift pump problems untill it was loping, then I looked at my gauge on the next pass and verified my problem.

 

[RustyJC]

Originally posted by 78repowerwagon

I disagree.

With which position are you disagreeing?



Rusty

 

[JPLB22]

I agree with RustyJC in the fact that any changes that Dodge made to the testing procedure were likely economical.



15w40 seems to be thinking the same as I was. But then I thought of the fuel used for injection and the fuel used for lubrication as two seperate systems. Take the injected fuel out of the equation, because inlet pressure has no effect on amount of fuel injected ( unless VP is starved ). Now with positive pressure at inlet, that says to me that the VP is full of fuel according to volume. With increased pressure to the inlet, that would just circulate the full volume of fuel faster. But how much faster would make a difference ??



Is this thought way out in left field ??



Jeff

 

[isb360]

How about adding larger diameter orifice banjos coming out of the fuel filter? All other things being equal, you should expect to see a drop in pressure but an increase in volume. To this engineer's brain, increase in volume aids both lubrication and heat transfer rate which should help longevity of the VP pump.



Tim

 

[78repowerwagon]

Originally posted by RustyJC

With which position are you disagreeing?



Rusty

What I am saying is any positive pressure is not good enough. Mine is cavitating the VP44 with 5psi at inlet. In other words, if you could measure internal pressure it would be a vacuum.

 

[RustyJC]

OK, that's what I thought. I think we're both saying that a low VP44 supply pressure, even though it might still be positive at the injection pump inlet, is not a good thing.



Rusty