Engine/Transmission (1998.5 - 2002) Cavitation is NOT your VP-44's friend!

[Gary - K7GLD]

SO, Joe Blow has PSI problems with his 24 valve fuel system, and coughs up the $$$ for a replacement Lift Pump (LP)... BUT, to his total disgust, the NEW LP displays exactly the SAME PSI fluctuation problem the original did! It starts off fine, and delivers full PSI up around 15 PSI or so for a few seconds - THEN drops down to almost zero. It will come back up near full PSI if Joe does the WOT bit - but as soon as max PSI of about !5 or so returns, so does the erratic fluctuation.



DAMN, what could be wrong - especially with *2* different LP's - and ONE of them brand new? #@$%!



It's called C-A-V-I-T-A-T-I-O-N!



And I made up a short video to demonstrate it in action...



OK perhaps not as laboratory-like as some might insist upon - but here's a video example of cavitation - focus on the PSI in effect, not the method it was obtained - it all works out the same, whether the PSI is obtained while the pump is installed on the engine, or on a test bench...



Click here to watch Cavitation



It's worth mentioning that even with NO restriction related to clogging, kinked lines or others, these Carter pumps WILL cavitate at 15 PSI or so, it's just their nature and design - aggravated by use of diesel fuel as the liquid being pumped instead of the gasoline for which they were originally used.



In many cases. it's NOT an issue of the pump being poorly designed or defective, simply an issue of the pump being placed at the wrong location on the truck, and the wrong application - these pumps will last nearly forever if PROPERLY installed on a diesel rig, JUST AS they do on gasoline vehicles!



Even if the OEM system on our trucks are relatively free and open - the solution to cavitation is to either provide for better overall system flow, or provide a method to reduce overall system PSI to some point BELOW 15 PSI or so, such as a PSI regulator.



The pump used in my video demo has operated flawlessly on my truck for 50K miles - but the rest of my system has enough mods to not allow cavitation problems, such as relocating the LP down to the frame, back near the fuel tank - that same pump pictured will operate pretty much endlessly down at 12-13 PSI - but you can see what another 2 PSI causes to happen!



But to be totally fair where criticism of the Carter is concerned, just try to run a FASS, Holley or Walbro mounted up high on the engine like the OEM Carters are, and see how well THEY hold up over the long haul! :-laf

 

[klaybus]

Gary,

I just watched your video. Very interesting. I too relocated mine down to the frame & installed Vulcans 1\2" BIG line kit. I saw much improvement with that setup & was hard pressed to pull my pressures down but like many have seen, some LP's last a long time while others APPEAR to die an early death. In August of last year I installed the Walbro system from Glacier & thus far it has performed flawlessly. I know their (2) different animals but... . I don't know how long it will last but so far so good! It's only been 9 months but it's been promising so far. Time will tell.



Clay

 

[rweis]

I'm speechless, NICE video.



"Keep the flow GOING" and add a little piece of clear tubing to the line as it goes back to the tank so you can SEE if cavitation is happening.



Impressive, first "testing" video I have seen.



Bob Weis

 

[Gary - K7GLD]

I have received a followup PM in regards to my above LP test video - and the questions and comments it presents are worth sharing with the rest of the members:



=====

Hi Gary,



I posted a response to your video, but I wanted to get your thoughts on a couple more questions.



1. With a LP that starts out at 15 psi, goes into cavitation, pressure drops to 3 psi... what happens as you slowly release the restriction? Your video cut that part just a bit short, so I am wondering if the pump ever recovers to a smooth pumping state? I am thinking this would simulate going from idle to higher rpm where there is more flow required.

USUALLY the PSI will rise back to it's "normal" value when PSI/flow is restored - BUT, it's not uncommon for the cavitation air blockage to NOT allow restoration of PSI, and the pump must be completely shut down and restarted for normal operation to resume - in fact, that was EXACTLY what happened in my demo, and that's why there was a break between the first and second section of the cavitation demo - the pump would not resume proper operation when I removed the restriction, and I had to shut it down to continue the demo.

2. Where could there be a restriction on the output side of the LP if you are reading 15 psi on the output side of the fuel filter? From there it goes straight into the injector pump. I'm thinking that the cavitation you demonstrated on the bench may be for pumps that will actually reach 15 psi, but if you have a pump that tops out at say 13 psi, it will not cavitate. What I am saying is that maybe there is not a restriction on the output side, it's just that the pump is capable of 15 psi at idle, which in turn leads to cavitation. Bottom line is back to your point, the pump is not a good fit for this application.

The VP-44 itself is a "restriction" down at low RPM (idling) engine operation - and the actual PSI seen back at the LP can easily be at the threshold of cavitation under those conditions - especially if assembly tolerance of a specific LP bypass valve places it's bypass PSI at some value ABOVE 15 PSI. BUT, you are correct - if the pump will only see a PSI of 13 or so, danger of cavitation is reduced - too bad the bypass section of these LP's isn't adjustable!

3. What happens on your bench test when you install the "by-pass"? If you put the pressure gage after the by-pass, what is the initial downstream pressure and then I would love to see what happens to that pressure as you crank down on the clamp. I would expect that you would not see cavitation and the sudden drop from 15 psi down to 3 psi. My guess is that you may not see the 15 psi initially either.

If the operating PSI of the LP is reduced by ANY method to a value below it's natural cavitation point, it will not cavitate.

I have two pumps I want to test. One demonstrated exactly the same characteristics as your bench test. The second one came off my brother's truck and it was registering 0 psi at idle and negative pressure at higher rpm. New LPs were installed in both cases and the situation was remedied. I now realize that at least one of the pumps is probably still good, not sure what is up with the other one.

 

[BILLVO]

People tend to forget the basics. Nice reminder to all Gary.

Great way to explain it Gary.



Good info for everyone to see.



It is basic hydraulics or "Fluid Power" 101 that any and all techs should have had in college.



No matter what kind of pump or what kind of fluid any restriction on the inlet side of the pump will cause cavitation. Another good point Gary had is the type of pump also will contribute to cavitation.

 

[GHolcomb]

I did have Fluid 101 in college and the eye opener for me is just what Gary demonstrated... Discharge Cavitation. I must have missed that day!



Here is a nice definition:



Discharge cavitation - Output side is restricted

Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. (kind of like a blender) As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater (similar to what occurs in a venturi) which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.



More videos!



Greg

 

[BILLVO]

I think your thinking about water pumps,talking impellers ect. We are talking gear, piston and vane pumps. But cavitation in any pump is harmful to it as you are saying.



Closed loop is what we are dealing with, versus open loop systems as you are discribing if I read what your saying correctly.



"Delta pressure" is what was taught to understand cavitation in a sysyem on large hydraulic simulators. It was very similar to what Gary did in his video.



Don't forget cavitation also creates heat which we all know is a killer of the vp44 also.

 

[MBowman]

This discribes all my lift pump failures, but one problem. With a new LP everything would be fine for 6-12 months. Then it will cavitate once, and perform exactly as you have shown until finally replaced again. Why will I get 6 -12 months before seeing any cavitation, if cavitation is the problem?

 

[Bob V]

Sorry, the first test is NOT cavitation. It is either the internal relief valve opening with a bad ball and or seat. Or, most likely, a bad Vane sticking under a load. You can not cavitate a pump by restricting the output. Look up what cavitation is. . One other thing when was there the most flow? When the gage was at 10psi or when it was at 15psi? A pump does NOT make pressure, restriction does, the pump just the volume.

 

[GHolcomb]

I think your thinking about water pumps,talking impellers ect. We are talking gear, piston and vane pumps. But cavitation in any pump is harmful to it as you are saying.



Closed loop is what we are dealing with, versus open loop systems as you are discribing if I read what your saying correctly.



"Delta pressure" is what was taught to understand cavitation in a sysyem on large hydraulic simulators. It was very similar to what Gary did in his video.



Don't forget cavitation also creates heat which we all know is a killer of the vp44 also.

My point was to say that I learned from Gary's demonstration that you can have cavitation from restrictions on either side of the pump.



I always thought that cavitation was caused by a restricted input, but as Gary showed a restricted output and resulting high pressure causes cavitation as well.



Greg

 

[Bob V]

I did have Fluid 101 in college and the eye opener for me is just what Gary demonstrated... Discharge Cavitation. I must have missed that day!



Here is a nice definition:



Discharge cavitation - Output side is restricted

Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. (kind of like a blender) As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater (similar to what occurs in a venturi) which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.



More videos!



Greg

Remember that this is a vane pump with a internal reliefe valve, not a "blender" (cintrifical pump)

 

[GHolcomb]

Remember that this is a vane pump with a internal reliefe valve, not a "blender" (cintrifical pump)

A couple of questions here:



So in a vane pump, if there were no internal relief valve, the fluid would have nowhere to go under a restricted output situation and would stop the pump?



Second question... where is the internal relief located and how does it work?



Greg

 

[Bob V]

A couple of questions here:



So in a vane pump, if there were no internal relief valve, the fluid would have nowhere to go under a restricted output situation and would stop the pump?



Second question... where is the internal relief located and how does it work?



Greg

In theory it would stop the pump but reality there is enough internal leaks that it would probably keep running. The relief is a spring loaded ball in the outlet that is ported to the pump inlet

 

[GHolcomb]

Sorry, the first test is NOT cavitation. It is either the internal relief valve opening with a bad ball and or seat. Or, most likely, a bad Vane sticking under a load. You can not cavitate a pump by restricting the output. Look up what cavitation is. . One other thing when was there the most flow? When the gage was at 10psi or when it was at 15psi? A pump does NOT make pressure, restriction does, the pump just the volume.

In Gary's video, why would you think the ball or seat is bad in the relief valve? What effect would that have? Or in other words, how would a relief valve behave in this situation with a good ball and seat?



By a sticking vane, do you mean that a vane may not be fully extending and therefore letting some fluid pass by?



Greg

 

[Bob V]

If the relief is sticky and once it opens it doesn't fully close while it is bypassing.

The only thing the move the vanes out in these pumps is centrifugal force (and these are not big vanes). As the vanes and lands wear, and they are loaded, the vanes can stick in the retracted position. Think about it. If the output restriction is causing cavitation, how does moving the pump close to the tank help? If the pump was Cavitating why did the motor speed up?

 

[JKinney]

As posted in these forums I have had my share of problems with lift pumps, but today I had a whole bunch more. I have read on here that people have blown air into the tank and have noticed improvements in flow. So I tried it, and learned thats about the dumbest thing Ive ever done. It blew the whole tank filter apart causing no fuel flow below 3/8s of a tank. I just pulled the tank, took the sender out and put it back together. I also did snap the cloth filter off the bottom and cleaned it and got it back on, and yes it was a little dirty but plugged.

 

[NoSeeUm]

Thanks allot Gary, very well done. I had 2 Carter Lp's do that exactly same thing. Run fine for a few seconds then pop down to 5 pounds. The second one would at times flunctuate between 5 and 15 psi.

Sorry, the first test is NOT cavitation. It is either the internal relief valve opening with a bad ball and or seat. Or, most likely, a bad Vane sticking under a load. You can not cavitate a pump by restricting the output. Look up what cavitation is. . One other thing when was there the most flow? When the gage was at 10psi or when it was at 15psi? A pump does NOT make pressure, restriction does, the pump just the volume.

I am more or less with Bob here. My gut tells me this is right. In the video it sounds like the RPM goes up when the pressure drops to 5 psi, either that or the pump is just loading up with the pressure control valve full open. I can't tell. It definetly sounds like the pump gets air bound, which is not really cavitation.



Have you considered doing an amp draw test under the same conditions?



Jim

 

[Misenheimer]

If the pump was Caveatting why did the motor speed up?



The pump will speed up when it cavitates because it can do no work and the motor runs free. Put your hand over the suction side of a vacuum cleaner and it will speed up. These pumps are low level vane pumps and just a step above a centrifugal and cavitates from restriction on either side.



My question now is the pump on my 1999 2500 lost pressure after a few minutes running then if it was stopped it would pump again for a few minutes and come and go. I ran it with a regulated power supply for an hour on the truck and it ran without a problem (fuel got warm after an hour) but when the truck supplied the power it would cavitate. When I start the truck the battery voltage is low from pre heat and cranking and the pump will run as long as the heaters are pulling the battery down but when the alternator gets the battery's up to the 13. 7 - 14. 2 range the pump drops out. Is the higher voltage increasing the pump speed and pressure to the point of cavitation in an old pump? Will a new pump cavitate at a higher pressure then as it wears the point of cavitation comes down?



Just a thought

Robert

 

[BILLVO]

Put your hand over the suction side of a vacuum cleaner and it will speed up.



Robert

This is another good point here. Many have argued is it working harder or not as hard. Awnser it is not working at all because there is no air to move thats why it speeds up. This is why a lot of turbos fail when the air filter gets really plugged "We see lots of this at the coal mine I work at" it overspeeds the turbo and shafts break and wheels come apart.

 

[Misenheimer]

Remember that this is a vane pump with a internal relief valve, not a "blender" (centrifugal pump)

I just took an old pump apart and there is no internal relief valve in the lift pump. You can see the impeller from both sides, inlet and outlet. The only relief valve is in the return line from the VP44 and it has a small hole to circulate fuel drilled into the side of it without it being popped off.



Gary's demonstration was valid



Could the pump be made to cavitate easier? sooner? with less restriction with higher voltage? 15v Increased rpm?