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Fuel pump cavitation, which is my fault
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Wildthings
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PostPosted: Thu Apr 01, 2021 4:48 pm    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

davlance01 wrote:
dgbeatty wrote:
A very major problem is the incorrect pump being installed. Without naming offenders suffice it to says that many "similiar" pumps are being installed. They are too good with much higher flow which in this case is not a good thing.


Got me wondering if that is what the resistors are solving?


Yes that is why some people use them, to slow down the pump.
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dgbeatty
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PostPosted: Thu Apr 01, 2021 8:17 pm    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

Yes the resistor, although crude, is derating the pump down to the original spec. The tanks' internal sump, and filters if still OE, cannot supply the higher than spec volume and creates a slight vacuum. Viola, high temps, higher voltage after drving a while, current fuel formulations, a touch of vacuum and bingo cavitation. The only things we can control is pump volume. OE spec pump and/or lower voltage and the vacuum goes away and so does the cavitation.
Note many new cars have variable speed pumps or staged multiple pumps. They also manage a slight pressure inside of the tank.
I have spent many hours researching this problem with extensive instrumentation. I am convinced this is the problem but ready to listen to other ideas and solutions.
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PostPosted: Fri Apr 02, 2021 2:56 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

The best part of this thread is now m when I’m late for dinner, I can blame “ fuel pump cavitation” instead of running out of gas.
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PostPosted: Fri Apr 02, 2021 5:53 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

dgbeatty wrote:
Yes the resistor, although crude, is derating the pump down to the original spec. The tanks' internal sump, and filters if still OE, cannot supply the higher than spec volume and creates a slight vacuum. Viola, high temps, higher voltage after drving a while, current fuel formulations, a touch of vacuum and bingo cavitation. The only things we can control is pump volume. OE spec pump and/or lower voltage and the vacuum goes away and so does the cavitation.
Note many new cars have variable speed pumps or staged multiple pumps. They also manage a slight pressure inside of the tank.
I have spent many hours researching this problem with extensive instrumentation. I am convinced this is the problem but ready to listen to other ideas and solutions.


Original electric fuel pumps will collapse original gas tanks.
This is related to vent issues of course.
I recently discovered that my original 1990 German Bosch fuel pump (as well as my Yugoslavian replacement Bosch pump) can suck and compress air to 26psi!

Dave
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PostPosted: Fri Apr 02, 2021 6:29 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

dgbeatty wrote:
Yes the resistor, although crude, is derating the pump down to the original spec. The tanks' internal sump, and filters if still OE, cannot supply the higher than spec volume and creates a slight vacuum. Viola, high temps, higher voltage after drving a while, current fuel formulations, a touch of vacuum and bingo cavitation. The only things we can control is pump volume. OE spec pump and/or lower voltage and the vacuum goes away and so does the cavitation.
Note many new cars have variable speed pumps or staged multiple pumps. They also manage a slight pressure inside of the tank.
I have spent many hours researching this problem with extensive instrumentation. I am convinced this is the problem but ready to listen to other ideas and solutions.


I am not savvy enough to know how to do a test unless it involves sitting there with a bunch of appropriately sized resistors and swapping them out until the pump goes quiet. What should I be looking to get?
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PostPosted: Fri Apr 02, 2021 8:15 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

I don't know that I'd want to run my fuel pump through a conventional resistor. It would need to be something with a heat sink or ballast for me to feel comfortable.

There is a gauge thread, where somebody linked to a variable resistor box with a knob. That might be the easiest way to focus in if you go that route.

I'd be more inclined to look at improving the inlet fuel flow personally. I described a way to do it earlier.
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Wildthings
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PostPosted: Fri Apr 02, 2021 8:58 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

dobryan wrote:

I am not savvy enough to know how to do a test unless it involves sitting there with a bunch of appropriately sized resistors and swapping them out until the pump goes quiet. What should I be looking to get?


You would need something of moderate size. Maybe an ignition resister from some older rig or a resister used in conjunction with a heater or cooling fan.

I'm with Mark on this in trying to find out what is actually wrong first.
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PostPosted: Fri Apr 02, 2021 9:16 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

There are basically three ways to control the pump.
1. OK is a ballast resistor. Low cost but subject to voltage variations.
2. BETTER is a PWM speed controller. Fully adjustable with steady output (depending on quality of design). While overkill I would choose a 15-20 amp unit.
3. BEST is a PWM regulated by RPM. At idle low voltage increasing to full voltage at redline. Not subject to supply voltage variations or temperature swings. Robust and durable!
This is the one I chose. tinyurl.com/yzu2pmjw . Amazon appears to have the best price.

Like so many things that work well it isn't cheap. To address Dave's question regarding resistor values I would choose a 50 watt metal encased wire wound resistor. The value can be derived using ohms law. Always install on the ground side, safer incase of an internal short.
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PostPosted: Fri Apr 02, 2021 10:05 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

I wonder if you should simply bite the bullet and order a fuel tank for the newer models that has the 12mm fuel pump outlet?
Get a better flow to the pump.

Dave
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PostPosted: Fri Apr 02, 2021 10:07 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

djkeev wrote:
I wonder if you should simply bite the bullet and order a fuel tank for the newer models that has the 12mm fuel pump outlet?
Get a better flow to the pump.

Dave


I have had times when the fuel pump whined on my busses and they all have new later tanks.
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PostPosted: Fri Apr 02, 2021 10:34 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

Forum;

(Only) if this noisy Fuel Pump issue is not really an indication of another issue and reducing its supply V is a valid modification...

...Dropping Resistors are a brute force way to reduce the voltage applied to a load. I recommend them on vintage Volvos when doing conversions from 6V to 12V, and for instance, the 12V rated Blower or Wiper motor are not available...by using the original 6V motor and wiring an equivalent DR in series (R value determined by current consumption test, Ohms Law and some simple math Kung-Fu), the DR drops 6V, leaving another 6V for the 6V rated Wiper Motor.

Of course, for the purpose of quieting down a noisy 12V FuPu, dropping just a couple of Volts and running it on 10V may be just what's needed! Practical tests are always advisable and necessary! DRs are not so electronically elegant...in fact somewhat crude as the DR generates the equivalent heat in W as the Load is doing in work (in the 12/6V instance, not in the 12/10V instance!), but perfectly effective, and designed and installed right, reliable as a rock...which I much prefer to some china'sbest electronic PWM switcher.

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

Source: https://www.sw-em.com/Vintage_Volvo_6V_to12V_conversion.htm

I have a selection of industrial strength 50W Dale Rs in stock in the style shown (in single to tens Ohms), if you figure out what value R you need for the VW FuPu application, contact me directly...I might be able to help with a modestly priced DR (or combination of more than one to get a specific R value). I even have some high Wattage variable Rheostats, which could be used during trials, to set and start with a beginning value, then diddle up or down to determine the permanent value to install. See: https://www.sw-em.com/Vintage_Volvo_6V_to12V_conversion.htm#Dropping_Resistors

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PostPosted: Fri Apr 02, 2021 11:09 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

djkeev, sorry I do not know your name.
Unfortunately the 12mmm tank will not solve the problem. The only 100% lower cost solution is an internal submerged pump. That involves fabrication etc and incase of a pump failure tank removal. Regardless I have considered it. I have the high capacity that was used in SA and the 6cyl WBX engined vans and am hesitant to cut into it.
Part of the problem somewhat unique to Vanagons is the fact that the fuel pump must begin lifting the fuel at roughly the 1/2 full. Try it sometime. At 1/2 full or a bit less disconnect the fuel pump. How much fuel drained out! Now try it again with just a breath of tank vacuum. Pretty discouraging.
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PostPosted: Fri Apr 02, 2021 11:22 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

dobryan wrote:
djkeev wrote:
I wonder if you should simply bite the bullet and order a fuel tank for the newer models that has the 12mm fuel pump outlet?
Get a better flow to the pump.

Dave


I have had times when the fuel pump whined on my busses and they all have new later tanks.


i suspect the intake screen mesh size is finer than OEM and causing restriction. but there are two variables at work here... replacement pump performance (fewer PN's with wider spec range) and screen restriction (doubt mesh sizes were matched perfectly with OEM). both are wild cards in today's replacement parts world.

i also wonder is this truly cavitation people are hearing? no one has dissected a pump to observe the impeller pock marks from cavitation. if it is the original Bosch pump, maybe loose bushings or pump vanes after 30 years. if new replacement pumps, maybe poor tolerances. maybe it IS cavitation but we should be careful with that term.

installing a dropping resistor seems a pretty good solution to lowering noise. i would add that this should be done in conjunction with a dead head and regulated pressure test to make sure the pump isn't being slown down too much.
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PostPosted: Fri Apr 02, 2021 11:26 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

VW knew they had a problem starting with the VW GTI, These came standard equipment but hard to find now. We used them in our race cars with a simple diaghram pump outside the tank to fill it. It was connected to the pump inlet with the correct 12 mm fitting, the fuel system return came back to it and the top fitting was a return to the tank. These worked great for having an ample supply of fuel available for running the engine. The system returns at least 2 quarts a minute unused.

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PostPosted: Fri Apr 02, 2021 11:43 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

that part makes total sense. with the length of the fuel lines, i really don't think one need worry about returning the fuel to the tank for cooling.
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PostPosted: Sat Apr 03, 2021 8:24 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

Dan
I have not measured fuel temps with a non return system. I do have limited experience with Mercedes fuel coolers, standard equipment for a few decades. They were required to keep the expansion rates under control. 1. When a hot engine was turned off the fuel would expand so much that the fuel rail would overpressure and push fuel past the pressure regulator resulting in very difficult hot restarts. 2. The fuel tank contents would expand faster and more than the large charcoal cannister could handle. The solution was an integrated return fuel cooler that used the low pressure refrigerant line to cool the return fuel.
Note: MB never had cavitation or starvation because the pump assembly was beneath the fuel tank. As a result there was always a slight positive liquid pressure at the pump inlet. In those decades the tank was always mounted directly over the rear axle inside the body.
That is the short version!
Our Vanagon fuel tank temps often exceed air temps by 20-30f. Here in California's central valley on hot summer afternoons I have seen in excess of 140f at the tank when climbing at lower speeds.
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PostPosted: Sat Apr 03, 2021 8:45 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

Mark
The replacement tanks did not copy the OE tank very well.
1. No return screen
2. No intake filter at all! Thus the the need for an inline prefilter (stainless mesh)
3. A truly terrible cheap a** "sump" design.

The OE tank was for the most part very well designed and built with a nice sump and vastly superior to any aftermarket I have ever seen.
Perhaps a dead end system would be better but pressure regulation and additional pump control would be required to be fitted.
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PostPosted: Sat Apr 03, 2021 9:04 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

Are you getting any deposits in your fuel filter now?

I say this because my last vanagon daily driver, it's been a while, had similar issues. I changed about a bazillion fuel filters, tried to drain it, then finally pulled it down only to find the tank severely rusted in the top in the valley where the heat pipe would be on an air cooled. Of course I replaced it then started having the same symptoms again beginning with cavitation. Again I had rust in my tank. apparently the low quality (aka Chinese) replacement tank was rusting also.

Ultimately I had to put a tank liner liquid in it which solved the problem for years.

My next question is why are you running a pre-filter at all? Some VW mechanics stated that VW removed the pre-filter in later buses and maintenance. The impeller on a pump is really not going to get injured by items in the fuel. It's your injectors you're trying to save.
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PostPosted: Sat Apr 03, 2021 9:25 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

dgbeatty wrote:
Dan
Our Vanagon fuel tank temps often exceed air temps by 20-30f. Here in California's central valley on hot summer afternoons I have seen in excess of 140f at the tank when climbing at lower speeds.


well, that's pretty convincing!! my cold weather bias showing and those are rilly high temps where you'd want tank influenced cooling for certain. that's gotta be close to the boiling point of some of the fractions in the gasoline mix!

i know hot rodders will run fuel thru a fuel ice chiller for greater density but we don't run near the fueling delivery limit.
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PostPosted: Sun Apr 04, 2021 6:46 am    Post subject: Re: Fuel pump cavitation, which is my fault Reply with quote

nemobuscaptain wrote:
Are you getting any deposits in your fuel filter now?

I say this because my last vanagon daily driver, it's been a while, had similar issues. I changed about a bazillion fuel filters, tried to drain it, then finally pulled it down only to find the tank severely rusted in the top in the valley where the heat pipe would be on an air cooled. Of course I replaced it then started having the same symptoms again beginning with cavitation. Again I had rust in my tank. apparently the low quality (aka Chinese) replacement tank was rusting also.

Ultimately I had to put a tank liner liquid in it which solved the problem for years.

My next question is why are you running a pre-filter at all? Some VW mechanics stated that VW removed the pre-filter in later buses and maintenance. The impeller on a pump is really not going to get injured by items in the fuel. It's your injectors you're trying to save.


Yes, I usually pull something out of the bottom of the filter. From metal shavings to leaves and dirt. If you look at mesh size in simplex strainers and picture some sort of debris that size entering your fuel system you will understand that vw was absolutely wrong about removing the pre filter. There seems to be a strainer inside the tank on the original tanks but mine is after market and I never bothered to look inside the tank when I installed it. I've seen way too many baskets sitting next to simplex housings on the ground not being used to trust any fuel at gas stations. It's not the fuel brand it's the lazy people in the fuel pipeline I'm worried about.

Yes I'm worried about the fuel pump and the injectors, most vw mechanics worry me with some of the things they say. They remind me of diesel mechanics which all seem like know-it-alls that think the more they talk the smarter they are. Yes dirt will hurt the impeller.
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