Answer to lift pump and injector pump pressure riddle

[WMann]

I did a search under user name HVAC for lift pump and did alot of reading which has changed my opinion on how the system really works.

Ron Duncan posted some information he got from the shop manual which got me to thinking. He also asked this question: It doesn't make sense that the lift pump puts out 10 psi and the injector pump overflow valve opens at 14 psi and 70% of the fuel is supposed to be returned to the tank? Can anyone explain this?

I think I can answer this. First, the lift pump is not opening the overflow valve. It doesn't have the required pressure. The injector pump is the pump that is delivering the pressure required to open the valve. Now in english. Lets say (for example) the lift pump is providing 10 psi of pressure. It is moving a large amount of fuel through the fuel lines in order to achieve 10 psi. The injector pump however is pumping the fuel through much smaller passage ways inside the injection unit, creating much higher pressures. Therefore opening the overflow valve and returning roughly 70 percent of the fuel back to the fuel tank.

Remember PUMPS do NOT create pressure, RESTRICTION does. Example: when you have a dirty fuel filter, the pressure on the fuel tank side of the filter is higher than on the injector pump side of the filter, because of the restriction in the fuel filter.

The manual says that the VOLUME of the transfer pump (lift pump) will always provide more fuel than the fuel injection pump requires.

If this is correct then a pressure guage placed just before the injector pump that is reading only 1 psi under any load condition tells me that the lift pump is providing more fuel than the injector pump can use, because only 1 psi means the injector pump is under positive pressure from the lift pump.

If you are seeing a negative pressure before the injector pump in any load or throttle condition, then the injector pump is actually sucking on the lift pump trying to get more fuel, which I think we all know is a life threating enviroment for the injector pump to operate in.

Now I have a question. How can we find out the pressure comming from the injector pump? We know it is at least 14 psi if it is keeping the overflow valve open and returning 70% of the fuel back to the fuel tank. Also by returning this fuel and keeping the overflow valve open it is performing the task of cooling and lubricating the injector pump. Actually on second thought this is probably not something we even need to know.

Now we just need a more reliable pump that will copy the exact performance of a new properly functioning lift pump. That shouldn't be too hard to find, should it?

PS. With this new found belief in the way the system works, my previous post about using a fuel pressure regulator before the injector pump is totally wacked. Please dissreguard my previous transmission.

Wayne

 

[B.G. Smith]

I, for one am glad to see you exploring this subject. It looks like if HVAC has his fuel transfer pump regulated at 10psi that the return valve would never open/no cooling. The factory manual (99) doesn,t shed much light on the subject. Hang in there and good luck. bg

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White 99 3500, QC, 3:55, auto. most options except leather, Line-X bed liner, fiberglass running boards Rancho 9000s, bug shield and window vents, black vented 5th wheel tailgate, 100 gal. aux. fuel tank.

 

[sport bike]

Helipolit,

You right except for the pressure required for the injection pump to function properly. You can get a high rpm stumble if fuel pressure drops below 4psi. This is followed by a bad stumble or stalling of the truck when you let off of the throttle. I think what is happening is that your using most of the fuel to run the engine and not enough is used to cool the pump. This causes the pump to get hot and start to aerate the fuel when you lift. If you ever had your truck stall after a hard run,it,s hard to start because there is air in the lines. It,s like you ran out of fuel. The best way to fix this problem is to keep fuel pressure at or above 8psi.

John/sp

[This message has been edited by sport bike (edited 01-18-2001). ]

 

[Dane]

Good discussion!
One point about measuring fuel pressure and the venturi effect. If the gauge is installed in the path of fuel flow the pressure measurement may not be accurate due to the venturi effect. This was kicked around a bit in the PrimeLoc thread a while back.

Sport bike - I've experienced the fuel starvation you described - engine stumbles and dies at higher rpm when you back off the throttle in the higher gears. It was on a 1100 Honda. The problem was indeed fuel starvation as the cause was a plugged fuel filter.

Can some one tell my why just installing an additional fuel pump at the fuel tank would not be helpfull?

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2001 HO 6 speed Regular Cab SLT 4x4 3. 54 anti spin 2500. Used for the daily grind and sneaking away to some secret Baja beaches toting a cabover

 

[Ron Duncan]

Heyguys,
The lift pump actually supplys fuel to the injector pump unit while the engine is running at low rpms. Once the engine starts reving up a gear pump in the injector pump unit takes over and supplys a larger amount of fuel to the high pressure injector pump. The excess from this GEAR pump does the cooling and is returned through a 14# relief valve. Remember, there are 3 pumps.
1. The lift pump to get fuel to the injector pump unit at low rpms.
2. The gear pump connecteed to the injector pump drive shaft starts working at higher rpms. When this pump is sucking the lift pump doesn't do so much. The pressure between the gear and lift pump is no longer so important as the gear pump can suck more than the lift can supply.
3. The high pressure injector pump that actually feeds the injectors, a not so large amount of fuel (unless you are HVAC and others) feed the engine.
Hoe this makes sense!
Ron

 

[WMann]

If to low a pressure from the lift pump can cause this stumble at high rpm's, then I think my previous thoughts about having a higher pressure lift pump and a regulator set at ten psi or so right before the injection pump is not such a bad idea after all.

Lets see if I can give an example of what you are saying in order to make it easy to understand for everyone.

Lets say that the truck is running down the highway at 80 mph under a steady constant load.

In the first example the lift pump is providing 10 gallons of fuel per minute at 10 psi. The injection pump is feeding the engine 3 gallons of fuel per minute to maintain this speed. That leaves 7 gallons per minute to bypass through the overflow valve, cooling and lubricating the injection pump.

In the next example the lift pump is providing only 4 gallons of fuel per minute at 4 psi. The engine is still requiring 3 gallons of fuel per minute to maintain this speed, so this only leaves 1 gallon of fuel per minute to cool and lubricate the injection pump.

Using this simple formula its easy to see how low output pressures from the lift pump can starve the injection pump of the excess fuel it needs for cooling and lubricating.

Thinking about this stuff can drive you crazy.

Has anyone tried contacting the manufactor of the injection pump and getting detailed information about how the thing really works and what would be the perfect setup for providing fuel to the pump if money were no object? They might give us some very usefull information.

Thanks for the input.

Wayne

 

[WMann]

Hi Ron. thanks for the reply.

We use a hand held gear pump with a rotating handle to pump fuel into our R/C Helicopters. If that pump handle is being turned, no matter how slow, it is pumping fuel. The faster you turn the handle, the more fuel it pumps. Also as a side note, if we use larger fuel line on both sides of the pump, it pumps a much larger amount of fuel when the pump handle is being turned at the same speed as with the small line. Also when using the smaller fuel tubing on both sides of the pump, if you try and spin the pump too fast, it will actually cavitate causing the fuel flow to slow down. When this happens it is telling me that there is too much restriction in the lines to supply enough fuel to keep up with the pump. So it has no choice but to cavitate since it is being driven too fast for the amount of fuel that is available.

I would assume that the gear pump inside the injection pump is is being driven mechanically, which means if the engine is running, no matter how slow, that pump is pumping fuel.

I sure would like to know from the people that designed this system what the chain of command is regarding the three fuel pumps. Meaning who is suppose to be feeding who, what, where and why.

I hope we all are able to look back on all these discussions ten years from now and think about how much fun we had working out all of these little annoying problems with our trucks.

Wayne

 

[B.G. Smith]

Hey Ron, this is the first time I have heard mention of a gear pump in the injection pump? Are you talking about the VP44? It sure would be nice if we could get an exploded view(no pun intended)of the pump. I e-mailed a guy tonight who sent me some info on the VP44 early last year. If I get anything new I will let you know. bg

 

[sport bike]

Ron,

I dissagree with you. If you have a truck thats running low fuel pressure,It will hurt performance. When I first started to bomb my truck everything was fine,then I hit a point where I started to get a miss at high rpm. I decided to check my fuel pressure under load. I found out that it would drop to 0psi. As the pressure was falling below 4psi the boost would start dropping and you could fell the power loss and stumble starting. So I came up with the idea of upgrading my fuel lines. That took care of the stumble and power loss. As I have increased power levels,I have to keep updating the fuel supply to keep the truck from stumbling. Every truck is different. Some seem to be fine and others have weak lift pumps and need help. All high hp ISB,s need to do some kind of upgrade on there fuel system to make that kind of hp and live. Your truck souldn,t have any problems with fuel delivery with just injectors in it. The clip on boxes are the hardest on the pump because they ask the pump to work harder. As long as you keep the fuel supply up,there is no problems. I think the VP-44 is very durable and that there is still more potential for hp in it.

 

[Ron Duncan]

Sport Bike,
I'm in agreement with you. I think it depends on the truck and the method of bombing. I have always been scared of the boxes due to the extra strain they put on the injector pump. That's why I went the injector route.
Lots of guys seem to be happy with just canning the banjoes bolts or drilling them out.
I remember Nowell (with a zillion horse power and every enhancement he could get) went to, I believe, a big Holly pump. He saw improvement on the dyno! Proof is in the pudding, as they say.
I'm not trying to say stock is for everyone, I'm more interested in understanding how the system works.
I also don't think us mildly modified guys have to stay awake at night worrying about frying our V44, but bigger is better, and I may spring a few dollors in the future just to be safe myself! That is if anything is left after building my Jeep, ha ha...
Ron

 

[sport bike]

Ron,

If your not having a fuel pressure problem,then there is no need to upgrade.


John/SP

 

[Bart Timothy]

I've seen the question asked about an extra inline pump added to the system a couple of times but haven't seen any good answers yet. This seems like an easy no brainer fix to fuel pressure problems to me. Plus it would give extra reliabilty of an extra pump should the main lift pump fail.

The only problems I can see with this fix is, as I understand it, the lift pump operates at 2 different pressures - 8 lbs at cranking and 10 psi while running (??). I think someone said this was done to keep from flooding the engine while starting?

 

[Ron Duncan]

Yes the gear pump is in the VP44.
This stuff was kicked around a bunch about 8 months ago. We were all in a panic our injector pumps were going to fry due to the crappy lift pump pressures.
I talked to Lawrence at Diesel Dynamics about the problem and he kinda laughed and showed me the secret (at that time) gear pump in a dissasembled V44. He said he had never heard of a V44 failure with the level of BOMBing being done at that time. He also agreed research should be done.
I also talked to the major "hot box" folks at May Madness. They said they also had no problems with the stock pump but some folks were adding an extra inline pump just to be sure.
There are two threads on this subject going at the same time (what's new).
In the other thread I asked a question:
If one pump is supplying 10# pressure into a manifold and another bigger pump is sucking from that manifold, what would the manifold pressure be.
I have always maintined the true test would be to monitor the pressure in the VP44 body where the 14# return valve is located or to somehow monitor the excess fuel being returned to the tank. The outfit that makes the add on fuel filter gizmo did some good testing after being accused of restricting fuel flows. You might find the thread using search features.
I quit worrying about it and tow my 30' fiver just fine... .....

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'99 Quad long bed, 4x4, 5 speed, 3. 55 limslip, DD stage IIs/boost module and elbow, US Gear D-brake, Mag-Hytec, 4" exhaust, gauges, stock filter, air bags and a bunch of other stuff. Pull a 30' Wildwood
fifth wheel.