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Cavitation Is Your VP-44's ENEMY!!

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The following was originally posted by Gary - K7GLD


This info is probably "old news" to some of the veteran members here who also are active on the TDR group - but other Dodge/Cummins owners might find it useful. Responses in other threads tend to get hung up on the reference to "cavitation", and it's definition - don't get all hung up on that - stay focused on the real issue, SEPARATION of vapors from the fuel, and it's effects on the LP's ability to function properly!


LP Cavitation:

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 (Link No longer works-11/19/08)

It's worth mentioning that even with NO restriction related to clogging, kinked lines or others, these Carter pumps WILL cavitate at about 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! :eek:

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!


HOOKAYYyy ‑ some suggestions ‑ and a few criticisms and disagreements on "correct terminology" ‑ the most reasonable request was for another video displaying the actual effect of raising the fuel source up at the same level, or above the pump ‑ will it still cavitate? Let's see:

Click here to watch cavitation‑‑‑again (Link no longer works 11/19/08)

There are many Internet source discussions on what cavitation is ‑ what causes it, and how to avoid it ‑ here's a quote from one:

Cavitation means that cavities or bubbles are forming in the liquid that we're pumping. These cavities form at the low pressure or suction side of the pump, causing several things to happen all at once:

We experience a loss in capacity.

The pump can no longer build the same head (pressure)

The pump's efficiency drops.

The cavities form for five basic reasons and it's common practice to lump all of them into the general classification of cavitation. This is an error because we'll learn that to correct each of these conditions, we must understand why they occur and how to fix them. Here they are in no particular order :


Air ingestion (Not really cavitation, but has similar symptoms)

Internal recirculation

Flow turbulence

The Vane Passing Syndrome

Vaporization .

A fluid vaporizes when its pressure becomes too low, or its temperature too high. All centrifugal pumps have a required head (pressure) at the suction side of the pump to prevent this vaporization. This head requirement is supplied to us by the pump manufacturer and is calculated with the assumption that fresh water at 68 degrees Fahrenheit (Twenty degrees Centigrade) is the fluid being pumped.

Since there are losses in the piping leading from the source to the suction of the pump, we must determine the head after these losses are calculated. Another way to say this is that a Net Positive Suction Head is Required (N.P.S.H.R.) to prevent the fluid from vaporizing.

We take the Net Positive Suction Head Available (N.P.S.H.A.) subtract the Vapor Pressure of the product we are pumping, and this number must be equal to or greater than the Net Positive Suction Head Required.

To cure vaporization problems you must either increase the suction head, lower the fluid temperature, or decrease the N.P.S.H. Required. We shall look at each possibility:

Increase the suction head

Raise the liquid level in the tank

Raise the tank

Pressurize the tank

Place the pump in a pit

Reduce the piping losses. These losses occur for a variety of reasons that include :

The system was designed incorrectly. There are too many fittings and/or the piping is too small in diameter.

A pipe liner has collapsed.

Solids have built up on the inside of the pipe.

The suction pipe collapsed when it was run over by a heavy vehicle.

A suction strainer is clogged.

Be sure the tank vent is open and not obstructed. Vents can freeze in cold weather

Something is stuck in the pipe, It either formed there, or was left during the last time the system was opened . Maybe a check valve is broken and the seat is stuck in the pipe.

The inside of the pipe, or a fitting has corroded.

A bigger pump has been installed and the existing system has too much loss for the increased capacity.

A globe valve was used to replace a gate valve.

A heating jacket has frozen and collapsed the pipe.

A gasket is protruding into the piping.

The pump speed has increased.

Install a booster pump

See any there that might apply to our truck's fuel system?

Hopefully SOME of the more open minded guys reading and viewing what I have provided can benefit from it, and put it to good use!

AGAIN, the intended point is NOT what to call it ‑ but how to AVOID it!


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