cavitating pumps - need help!

DieselSJ · Post by **DieselSJ** » Sun Mar 26, 2006 2:31 pm

Here is an interesting problem we are having with the fuel pumps. A pump will run for about 10 minutes, then suddenly will get noisy and fuel pressure will drop. If we flip the switch to change pumps, fuel pressure comes back immediately and that pump will run for about 10 minutes. Again we flip the switch back to the original pump and pressure comes back immediately.

The pumps are mounted in parallel on top of the fuel cell. Line comes out of the top of the cell, into a "T", into each pump, then into another "T" and into the filter.

We had the cell apart after the last race and the bladder is fine. We also replaced the pickup with one that has a bigger inlet. Pumps are staying cool - they are cool to the touch the entire time they are running. I don't think it is a pickup problem as the fuel pressure comes back immediately upon switching pumps (they can be swithed with the engine running).

We thought maybe we were getting some type of feedback loop through the second pump, so we removed it from the system and still have the same problems.

The pumps are both Walbro units - GSL394. Fuel pressure we are running is 50 psi. Could they both just be failing? Would we be better moving to a GSL392?

We are stumped. Any thoughts or suggestions?

woh · Post by **woh** » Mon Mar 27, 2006 10:15 am

I can't explain it, but try removing the gas cap. Sounds like you are creating a vacuum.

DieselSJ · Post by **DieselSJ** » Mon Mar 27, 2006 10:28 am

Thanks. Thought about that, but the fuel cell is well vented. We thought maybe the roll-over valve in the vent was stuck, but it all checks out OK. I think if we had a vacuum problem then the fuel pressure would not recover immediately when we switched pumps.

We are still stumped.

DieselSJ · Post by **DieselSJ** » Mon Mar 27, 2006 6:14 pm

Let me add a few more details about the setup. We are running a semi-returnless system. By that, I mean that the regulator is next to the pump back at the fuel cell. The fuel is pumped into the regulator, then to the fuel rail. The return to the tank is from the regulator. There is no return coming from the fuel rail. We are using an Aeromotive regulator and they said that the setup should work fine.

bleoh · Post by **bleoh** » Mon Mar 27, 2006 7:24 pm

Could it be electical? Are you getting full power to the pump when it acts up?

woh · Post by **woh** » Mon Mar 27, 2006 7:54 pm

Are the pumps below fuel level?
How do you know it is cavitating, and not stalling?

How about we try a couple of experiments?

1. Turn on pump 1, when it cavitates, turn it off. Wait 10 sec and turn it back on. Does it recover?

2. Disconnect pump 2 at the input, and block the fuel line. Now try Pump 1. If pump 1 now works, then try it again with pump 2 connected at input, but disconnect output and block pump 2 output (airtight)

3. Put a fuel pressure gauge at the T and monitor the pressure as the pumps are turned on. If you can, also monitor the output at the pump at the same time.

4. Monitor the current to the running pump before and after cavitating.

DieselSJ · Post by **DieselSJ** » Mon Mar 27, 2006 10:16 pm

bleoh - we are getting full power to the pumps. They are wired directly to the main power distribution block (through a relay). We verified that we have full power to them all the time.

woh - the pumps are not below fuel level - they are mounted to the top of the fuel cell. The fuel pickup comes out through the top, through about 3" of -6 line, the "t", then about 3" more -6 line to the pumps. Then out of the pumps, into a "t" then into the regulator. Again, all -6 stuff.

How can we tell the difference between cavitation and stalling? What causes stalling?

To answer the rest of your questions -
1 - we did not try that yet.
2 - already tried that - eliminated the "t"s and ran a single pump and we still have the problem
3 - we have a gauge on the system all the time - pressure comes up to 50psi very quickly, like maybe one second or so. So, we can see when the pump starts to cavitate because we lose pressure very quickly
4 - have not monitored current draw yet. Would it be correct to assume that cavitation would make current draw go down, while stalling would pull more current?

Thanks for all the help - it is greatly appreciated.

woh · Post by **woh** » Tue Mar 28, 2006 6:00 pm

I'm only familiar with the Bosch roller-cell pumps. They do not like to be above the fuel and will not self prime. This indicates to me that the pump is not able to 'pull' up a sufficient vacuum to lift the fuel. I suspect the pump can not maintain a low enough pressure to lift the fuel. This is especially true if the output pressure is high.

Yes if it is working very hard the current will go up. If it is cavitating and there is air inside the pump the current should go down and the speed go up. I was thinking that the ouput line may have a blockage that takes time to develop???

We still don't know what the pressure is at the input when it cavitates.

tlivingd · Post by **tlivingd** » Thu Apr 20, 2006 10:34 am

I get the "T" on the outlet. but you have a "T on the inlet? so both pumps are attempting to suck thru the same straw? You maybe limiting flow based off of your fuel line size and the pump cant draw well against it since there is too much restriction.

Something else to consider is you don't have any check valves in the system. and if one pump is stronger than another you will cause more head pressure against the weak pump.

the pump that continues to cavitate can you run it forever if the other pump isn't running?

-nate