TheSamba.com :: View topic - Fuel Pump cavitation (solution is close, see pics)

Thu Apr 16, 2015 8:35 am

I didn't want to hijack this recent thread from
http://www.thesamba.com/vw/forum/viewtopic.php?t=619556 from BlueGrasser dealing with his loud fuel pump but I've had a similar noise problem with two different bosch fuel pumps, both noisy presumably from the cavitation caused by going from
Narrow outlet fuel tank -> plastic pre-filter -> fuel pump -> canister filter

For a variety of reasons the plastic pre-filter sits between tank and pump:
1.) it functions as an adapter of sorts going from 7mm (or so) tank nipple to 11mm (or so) fuel pump nipple
2.) it keeps crud out of the fuel pump

I read somewhere on thesamba about the suggestion of putting a longer piece of wider diameter hose between the pre-filter and pump and looping it up with some zip ties. So today I tried that.

Note here, that the pre-filter is new. Changing out the pre-filter does not seem to change the level of noise from the pump.

When I first installed this new pump I used only a few inches of 11.1mm hose, today I replaced that with 3' of coiled up hose.

original short length of hose:

When I first started the van after replacing with longer length the pump was almost completely silent and i thought I'd solved the cavitation problem but after turning it off again and turning it back on the cavitation was back, but intermittently.

longer length of hose:

This leads me to think that perhaps an even longer length of hose might solve the problem but this is a fluid dynamics type of engineering problem that is way out of my league (figuring out how long of a hose you'd need and whether or not it could be secured in such a way as to not be in the way and obtrusive.)

Thu Apr 16, 2015 8:54 am

I would bet this would solve all of these cavitation problems. You plumb the FI return in to it and then on to the tank return. This keeps a good supply of fuel on top of the pump inlet.

Thu Apr 16, 2015 9:43 am

Just go ahead and ditch the white filter if you have the big can filter after the pump. You can put it back later if you really think you need it, but the later vans didn't have one before the pump and it didn't seem to adversely affect the pump that I've heard.

Thu Apr 16, 2015 9:47 am

Your still drawing fuel through a 7mm straw with the prefilter removed. The pump is capable of drawing more fuel than the small outlet on the tank can provide. The noise is from cavitation. In other words the pump is starving for fuel.

Thu Apr 16, 2015 9:54 am

I think you're missing the solution...just kick it!

Thu Apr 16, 2015 10:08 am

OK, so it looks like the 11mm hose length is irrelevant according to a Facebook buddy of mine who it turns out is an engineer of the fluid dynamics sort!

Here are some of his notes from our Facebook thread:

"The best solution if you really are having cavitation and the pump is the tight pump and it’s working properly…
Install a second fuel line from the tank to the inlet of the pump.
***Even if you use two 7mm fuel lines you won’t have enough cross sectional area.
You need at least a 7mm hose and a 9 mm hose going into the fuel pump.
I have not done a headloss calculation but off the napkin you need at least 95 square mm to equal the area of an 11 mm hose."

"also that 3' of [11mm] hose is acting like a little storage reservoir. The reason why your cavitation seems to decrease is because you have likely increased the amount of time you can hold the throttle down before the 11 mm line empties. Once it empties, though - cavitation."

as for making the 7mm line IN FRONT of the pre-filter (to the tank) as short as possible:

"Every additional inch of hose translates to more loss which takes more energy to move. Think of the extreme, if you take a drinking straw that is 8 inches long vs the same size drinking straw that is 30 feet long, you will have to do a lot more work to get soda coming out of the 30 foot straw. Cavitation happens on the Suction side of the pump, not the pressure side."

So, i'm going to go back to my shorty 11mm hose and shorten the 7mm up as best I can and see if that improves things at all.

Also, as for kicking the ball, i could also just leave the van at home and take the sweet black Cozy Coupe truck that's in the photo, too :lol:

Thu Apr 16, 2015 10:43 am

If your plan does not yield the result, I would try the part I showed above. It came as standard equipment on the VW Fox, Mid 80s Cabriolets, and Sciroccos of that era. You would install it in place of the filter you have, route the return from the engine, to the upper fitting, with the blue ring and then out the top to the return on the tank. Big end down. The system flows about a quart every 30 seconds, so by tying in the return, there is always plenty of fuel for the pump.

Thu Apr 16, 2015 11:59 am

OK, so based upon the information i got from my engineer friend I:
- dropped the pre-filter
- inserted a 1/4" hose into the 11.1mm hose and secured with a clamp (used some WD40 to get it in there. seems to be a secure connection but would appreciate any input
- used the shortest possible length of narrow hose and joined up to the fuel pump with no extra loops.

just went for a test drive. the violent cavitation noise is now gone but it is still noisy (sounds more like a light whine or static) so I'm guessing that the narrow outlet on the tank is still going to starve the pump a bit no matter what but it does seem much improved.

Also, rsxsr, i'm looking for that 533201511a part and can't find anyone who has it available. Any ideas?

I notice that van cafe sells a $20 aluminum 7mm to 12mm adapter. that may be a better long term solution than having the narrower hose inserted into the wider one. Any input on this appreciated. Thanks.

Thu Apr 16, 2015 12:14 pm

Came across this thread. I would have guessed the dealer could still get them. Guess not. We use them on the race cars to avoid cavitation under cornering. http://forums.vwvortex.com/showthread.php?6908226-...533201511A

Thu Apr 16, 2015 3:06 pm

Just as a point of reference, my noisy fuel pump quieted down immediately after swapping out that clogged fuel pre-filter.

Did you happen to cut yours open and see what it looked like inside?

It could give you some indication of what it's like inside your gas tank. Could be gunk is clogging up in there, and since it is gravity fed to your pre-filter and pump, this would cause problems.

I'm a little skeptical about how you have adapted from the 7mm to 12mm. By just inserting the smaller hose into the larger hose you have to compress the opening of the smaller one to tighten the larger one over it. Look how the GoWesty one has two clamps. It is clamping onto the metal part you saw at Van Cafe.

Also, if you remove the pre-filter make sure you have the larger high pressure filter after the pump (mine had both).

I plan to ditch the pre-filter after I'm sure I have solved the issue of junk getting into my fuel supply (new tank and seals and such).

Thu Apr 16, 2015 4:01 pm

There is a screen in the tank that is not serviceable, it is possible that it has clogged. If you recirculate the fuel through a device such as this, you will be drawing far less fuel from the tank even though the same amount of fuel is going through the pump, at least I believe this is how this item works.

Also make sure your tank vents are clear, this would include the charcoal filter. If the tank can't breath it will cause the pump to growl.

Removing the original white cube filter and adding the round canister filter after the pump worked for me. Others have had success by putting a ballast resister in line with the electrical feed to the pump, slowing the pump down do it draws less fuel from the tank.

Maybe the best solution in the long run is to buy a later style tank and convert everything over that you need to make it work.

Thu Apr 16, 2015 4:11 pm

BlueGrasser wrote: ...I'm a little skeptical about how you have adapted from the 7mm to 12mm. By just inserting the smaller hose into the larger hose you have to compress the opening of the smaller one to tighten the larger one over it...

I agree, could be more restrictive. I suppose a simple adaptor could be made by just punching a big hole in the innards of the plastic cube filter.

I still use the pre-filter and get no cavitation - though it does get noisy when it wants to be changed (like a little baby).

Thu Apr 16, 2015 4:42 pm

thanks all for your eyes/thoughts on this, i appreciate your help.

- i ordered the 7mm to 12mm adapter from van cafe which ought to address any restriction caused by joining the two hoses together.

- i cut open the square fuel filter that was on there for the past 2k miles and it looks perfectly clean, not even a tiny bit of sediment.

- the noise has definitely reduced with the no pre-filter and two joined hoses but it's still noisier than it ought to be and i'm hoping the adapter fixes that.

- if this doesn't fix it,i will try the "re-circulating" filter piece assuming i can find it somewhere.

- the tank is fairly new and i preemptively changed the lines and such on the overflow tanks last year (they were not leaking but i was changing lines anyway so did them), so i'm not really itching to take it down to see if it's possibly clogged on the inside but it seems like it might be the next logical step here.

Thu Apr 16, 2015 5:06 pm

The VW cars that I checked on that used that recirculating part also had a feed pump inside their tank to supply that part with fuel. Then the main fuel pump does the high pressure work drawing from that part. I wonder how well it would work in a Vanagon when the fuel level was low in the tank? How is the Vanagon pump supposed to suck just fuel when the return line inside the tank gets above the fuel level?

This place says they have them, $90 plus shipping.

http://www.germanautoparts.com/partsearch

Mark

Sjwillis wrote: ....if this doesn't fix it,i will try the "re-circulating" filter piece assuming i can find it somewhere.....

Thu Apr 16, 2015 7:22 pm

Just a thought..there is an abundant amount of filtered, low pressure fuel returning to the fuel tank on the driver's side via the hose up near the fuel tank.

Put a "T" fitting in the return hose and connect a hose going to the other side on the Vanagon with the hose end up-stream of the fuel pump on the passenger side with a "T" fitting..right where your pre-filter would be.

Would take some fiddling due to the different size fuel hoses, making some "T" fittings from the hardware store or maybe some AC "T" fittings..but it could be done without a high cost.

Once done there would be a constant supply of fuel delivered at low pressure right at the intake of the fuel pump as well as the gravity feed from the fuel tank. No more fuel pump captivation issues. The 7mm inlet tank hose hose won't matter. There would be plenty of fuel available for the pump.

Thu Apr 16, 2015 8:58 pm

The adapter I used whilst trying to solve pump cavitation noise was a fairly pricey item. $40 Cnd. IIRC.

It made no difference (it being used in conjunction with pre pump filter delete)

In hindsight, maybe I could've attached that moulded rubber adapter to the tank outlet?
Anyhow...

2 filters, one filter, made no difference. This was with near new tank, hoses etc.

https://picasaweb.google.com/musomuso/FuelFilterUpgrade#

Thu Apr 16, 2015 9:14 pm

There is no good reason that I can think of as a professional engineer that the length of hose from the filter should make the fuel pump cavitate. The size of either hose is sufficient to provide fuel to the pump. There is something else going on here, possibly a restriction within the fuel tank itself....

Fri Apr 17, 2015 2:44 am

dobryan wrote: There is no good reason that I can think of as a professional engineer that the length of hose from the filter should make the fuel pump cavitate. The size of either hose is sufficient to provide fuel to the pump. There is something else going on here, possibly a restriction within the fuel tank itself....

I agree with this idea.
Let me ask "How long has this cavitation been going on?"
There are thousands of vanagons out there, surely they are not all cavitating.
What has changed from the original equipment this one left the factory with?

The age factor will have debris in the tank, but I think you said it was new tank.
Can you run a test hose from another source of fuel to the pump and see if there is cavitation?

Does it make a difference if your tank is full or empty?

Is this a new fuel pump - are it's spec's the same as the original? i.e. is it looking to pump more fuel than the original pump?

Fri Apr 17, 2015 4:10 am

dobryan wrote: There is no good reason that I can think of as a professional engineer that the length of hose from the filter should make the fuel pump cavitate. The size of either hose is sufficient to provide fuel to the pump. There is something else going on here, possibly a restriction within the fuel tank itself....

The longer and smaller the hose, the more friction losses related to the fluid flow therefore the greater the pressure drop and therefore the lower net positive suction pressure available to the pump, therefore the higher the risk of cavitation.

Fri Apr 17, 2015 4:43 am

Mellow Yellow 74 wrote: dobryan wrote: There is no good reason that I can think of as a professional engineer that the length of hose from the filter should make the fuel pump cavitate. The size of either hose is sufficient to provide fuel to the pump. There is something else going on here, possibly a restriction within the fuel tank itself....

The longer and smaller the hose, the more friction losses related to the fluid flow therefore the greater the pressure drop and therefore the lower net positive suction pressure available to the pump, therefore the higher the risk of cavitation.

I think Mellow Yellow 74 is referring to adding a loop of large diameter hose prior to the pump, in my mind there is little reason this would make any difference in how the pump works. Don't know why this might have worked for anyone as all it does is add a bit of pressure drop to the system. It is not effectively going to store any extra fuel for the pump or in any way buffer the inlet pressure. Trying to shorten the length of the small hose leaving the tank and lengthen the larger hose may have a very slight effect on the pump inlet pressure, but there is still a good length of small diameter tubing inside the tank with high entrance and exit losses.

The cube filter and small hoses work just fine on a Bay, but gave problems on a Vanagon. At least a few things changed between a Bay and a Vanagon.

-On a Bay the tank sits high above the filter and pump giving more static head

-The outlet for the Bay tank is just an open pipe, whereas the Vanagon has a screened outlet

Very fine material that is not readily available to the naked eye seems to clog the cube filters enough to cause the pump to growl. On my one trip to Mexico years ago before I swapped over to the newer style filter system I could never see any appreciable buildup in the cube filters when I changed them out and cut them open, but for the filters I backflushed I could get almost as many miles out of them as I did out of a new filter.