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Fuel Tank Noise - Ideas? Fix?
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camo westy
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PostPosted: Sat Feb 02, 2008 7:40 am    Post subject: Fuel Tank Noise - Ideas? Fix? Reply with quote

Van 1985. 200K miles on the van/tank.
Stock fuel tank and pump, Suby motor (noise was there with the WBXer)
Synptoms;
When I start the motor, fuel pump cycles as usual, normal whirring noise.
After the motor starts, usually about 5 seconds, the louder noise starts.
I even know the base cause of the noise; bubbles.
The noise starts when the bubbles arrive to the pump. I put a clear pre filter between the pump and the tank, 5 to 20 seconds after start up, a stream of bubbles can be seen coming through the clear filter, when the bubbles go through the pump, they create a noisy whirr.
Same symptoms if the tank is full to the brim, or near empty.
I removed the 'pre filter' and that makes no difference.
Tank has new hoses, and new clamps, double clamps!
Lines on the tank look normal.


So any ideas where the bubbles come from?
Anyone ever experienced this and fixed the issue?
Seems like a new tank is in order, but I still want to know where the bubbles come from? I'd hate to spring for a tank, then still have the problem.
I cant imagine what the issue is unless there is a tiny hole in the suction line of the tank.
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ValleyHappy
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PostPosted: Sat Feb 02, 2008 8:49 am    Post subject: Reply with quote

Double check what you have between the tank outlet and the pump. I suspect something in that interval is sipping air under neg. pressure when the pumps spinning. Things may look dry and sealed enough not to leak fuel, but not sealed enough to prevent sipping air when the pumps running.
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Wildthings
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PostPosted: Sat Feb 02, 2008 11:13 am    Post subject: Reply with quote

Sounds like you have an internal blockage inside the tank. Someone posted a picture of a cut open tank a while back that would show you what the insides actually look like. I suspect that there is something blocking the outlet. Back blowing it with air might help, as might forcing a piece of wire back up through it.
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foodeater
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PostPosted: Sat Feb 02, 2008 11:54 am    Post subject: Reply with quote

Getting a new tank do the trick, I've got this planned for the summer, for the same reason.
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wbx
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PostPosted: Sun Feb 03, 2008 12:29 am    Post subject: Reply with quote

Could the bubbles be coming from the pump (cavitation due to a blocked exit line from the tank) and traveling backwards through the line back up to the tank? I'm just wondering if you could be looking down the wrong path...

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tencentlife
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PostPosted: Sun Feb 03, 2008 11:59 am    Post subject: Reply with quote

Sounds like fuel line cavitation. Does the prefilter have a smaller inlet on the tank side than the outlet on the pump side? This will cause a feed pressure drop thru the filter, and with the hi-volume pump pulling on it, the bubbles form. If this is the case, they are not air; they are bubbles of gasoline vapor "boiled" out of the liquid due to the suction of the pump after the restriction presented by the small filter inlet added to the resistance of the filter (remember how water will boil at a lower temp at a high altitude, and vice versa? The same principle of boiling points is at work here. That is what cavitation is).

First thing I would do is remove the EEC charcoal canister from just inboard of the right rear wheel. Make sure air can be blow into it and drawn out by blowing and sucking on the ports on top. It should breath very freely, with no noticeable resistance to airflow in either direction.

While it's off, remove the fuel filler cap, and shoot some compressed air into the plastic hardline that attached to the canister, the line that leads forward to the fuel tank. You should hear air hissing into the tank.

If any part of the EEC system is blocked, the fuel pump will draw down a low pressure within the tank, which will exacerbate the feed pressure drop caused by the prefilter and mismatched hose sizes.

If all that checks out, you can volunteer to be a guinea pig for an experiment I've had in mind for someone using an early tank and/or prefilter who is having cavitation issues. If your prefilter has the small inlet and large outlet, the outlet being the same diameter as the pump inlet, try this: rather than going directly from the prefilter to the pump with the piece of 11mm hose, try using a piece of the same size hose but more like 18" long, making it into a loop before it gets to the pump. Tie the loop up securely so it can't hang down. The longer the extra hose the better.

I don't know if this will help or not; it's just a theory I have that the longer length of large hose will create a pressure buffer of sorts that may allow inlet flow to normalise, preventing cavitation at the pump.

Of course the prefilter must be clean as well, since its restriction to flow is part of the total feed pressure drop.

If you decide to try the extra loop of hose, please report back and tell us what happened.
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pushkick
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PostPosted: Sun Feb 03, 2008 8:21 pm    Post subject: Reply with quote

here is a picture of the tank i took out of my 1990 vw 2.1l camper

Image may have been reduced in size. Click image to view fullscreen.


hey .10 are you saying that if there is a leak in the expansion filtration system it could cause cavitation problems?
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tencentlife
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PostPosted: Mon Feb 04, 2008 8:52 am    Post subject: Reply with quote

Quote:
hey .10 are you saying that if there is a leak in the expansion filtration system it could cause cavitation problems?


You mean in the EEC system? No, on the contrary. It has to breathe.

There's two systems here, actually, that share the same hoses.

The two expansion tanks are to allow fuel to change volume with temperature, because it does that, a lot. I've filled my tank on a summer morning, with cool fuel coming out of the ground, and without driving much, seen the expansion tanks a third full of fuel later in the heat of the day (which leads to an aside: how many know that in warm weather, you will get more fuel for your money if you fill in the morning?)

The Evaporative Emission Control (EEC) shares some of the same hoses to vent fuel vapors to the charcoal canister (it's not a filter). When the car is parked, the vapors find their way there by diffusion, as there isn't any active flow of air or vapors thru the lines. The activated carbon (charcoal) attracts and absorbs the vapors, and then when the engine is running above idle it purges the vapors from the charcoal by applying engine vacuum to the top of the canister. Fresh air is vented up thru the bottom of the canister, thru the charcoal matrix, and into the intake manifold, absorbing the vapors and burning them. So the EEC charcoal canister has to be open to air at the bottom, where there is a vent line attached. Some vans have the vent line disappear into a hole in the frame rail as a way of protecting the inlet from dirt, some just leave it open, but it has to breathe freely from the bottom either way.

So the expansion tank/EEC hoses need to be tight at the fuel tank, expansion tanks, and in between, to prevent liquid leaks, while one plastic line tees off and goes back to the EEC canister. The canister then also acts as an air vent for the tank itself, as you don't want to draw a vacuum in the tank. It lets air pressure in the tank equalize with the atmosphere. Otherwise, as the engine uses up fuel in the tank, a vacuum would accumulate there and cause the pump to work harder to draw fuel in, and it will end up cavitating because it is being starved.

So that's why the EEC canister needs to be checked to be sure it is vented at the bottom, and that there isn't any blockage in the vent lines between it and the tank. Thats so the tank and the whole expansion/EEC system can equalize with atmospheric pressure.

That's one potential problem. The other is the problem of having the fuel feed nipple on the tank smaller than the pump inlet line. This is a classic problem in pump dynamics that will cause pump cavitation if the pump's flow rate is high enough. These pumps flow 5 times what the engine uses at average output, so the flow rate is pretty high. The older tanks, and some replacements, had a 7mm feed nipple, while most of the pumps have an 11mm inlet nipple. That's just a recipe for cavitation, and it's hard to believe that such an obvious design flaw got put into production, but hey, they left an obvious stress riser in the 3-4 gear slider hub, too, which is why those fail, so I don't take much truck with the infallibility of German engineers!
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pushkick
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PostPosted: Mon Feb 04, 2008 1:56 pm    Post subject: Reply with quote

i dont know if you remember the thread but i had the problem with the new tank from bd being 7mm instead of 11mm and after much discussion with ron salamon he just kept doing the corporate salute pointing the finger at other people and just kept saying that i didnt need it. i went to bosch and volkwagen and they both said they recommend the 11mm esit pipe. so to solve the cavitation problem i took a 1/2" hose from fuel pmp and ran it to the 7 mm outlet. i made a bushing to go over the 7mm outlet pipe and small enough to let the 1/2" hose slide over it and this seemed to help on the cavitation noise. previously i made the reducer at the pump and this was a big mistake. lost a pump within a week. i am thinking about having a custom tank made next time.
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PostPosted: Mon Feb 04, 2008 1:56 pm    Post subject: Reply with quote

i dont know if you remember the thread but i had the problem with the new tank from bd being 7mm instead of 11mm and after much discussion with ron salamon he just kept doing the corporate salute pointing the finger at other people and just kept saying that i didnt need it. i went to bosch and volkwagen and they both said they recommend the 11mm esit pipe. so to solve the cavitation problem i took a 1/2" hose from fuel pmp and ran it to the 7 mm outlet. i made a bushing to go over the 7mm outlet pipe and small enough to let the 1/2" hose slide over it and this seemed to help on the cavitation noise. previously i made the reducer at the pump and this was a big mistake. lost a pump within a week. i am thinking about having a custom tank made next time.
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tencentlife
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PostPosted: Mon Feb 04, 2008 7:19 pm    Post subject: Reply with quote

Quote:
previously i made the reducer at the pump and this was a big mistake.


Good, this supports my idea. When you moved the change in inlet pipe diameter closer to the pump (at the pump in this case), cavitation worsened. And moving it away lessened it. So it may be true that if you used an extra long inlet hose adapted to the tank fitting, either by bushing or using one of the dual-sized cube filters, to let the flow normalize after the change up in diameter, it could take care of the cavitation problem.

I have a late-model. Anyone with an earlier tank willing to try this, who has had cavitation problems?
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camo westy
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PostPosted: Tue Feb 19, 2008 7:06 pm    Post subject: Reply with quote

Ten Cent.. I'm the origional poster (is that a real word?)... anyway, I will try the 'Anti-Cavitation' transition hose trick this weekend. I have plenty of hose.
I definitely have a 7mm tank outlet, and a 11mm pump inlet and outllet. Freaking reducers at both ends of the pump.

From the look of the inside of the fuel tank pictures, I may get a new tank anyway.

Does anyone know where to get a 11mm outlet line tank?

(Dont suggest BD as a source. Their poor service, slow shipping, miscommunication and incorrect parts shipped finally drove me away)
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tencentlife
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PostPosted: Tue Feb 19, 2008 8:02 pm    Post subject: Reply with quote

Great. I'd sure like to know if that helps with your cavitation problem. Be sure to post an update after you have some time with it.

It seems like late-model tanks are hard to find new, ones with the right 11mm outlet nipple. I guess some of the vendors are just offering the old-style with small nipple, and claim it won't cause any problems. Judging by the frequency of cavitation complaints with that combo, I can't see how they can claim that in good faith.
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camo westy
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PostPosted: Tue Feb 26, 2008 12:33 pm    Post subject: Reply with quote

Bubbles are on the inlet side of the pump.

I am going to remove the 'big filter' that is past the pump (pressurized) and try that... that is the way this van was delivered.

Next I'm going in search of the source of thebubbles, since the bubbles occur with a full fuel tank, the air has to be entering either at a pinhole on the steel line on the outside of the tank, or around the flex line to steel line connection before the pump.

Those photos of the collapsed in tank filter have me thinking the suction line 'inhales' a portion of plastic pre-filter debris, thus creating enough vacuum in the line between the pump inlet and the newly inhaled blockage to suck air bubbles from a normally well sealed hose.

Funny thing is, the van runs fine, you'd think it would starve for fuel????
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PostPosted: Tue Feb 26, 2008 6:48 pm    Post subject: Reply with quote

They're bubbles of gasoline vapor. The pump suction is "boiling" the fuel. That's what cavitation is. The reason it runs fine is because there isn't any actual air in the line; the vapor bubbles are being recompressed to liquid gasoline on the outlet side of the pump.

Think about it: anywhere air could be drawn in below the liquid level in the tank, fuel is going to leak out.

Removing the big metal filter and the resistance it presents to flow may actually worsen the cavitation. Try it and see; I'd be interested to know. At any rate you don't want to run for long without that filter, if you'd like your injectors to last.
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PostPosted: Sat Mar 01, 2008 12:23 pm    Post subject: Reply with quote

tencentlife wrote:
Anyone with an earlier tank willing to try this, who has had cavitation problems?


I gave it a shot. I've got an '83 WBX with the original fuel filter setup: tank to square fuel filter to pump (no cylinder filter after the pump). I didn't have the problems that the late-model guys mentioned about the change in outlet sizes on the tank--my busdepot tank is the same outlet size as my original tank.

I recently put in a replacement tank because mine rusted out from the inside. I expected the new tank to solve my buzzing fuel pump problem but even with the new tank in it still buzzes like there's no tomorrow. I'm counting down the days until my pump burns out.

So the 'longer hose' option between the tank and the square filter. . .

Unfortunately a no go. She still buzzes. I hope this isn't a manufacturing issue with the replacement tanks. Is there any way to fix this or will I just have to budget for burned out fuel pumps?
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PostPosted: Sat Mar 01, 2008 5:38 pm    Post subject: Reply with quote

Quote:
So the 'longer hose' option between the tank and the square filter. . .



Uh, nope.

Quote:
When you moved the change in inlet pipe diameter closer to the pump (at the pump in this case), cavitation worsened. And moving it away lessened it.


Aren't you using the cube filter that has a 7mm inlet, right off the tank, and 11mm outlet, to the pump? My idea was to join the filter to the tank, with a short stub of 7mm hose. Pretty much the way it already was. The cube filter is the change in pipe diameter. So it's after the filter, you use a longer loop of 11mm hose. So the change in effective pipe size (across the cube filter) is placed farther away from the pump inlet. The idea is to give the flow a chance to normalise after the step-up in pipe size.

If you used the longer hose off the tank, but put the cube filter close to the pump, I would expect it to still cavitate.
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TooManyDubs
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PostPosted: Sun Mar 02, 2008 8:20 am    Post subject: Reply with quote

Oh boy, I totally misread that--sorry. I need the longer 11 mm hose between the cube and the pump--got it. I'll report back as soon as I make the swap.
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PostPosted: Sun Mar 02, 2008 10:45 pm    Post subject: Reply with quote

If this works I'll be doing it too. sounds like a nice cheap (free, even) fix to my fuel cavitation problems. Thanks for a good idea 10c.

-Glenn
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PostPosted: Mon Mar 03, 2008 8:05 am    Post subject: Reply with quote

Well, having more than one Guinea pig would provide better proof of its working or not, so maybe you should give it a shot, too. It's just a theory, but a cheap fix if it helps.
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