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Fuel starvation, cavitating fuel pump.
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dgbeatty
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PostPosted: Fri Sep 06, 2019 7:56 am    Post subject: Fuel starvation, cavitating fuel pump. Reply with quote

We had fuel cavitation problems in the middle of Nevada. I changed the pump and filter in a McDonald's parking lot in Fallon, NV (pavement temp 153f). The problem reoccured 50 miles later. We spent the night and scurried home early the next day while it was still cool without further problems.
Once home we installed a new tank believing that to be the problem. Upon inspecting the old tank with a remote camera it was spotless inside (the original OE tank). At this point my hypothesis was the the screens assemble or the internal plastic line(s) had failed in an area I was not able to inspect. I installed a fuel resistant clear hose on the fuel pump intake to observe. Sure enough it was continuously cavitating with a full tank at idle.

CONCLUSION Two factors created the problem.
1. We use LFP batterys and as such our alternator is set to 14.8 volts.
2. We use a Pierburg fuel pump. These pumps have essentially the same specs but are renowned for their toughness. They do provide a very slightly higher delivery quantity.
Combined these two changes increased the fuel delivery quantity to the point where it exceeded the ability of the tank to deliver a solid stream of ETHANOL. On pure non California gasoline the problem does not come into play.

SOLUTION.
Reduce the pump speed. A 30amp PWM speed control set to 10 volts achieves the specified return fuel flow. Cost $10. Never doing anything simple I installed a PSC (pump speed control) that calculates pump speed based on RPM. Cost (you don't want to know).

This has been a problem for the past three years with various changes made. It was my punishment for not following the full test procedure
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MarkWard
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PostPosted: Fri Sep 06, 2019 9:27 am    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

What size outlet did your replacement tank have?
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dgbeatty
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PostPosted: Fri Sep 06, 2019 10:06 am    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

The van is an 87 Westy. Both the OE and replacement tank are 12mm outlets.
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dobryan
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PostPosted: Fri Sep 06, 2019 10:15 am    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

I cannot believe the outlet at 12mm could be the issue. Something else in the tank system must constrict the flow.
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DanHoug
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PostPosted: Fri Sep 06, 2019 11:46 am    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

dgbeatty wrote:
I installed a fuel resistant clear hose on the fuel pump intake to observe. Sure enough it was continuously cavitating with a full tank at idle.


we should be clear here... that is NOT fuel pump cavitation per se'. pump cavitation occurs on the low pressure side of vanes, rollers, impellors, etc creating a spot vacuum bubble that slams shut immediately and can erode metal on the pump mechanism.

what you are seeing on the intake side via a clear tube is either the fuel boiling in the line due to greatly reduced pressure from a line/tank blockage OR air entrapment from an air leak. since the fuel return line dumps into a chamber with a fine mesh over it, sometimes if air does get in there, it has a hard time finding its way out and keeps recirculating.

but... dollars to donuts, your fine mesh tank screen that resides above the fuel tank outlet and recirc return is clogged. this is a plastic screen that can get clogged with a variety of organic or inorganic materials, creating a high vacuum condition and boiling the fuel on the intake side of the pump creating the 'cavitation' bubbles you are seeing.

there's an easy way to test for screen blockage... Tee a vacuum gauge into the intake line and look at the reading. seeing a drop of a few inches Hg would be normal as there is always some flow restriction. if the vacuum gauge is reading 10-15-20 " Hg, i'd be very suspect of the in tank filter screen being blocked. mind you, i've NOT ever measured my intake pressure so these are guesses as to the values one might see based on fooling around a lot with pumps. someone might have measured this on a clean tank screen.

or... far more crudely... remove the fuel cap and take an air nozzle and blow back into the intake. do it with a nearly empty tank as it can gusher out fuel from the filler. if you hear a pop, you've burst your clogged screen and fixed your problem!
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87 Westy, stock 2.1 w/ 4 spd
89 Westy, stock 2.1 w/ 4 spd, simply gorgeous


Last edited by DanHoug on Fri Sep 06, 2019 1:32 pm; edited 1 time in total
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MarkWard
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PostPosted: Fri Sep 06, 2019 12:44 pm    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

I think the point was that the alcohol added to the fuel was lowering the vapor point that in conjunction with the higher efficiency pump was causing the cavitation. Not an actual restriction per say. It makes sense to me.
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DanHoug
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PostPosted: Fri Sep 06, 2019 1:27 pm    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

MarkWard wrote:
I think the point was that the alcohol added to the fuel was lowering the vapor point that in conjunction with the higher efficiency pump was causing the cavitation. Not an actual restriction per say. It makes sense to me.


gotchya - the problem shows with certain fuel blends. but it just doesn't seem that he'd be running on the ragged edge of causing boiling in the line without some worse-than-designed restriction.

an interesting point i just came across ... fuel grade ethanol has a lower vapor pressure, that psi created in a sealed container at a certain temp, than gasoline. meaning, it has a lower boiling point than gasoline. HOWEVER, when combined with gasoline, it INCREASES the vapor pressure, aka the volatility (or lowers boiling point) of the ethanol/gasoline mix. in other words it evaporates more easily and would boil or 'cavitate' in a suction line more easily.

treatise on ethanol - gasoline blend effect on vapor pressure here:
https://ethanolrfa.org/wp-content/uploads/2015/09/RVP-Effects-Memo_03_26_12_Final.pdf
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87 Westy, stock 2.1 w/ 4 spd
89 Westy, stock 2.1 w/ 4 spd, simply gorgeous
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dgbeatty
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PostPosted: Fri Sep 06, 2019 2:07 pm    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

Perhaps I was not clear enough.
It continued to occur with a NEW tank.
YES the pump is cavitating caused by lack of solid fuel stream.z
No there are no fuel or air leaks.
Yes I have measured the pressure at the pump inlet, it is negligible.
Yes I have measured the tank pressure, essentially 0 differential.
Vehicle speed does not appear to be involved.

Somethings that I should have made clear.
It was August in Nevada, I was over 4000 elevation. It has never occurred at lower temps or elevation. 12,000 is fine as long as it is cold. 110f is fine in death valley. It is the combination of altitude, temperature and running time (minimum of 1 hour).

When it occurs the pump becomes much warmer. No the pump is normally quiet. It just occurred to me to perform some testing with a swirl tank installed. BTW this a known problem with the WRX-STI crowd. Many go so far as to add a surge tank with a swirl device and a second pump in the trunk. There is also a documented problem of fuel heating with recirculating fuel injection systems.

Dan Beatty PE
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DanHoug
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PostPosted: Fri Sep 06, 2019 3:13 pm    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

ah, thanks for the clarification Dan. you've done your homework. sound like you need an ice can cooler for the fuel line! better yet, a 2-tap chill plate in an ice chest with one line for nut brown ale. NSF approved, of course.

back in the day, my dad said they'd pee on the fuel line if the car vapor locked from high under hood temps.... probably be multiple obscenity charges today with the ethanol fuels.
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Wildthings
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PostPosted: Fri Sep 06, 2019 4:36 pm    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

Sure your fuel tank is venting properly? Perhaps a ventilation hose is blocked or the carbon canister is a mess inside.
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dgbeatty
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PostPosted: Fri Sep 06, 2019 7:34 pm    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

Dan Houg ,thanks for looking this little problem. I am on a test drive this afternoon. Sacramento to Weed as fast as I can go with a couple idling stops on the hottest asphalt i couId find. The van has run well, the pump has stayed cool and quiet. When I replaced the tank I insulated the front and top sides to protect it from radiator heat. That and the PSC appears to have hopfully cured my self induced problem. Just need to find a higher and hotter road to be sure. What has worked in the past is to load the van with the wife and dog and have a definate place far away as the destination.
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Wildthings
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PostPosted: Fri Sep 06, 2019 9:14 pm    Post subject: Re: Fuel starvation, cavitating fuel pump. Reply with quote

Other people drive these vans in all kinds of weather at all kinds of altitudes. If yours will not do what other vans do, your van must have some part of its fuel system that is out of factory spec. Finding what it is and getting it back to factory condition would most likely be the best policy in the long run.
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