TheSamba.com :: View topic - Some inline fuel pump lessons learned (cavitation, etc.)

Mon Aug 12, 2019 11:05 am

Long post, so here we go. As a long time lurker/not-account-holder I thought I would give back to the community and post some recent fuel pump experiences from my cross-country drive in my 84 Vanagon. There are a lot of great threads debating the issues with various fuel pumps, particularly with respect to cavitation or "buzzing" noises (search for details). I recently had an enlightening experience so I am sharing.

Back when I acquired the van in ~2014 it was running the OE ~1983 Bosch fuel pump with the square filter. One of the first things I did during the restoration was a full overhaul of the fuel system, including switching to a new tank with the 12mm outlet, replacing the fuel pump with a newer reman (not Bosch) pump, and going to the Bosch can filter at the outlet, replacing all the fuel lines, etc. This setup served me well for over 4 years of driving.

I've always carried a spare pump; it's a pump I got from NAPA that is a (probably Chinese) Bosch copy; it looks identical to the OE silver Bosch unit except it has a red painted housing and ring terminals (which the later Bosch pumps switched to for better connection reliability).

A few months ago after driving for a while, I'd be about to shut off the van and notice a slight intermittent buzzing or cavitation noise from the pump, so I went ahead and changed the pump (to my spare), which operated with no problems for a couple of weeks and a few hundred miles of driving.

Fast forward to last week, I am driving through rural Oklahoma in scorching temps (~102-103F), the engine is pulling strong, and I am running my in-dash AC full blast. Over time I start to hear an increasing cavitation noise from the pump, and panic ensues since I no longer have the spare I've always carried.

Background: cavitation is just a local formation of vapor bubbles where the pressure drops in pump machinery at a relatively constant fluid temperature. For most liquids, a change from liquid to vapor and vice versa can occur along a continuum of pressure and temperature, so to “boil” or vaporize a fluid, one can either decrease the pressure, increase the temperature, or both. The dividing line between vapor and liquid is the familiar curve on a phase diagram. The cavitation noise heard in pumps with insufficient head pressure (or excessive inlet temperature) is due to the suction creating small vapor pockets, which then travel through the pump and collapse [condense] violently, causing vibration, and for pumps cooled by their working fluid, overheating since vapor cannot readily absorb heat.

Hearing increasing cavitation noise, I pull over in a remote truck stop and in the 100 deg heat get under the van, disconnect the 12mm inlet line, verify unobstructed flow to the pump (gas on demand!), clean in the inlet screen with brake parts cleaner, and reassemble. The pump is properly isolated, and not I contact with the frame. 90% of the time, a cavitating pump (and subsequent pump vibration/thermal failure) is due to a cruddy gas tank, with rust, debris, or gelled fuel restricting flow into the pump, decreasing the head pressure such that the pump suction causes cavitation. No such luck here; all my fuel hardware is clean and new, and after reassembly the cavitation continues.

Then begins the search for a replacement pump. The first thing that comes up is an O’Reilly, so I divert about 90 miles to pick up this pump in Tulsa, which upon inspection is a bad interchange that is roughly the same size and shape but has about a 16mm inlet and a banjo bolt outlet - clearly not going to work without lots of adapting, which can’t be done on the road.

By this point the pump is screaming (I can hear it on the freeway) and I’m sure it’s going to quit at any moment. I divert again to Oklahoma City where low and behold the AutoZone distribution center has an actual Bosch clone, with the proper fittings and all that. It’s identical to my existing pump, except it has blue paint instead of red. I promptly put up in a hotel for the night, and after composing myself I swap the pump in the parking lot.

Key on - prime OK - no noise, quiet operation. Cycle the key a few more times, start up, and the pump is whisper quiet. I am proud of myself for about 60 seconds, and then the new pump, at idle, starts making exactly the same cavitation sound as the previous pump. I reach up and feel the pump, and it’s as hot as the pump I just pulled off, having only run about 2 minutes. I am confused and frustrated, but some more diagnosis is in order.

I have a spare filter so I go ahead and swap that too, thinking that a restricted filter might be causing the problem (although if excessive flow was the problem, a restricted filter would actually make it better, not worse - I’m just throwing parts at it).

With the van running, I start checking flow paths:

1. Restrict (pinch) the pump inlet line. Cavitation increases (expected).
2. Restrict (pinch) the pump outlet line. Pump struggles but cavitation continues.
3. Restrict (pinch) the passenger side return line. Pump struggles, and eventually I can restrict the flow enough that the pump stalls and the van dies. Good, this tells me the fuel system is not leaking, there’s probably no air entering the system, and the regulator is wide open (which it should be at idle).

I give up and sleep. I get up the next morning at 5 and start up the van. It has cooled overnight and the ambient temps are back close to 70. I start it up, and no noise - nothing. The pump is whisper quiet. At this point I have a hypothesis, so I start driving towards a shop in CO that has a couple of old spare OE Bosch pumps.

More data points ensue on the 6 hour drive:

1. The cavitation is totally absent until well after the sun is up, the ambient temps are close to 95, the AC is running, and I’ve been driving at freeway speeds for 3+ hours.
2. With the cavitation noise back in full swing, I check the pressure regulator by driving at low RPM, and low speed, and going on and off full throttle, effectively keeping the alternator voltage constant, the engine RPM constant, and varying the vacuum. I can hear the pitch of the cavitation sound change, which means the regulator is changing the return flow correctly.

At this point it occurs to me that the Bosch copy (of which I now have two “good” examples), is likely a garbage design that is more sensitive to temperature than the OE part, particularly under my high thermal load conditions. Keep in mind that the design of the pump is such that it is cooled by the fuel passing through it, which under normal conditions should be relatively cool, say 80F max or so. In this case, after hours and hours of driving at ambient temps of 100+F, we have the following problem:

- The fuel tank is located aft of the AC condenser and radiator, which is convectively heating the fuel tank.
- The engine is at nearly full load, with the AC on max in full sun.
- The ambient temp is very high.
- The fuel is circulating at several GPH through the engine compartment, getting warmer.
- Save for radiative losses, the fuel is essentially a closed thermal system and there’s nowhere for the heat to go.

Now, fighter aircraft happen to have this same problem, which happens to be a limiting factor in their mission lifetime. They use fuel as a heat sink for just about everything. I guess on the Vanagon, typical operating conditions don’t usually allow the fuel to absorb enough heat to present an issue.

Conclusion - hot fuel, passing through a pump with poor construction, excess power, and/or tight internal tolerances, causing cavitation. Exacerbating the problem is the use of 10% ethanol blend virtually everywhere, which when mixed with fuel has a higher vapor pressure and is more likely to capitate at a given temperature than pure gasoline.

Ultimately, I buzzed all the way to CO Springs (check out Mountain Bus Werks - super great folks), where I changed the pump with that ~1987 original Bosch (including swapping terminals again), and the problem completely went away. I haven’t loaded the van up to the “hot fuel” point again, but the intermittent cavitation even with “warm” fuel on the NAPA brand pump is virtually gone.

The NAPA/AutoZone pumps are selling for $130 or so in the store. Turns out you can get an identical pump from Amazon for $29:

https://www.amazon.com/Electric-Universal-Pressure-Installation-0580464070/dp/B07B5ZZR75

I believe (looking at the case, paint, etc) that these are exactly the same part, and NAPA/AutoZone is playing the $100 lifetime warranty game with an otherwise low quality pump. I haven’t tried a “new” Bosch pump yet, but these still may not be as good of a quality as the original Bosch pumps.

Some folks have discussed doing the “resistor mod” to sink power to a series resistor in order to solve this cavitation issue. I wouldn’t recommend that, barring a better method to control the power delivery to the pump (e.g., current limiter circuit) that doesn’t generate as much heat. Besides, this being most likely a thermal problem at its root, putting more [electrical] heat into the fuel doesn’t sound like a good solution.

My long term plan is to install a fuel cooler using a repurposed small transmission cooler. There are inline extruded aluminum coolers that can mount easily under the van, using the return line to cool the fuel before going back into the tank. This might be a good solution for long extended drives at very high ambient temperatures, although care must be taken to locate the cooler away from the radiator/AC condenser airflow and avoid making the problem worse by giving that hot air a more easy path for convective heating of the fuel. This may require something fancy, like an insulated duct with the cooler located inside with a NACA inlet on the side of the van.

I hope someone finds this lesson learned illuminating. Feedback/criticism is welcome. I owe due credit to some friendly commenters on the VanAlert app that first suggested the problem might be worsened due to the high ambient temperatures.

Mon Aug 12, 2019 11:31 am

Nice analysis. You should post more on this forum. Thanks.

Mon Aug 12, 2019 11:59 am

What an excellent first post. :) Welcome to your non-lurker status. :)

Mon Aug 12, 2019 3:10 pm

The resistor mod absolutely worked for me. Once, it was a terrible racket. Now, and ever since the resistor was installed, I don't even notice that I have a fuel pump. Going great since March 2016:

https://www.thesamba.com/vw/forum/viewtopic.php?t=...p;start=40

Mon Aug 12, 2019 4:21 pm

Good post! I agree those cheap pumps are junk after trying far too many of them out.

howunlikely wrote: Some folks have discussed doing the “resistor mod” to sink power to a series resistor in order to solve this cavitation issue. I wouldn’t recommend that, barring a better method to control the power delivery to the pump (e.g., current limiter circuit) that doesn’t generate as much heat. Besides, this being most likely a thermal problem at its root, putting more [electrical] heat into the fuel doesn’t sound like a good solution.

The resistor may get hot but its remotely connected via the ground so it doesn't add any more heat to the fuel. The pump runs slightly slower so theoretically isn't getting quite as hot since its not working as hard. The bosch pump puts out way more psi than our engines use so its wasted energy and heat in my opinion.

I do run the prefilter nowadays as well. I feel it protects the pump a tiny bit...

Tue Aug 13, 2019 10:39 am

Epic first post!

There is a thread here showing differences between the Bosch and aftermarket pumps.

On my '81, on a hot day, on a steep dirt road, my fuel pump was "screaming", engine power dropped. At top of hill, van idling, it stalled. Removing fuel tank cap allowed engine to start. At that time, the *aftermarket* tank, Bosch pump, square and round filters, etc. had relatively low miles. Fuel tank venting might be a factor in your noisy pump situation? From the sounds of your post though, I bet you addressed or checked the venting system when swapping to the later style fuel tank.

In hindsight, it's possible a small amount of debris has gotten in the fuel tank via a faulty expansion tank seal and at the time of the stall, debris might've randomly blocked the tank outlet but I have not dropped (cleaned) the tank since that time and fuel pump is quiet but at times will develop some noise but never the same as that hot day stall and engine hasn't stalled since.

I can't recall if I found this fault before or after that trip, but after installing that *aftermarket* fuel tank, and addressing a leak at filler pipe to tank, I found the issue shown below.

Neil.

On an '81 Vanagon. Fuel tank out, looking up at squished vent line to charcoal filter. I don't know for sure if I caused that issue. i.e. not sure if that piece of protective material is to protect the tank or that EVAP hose and if the new tank is a slightly different dimension thus pinched hose.

Tue Aug 13, 2019 10:58 am

That was really interesting. That might explain a slight buzz I get from my fuel pump occasionally. I’ll have to pay attention to see if, as I suspect, it’s under hot conditions.

Tue Aug 13, 2019 11:02 am

All too often Lifetime warranty parts at the box stores are junk. The warranty sells the parts to an inexperienced consumers. Good parts are getting hard to find because people navigate to cheap more than good.

Mercedes used a fuel cooler on many of their CIS/CIS-e engines. It was attached to the A/C low pressure line. I always thought it was just another overly engineered part that Mercedes put on their cars. The more the fuel cycles, the hotter it gets. The area I live in does not get that hot compared to other places but if you have A/C, it's probably on on the hottest days which the Mercedes method may be ideal.

I'm not certain adding a fuel cooler to the front of the vehicle is a good approach and may be unsafe with all that additional hardware. Having it up at the front of the car would also cause me to think what would happen in case of an accident.

Either way, you seem to be intelligent enough to think this out so I'm curious what you end up doing.

Sucks that you just now joined, better late than never I guess.

Tue Aug 13, 2019 11:34 am

Did you try removing the gas cap during any of these noise events? Welcome to the Samba.

Tue Aug 13, 2019 12:18 pm

Interesting read. I may try the resistor mod for $hits and giggles and see what happens. Thanks for the post on this ...

I get occasional noise with both my original Bosch and the replacement one.

On the troubleshooting side of things .. I was replacing some hose from tank to the fuel pump, a little less than 1/2 tank of fuel on board.

I was well poised for a gusher ... nothing came out !

Fual cap off for a test, still nothing. Blew into the tank via the outlet .. Air went in and then a bit of fuel came out .... then it stopped again by itself ..

I know the PO had the tank resealed and not replaced.

It is 'normal' to have these tanks stop flowing fuel ? I am thinking NOT...

I am thinking this would be my cavitation problem perhaps ? Like sucking a thick milkshake through a small straw.....

Filter is clean.

Tue Aug 13, 2019 12:25 pm

There is a pipe with a shallow arch inside and the outlet is one end of that arch. Once air gets up inside the arch, the tank will no longer gravity drain. Unless of course the fuel level inside the tank is higher than the top of the arch, then gravity will cause fuel to come out. A siphon effect then keeps fuel coming out, as long as the outlet is below fuel level.

Mark

Tue Aug 13, 2019 4:46 pm

If it is the original tank and it was resealed, there are fine mesh screens over both the intake and the return lines.

Any tank sealer would clog the fine mesh. I can totally see flow being restricted.

Dave

Mon Dec 11, 2023 8:30 pm

How do you wire it in? inline with the 12v or the earth wire?

Forthwithtx wrote: The resistor mod absolutely worked for me. Once, it was a terrible racket. Now, and ever since the resistor was installed, I don't even notice that I have a fuel pump. Going great since March 2016:

https://www.thesamba.com/vw/forum/viewtopic.php?t=...p;start=40

Tue Dec 12, 2023 11:32 am

Or use a higher quality fuel pump?

Bosch 044, which is available NOS, or it's direct replacement, are great pumps.