Engine/Transmission (1998.5 - 2002) Don't Understand - LP Pressures

[rblomquist]

Ok guys, can someone explain to me why low LP pressure is bad for the VP44?



As a layperson, I don't understand why 1 psi would be hard on the injector pump - it's still positive pressure going to the VP44. In other words, the VP44 is not having to "pull" fuel to it if there's a positive pressure up to it.



Thanks in advance.

 

[bmoeller]

Lower pressure = lower flow



The lift pump not only supplys fuel for combustion, but the fuel also lubes and cools the VP-44. At 1psi, the VP is still probably pulling some of its own fuel to compansate.

 

[2ndgen2500]

as he said, diesel is used as a lubricant in the vp, therefore if it is getting less, then burning up the pump, it also has to pull the fuel from the tank.

 

[Sticks]

I doubt that it is pulling any fuel from the tank at 1psi (as you said, still a positive pressure) but it sure as heck is not returning any fuel, therefore NO LUBRICATION. This is when the VP44 gets hot and starts to wear out items inside that were not meant to get worn.

 

[RBallard]

Here's my take on the whole situation. As everyone has stated, the VP is looking for flow to keep it cool. Do you see anyone selling a flow meter for our trucks? The pressure gages are easier to come by than flow meters and with a known orifice size, one can determine the flow rate through the orifice by knowing the k-factor of the particular orifice. I am sure that the k-factor for the banjo bolt has been determined, not sure if it is published, and based on the minimum flow requirements of the VP a minimum pressure has been set. Although 1 psi is positive pressure it is not providing enough flow.



I replaced my banjo bolt leading into the VP with the enlarged tapped version. Since the holes are enlarged, for the same given flow, the pressure will be reduced slightly. I will have to look around for the original bolts and conduct some tests to determine it's k-factor.





k-factor = Flow/Sqrt(Pressure)

 

[Gary - K7GLD]

In addition to the above responses - and maybe even more important, is that such low PSI on a relatively unmodded truck is a no-doubt-about-it sign of serious problems in a fuel supply - anything from clogged fuel filter, poor power connection or failing LP - consider low PSI a slap in the face wakeup call that a serious problem might be in your future!

 

[ntillm01]

I'd like to see someone measure the cooling that takes place. Like a temp gauge at the inlet, and another at the outlet downstream from the pop-off valve.



Somewhere in this forum someone measured return flow with bad pump, new pump, pusher pump, etc. I found it interesting that at higher loads/RPM, the return flow was the same no matter how much lift pump pressure/flow you had. Makes me reason that the low-side pump built in the VP44 is doing most of the cooling flow in those cases. The lift pump seems to cover the idle/part throttle driveability issues and cooling.



The rub is that when it quits, either mechnically or electrically, it then becomes a restriction in the system, which becomes a problem. Would be better off to have no lift pump at all when that happens.



My 0. 03

 

[Gary - K7GLD]

"I found it interesting that at higher loads/RPM, the return flow was the same no matter how much lift pump pressure/flow you had. "



That was true at MAXIMUM loads and decent fuel PSI (which was the main focus of that test), but unsure as to the effect of BYPASS fuel volume as VP-44 INLET PSI is reduced down near zero...

 

[15w40]

I run 5 psi pressure at the inlet to the VP44 at idle, WOT, empty, loaded, hot or cold. I guess I belong to the faction of members that believe any positive pressure is enough to keep the VP happy. Others insist that more pressure (up to 25 or so) is better because it helps cooling and has shown to increase power at levels above 500 HP. I don’t buy this and have been “slapped up the side of the head” for saying so. Well, I am not making 500 HP, probably somewhere around 400 with an early ‘98 VP with a tapped pump wire. My VP44 is still kicking and seems happy.



Below is a picture of the inlet structure of the VP44. The large arrow shows where the fuel enters, coming from the lift pump. The internal VP44 lift pump is there to pressurize the injection pump before the HIGH-pressure pump. If you look at the fuel passages, the smaller arrows indicate the direction of fuel flow. Notice that the only flow “IN” comes from the lift pump and the other passages are returning HIGH-pressure flow from the HIGH-pressure pump. I am talking about 17,000 PSI, give or take. The VP44 is designed to operate without a lift pump as long as it inlet is not restricted, creating a negative pressure.



Just my $0. 02.

 

[Gary - K7GLD]

I wouldn't be particularly concerned with VP-44 PSI down as low as 5 PSI or so - as long as it went no lower as measured right at the VP-44 inlet.



Higher PSI and efforts to greatly increase fuel flow and PSI by use of large fuel lines and fittings are largely overkill in relatively stock situations in my opinion, and inspection of the restrictions to flow of the VP-44 inlet passages *themselves* provide some verification that EXTERNAL large fuel lines will merely end at the VP-44 bottleneck at the end of their run.



*I* only run higher PSI because it's what I ended up with after installing my pusher pump...

 

[rblomquist]

Very interesting back and forth here. I'm soaking it all in but I still don't see how if you have positive pressure on the inlet side no matter what that pressure is the VP-44 is not having to "pull" fuel; it's still being fed.

 

[Lsfarm]

VP44 Cooling

I helped a fellow TDR member remove his fuel tank and rig up a temporary fuel tank inside the bed of his truck.



We used a five gallon container as a fuel tank and rigged up a supply and return line. what we learned was just how much heat is being pulled from the VP pump.



He took the truck for a ten mile drive, we measure the temperature of the fuel before and after the drive. The fuel started out at 55*F, and the fuel was at 105* when the truck came back. . So there is a lot of heat in the VP44 and the excess fuel is used to lubricate and cool the VP. Ambient temperature was 55*.



If you want to gamble with a minimum fuel supply to the VP, that's OK, it's your truck. But if the dealer test values are 10-12 psi idle, and minumum 8-10 psi at WOT, then I'm going to supply my VP at least that amount of pressure, to ensure at least the factory minimum fuel supply for cooling and lubrication.



JMHO, Greg L The Noise Nazi

 

[15w40]

I think Lsfarm has zeroed in on the question of how much pressure is needed to cool the injection pump. I am open for answers to this, but I don’t see how increasing the lift pump fuel pressure can flow more cooling fuel through the VP44. The internal VP lift pump is a positive displacement pump which will accept only as much fuel as it can displace, and that is dependant on engine RPM. My thinking is the flow rate at 2000 engine RPM into the VP44 with a lift pump pressure of 1 PSI is identical to the flow rate at 2000 engine RPM with a lift pump pressure of 50 PSI. If the displacement is the same with varying lift pump pressures, the cooling will be identical. Can anyone tell me where I am wrong?

 

[lrox]

How can I increase the amount of fuel pressure going to my vp44

 

[Sticks]

Originally posted by 15w40

I think Lsfarm has zeroed in on the question of how much pressure is needed to cool the injection pump. I am open for answers to this, but I don’t see how increasing the lift pump fuel pressure can flow more cooling fuel through the VP44. The internal VP lift pump is a positive displacement pump which will accept only as much fuel as it can displace, and that is dependant on engine RPM. My thinking is the flow rate at 2000 engine RPM into the VP44 with a lift pump pressure of 1 PSI is identical to the flow rate at 2000 engine RPM with a lift pump pressure of 50 PSI. If the displacement is the same with varying lift pump pressures, the cooling will be identical. Can anyone tell me where I am wrong?

I have seen no verifiable info on the fuel return line other than temperature (another issue that I fell is bad on the VP44 - Hot fuel - will explain next). If the VP44 only takes in as much as it can process at a given RPM, and only spits back to the return line a constant amount at the same RPM, then the LP is basically churning the fuel (building heat, and making froth) since the VP is not moving all that it is fed. I was under the impression that the excess fuel and pressure was run through the VP for cooling and lube then shot back to the tank. I. E. If the LP is feeding the VP at a rate of 5 gph, and the engine consumes the fuel at a rate of 1 gph, then the other 4 gph should be flowing back to the tank. Now if the LP flows at 20 gph, and the VP still at 1, then 19 gph should be flowing back to the tank. If this is the case then the higher pressure - more flow - would be better than less.



HEAT - Last Fall I was in the throws of the dreaded 0216 code and occasional other symptoms, on a replacement LP. The only time the code would set is when the ambient temperatures were above 90*F, or I drove the truck for more than 1 hour straight. This gave me verifiable data that the VP does not like heat, or hot food, and I am anxious for this coming summer to see if I have the same problems on the replacement VP. I have also heard from several members on this site that the hotter climate areas replace multiple times more VP44s than cooler climates - Sucks to live in AZ, TX, and other desert areas.

 

[slo-ryde]

Just recently I had the good fortune to talk to one of the Cummins tech support guys about a lot of different things. It must've been a real quiet day because we chatted for a long time. I tried to ask every question I could think of about the Cummins motor.



On the subject of the VP44, a bad lift pump is not a good thing but there are a whole bunch of these trucks running around with lift pumps that have been dead for a very long time. Most non-TDR owners have almost no clue when it comes to these things.



Things that will rip out the heart of a VP44 are water and air. Air in the pump will allow metal to metal contact, most definitely a bad thing. Water has a similiar affect but does the most damage to the injectors. The tech was saying the water eats away at the orifice in the injector. I've been told that by the time the water in fuel light comes on you're pretty well screwed. The high pressures in the VP44 cause the water to become superheated and vaporize, leaving no lubrication (back to the metal on metal problem).



What percentage of non-TDR CTD owners do you think drain the fuel canister at regular intervals?

 

[rblomquist]

Originally posted by 15w40

I don’t see how increasing the lift pump fuel pressure can flow more cooling fuel through the VP44. The internal VP lift pump is a positive displacement pump which will accept only as much fuel as it can displace, and that is dependant on engine RPM. My thinking is the flow rate at 2000 engine RPM into the VP44 with a lift pump pressure of 1 PSI is identical to the flow rate at 2000 engine RPM with a lift pump pressure of 50 PSI. If the displacement is the same with varying lift pump pressures, the cooling will be identical. Can anyone tell me where I am wrong?

Originally posted by slo-ryde

On the subject of the VP44, a bad lift pump is not a good thing but there are a whole bunch of these trucks running around with lift pumps that have been dead for a very long time. Most non-TDR owners have almost no clue when it comes to these things.

I was just thinking of both the above mentioned items before I read them. My old neighbor had a 99 except his was a standard trans. His LP was dead for at least 2 years I know of before I finally forced him to hook up to my briar hopper test set. His original VP is still working fine and that's with the vegas heat and all. Now, I don't suggest the LP is non-needed item but you gotta wonder how many thousands of CTDs are running around the country with dead LPs and the owners are no smarter for it.

 

[Gary - K7GLD]

In regards to heated fuel returned to the tank - ask yourself where ELSE heat might be transferred to the fuel besides the VP-44...



There ARE other sources...

 

[tgbol]

As it was explained to me another function of the inlet fuel pressure is timing. Control of timing advance is done by an electric valve that lets the inlet pressure rotate the advance curve. This is one of the reasons of the 0216 code being caused by low fuel pressure. In my case I plugged a fuel filter=no advance, set code.

 

[NevadaBackroader]

Sounds like the real problem is hot fuel being fed to the VP44. My tank seems to always have 5 gallons of fuel left when the low fuel light comes on. There may be some corelation to always driving with the tank more than 1/2 full and long VP44 life. Seems to me the more fuel in the tank the more it acts as a heat sink and the cooler the fuel will be going to the engine. I think I'll start making it a point to always refuel at 1/4 tank. That is, when I finally get back form this little adventure.