Auxiliary Fuel Pump Mystery

[Ernie Martin]

Short Version
Has anyone heard of an auxiliary fuel pump sounding exactly as usual but not pumping fuel? Could a defect in the mechanical fuel pump or steep turns in the prior flight inhibit the auxiliary pump from pumping fuel? How about the problem going away seemingly by itself?

Long Version
Last week we installed a freshly-overhauled mechanical fuel pump in the front engine of my normally aspirated 337G, then took the aiplane for a spin which included steep turns. The airplane sat until yesterday, when I tried to start it. The aux pump sounded as always -- perfectly normal. There was fuel in the tank, throttle wide open, mix fully rich, healthy battery, but no flow. Not only did the fuel-flow gauge showed no flow, but, thinking the gauge might be inoperative, I a) kept pumping and saw no excess fuel on the tarmac, and b) the engine wouldn't start and showed all the symptoms of starvation.

I confirmed fuel in the tank, took the cowlings off and confirmed that the throttle and mix levers were having the desired actuation in the engine, and tried to run the aux pump from the cross-fed right tank, without success.

Called the mechanic and his instant reaction upon hearing the sound of the pump was that it wasn't the aux pump. Since the only change had been the installation of the mechanical pump, he disconnected and reconnected all lines to that pump, disconnected and reconnected the fuel line that goes into the firewall to feed the fuel-flow gauge, and disconnected and reconnected the fuel line that goes into the spyder.

We then tried it and the aux pump worked fine. We're stumped!

At one point my mechanic wondered if the just-overhauled mechanical pump might have failed, inhibiting flow from the aux pump. But to me that doesn't make sense, because you'll recall that the aux pump is there not only to prime the engine for starting but also as a backup in flight for a failed mechanical pump -- so how can a failure of the mechanical pump inhibit the operation of the aux pump? Moreover, after the aux pump started working, the engine started fine and the mechnical pump worked fine.

Suspects: The aux pump failed and then recovered. There was an obstruction in a line. Last (and unlikely) suspect: the mechanical pump.

Because essentially all of my flights are to the Bahamas -- where there are no aux pumps for sale -- I don't want to get stuck, so I want to change the pump if indeed it was the pump that hiccuped.

Would be very grateful for any thoughts.

Ernie

[skymstr02]

There may have been an air bubble in the line preventing the pump from working. This is called cavitation. The pumps will not pump air. Was the fuel selector turned OFF after the previous flight?

Basically, I would chalk this up to loss of prime on the pump.

[jchronic]

Ernie-
I concur re pump cavitation. Not quite the same critical situation, but I once had the same type of occurrence with the auxiliary engine on my sailboat with a slug of air trapped between the primary fuel filter/lift pump (analogous to the airplane aux pump) and the engine-driven fuel pump. Bleeding the system a couple of times cured it.

That said, I'd fly the airplane locally an hour or so before taking off for the islands.

Ahab

[Ernie Martin]

Thanks to both. I think you're right.

Ernie

[hharney]

Ernie
Just out of curiosity did you move the fuel selectors at anytime during this incident? I think someone asked that above but you didn't mention anything about the selectors.

[Ernie Martin]

Yes. No one asked, but I mentioned it in my report, that I had cross-fed, but it still didn't pump.

Ernie

[Roger]

I vote for vapor/air lock. Keep in mind that the pumps will sound different if they have liqiud running through them, vs just air, which may be what sounded confusing to the mechanic.

[hharney]

Originally Posted by Ernie Martin

Yes. No one asked, but I mentioned it in my report, that I had cross-fed, but it still didn't pump.

Ernie

If you look at Skymaster O2 post he was asking if you happened to turn them off. When the selectors are off or in AUX the pump is not fed. Of course your vintage doesn't have AUX, right?

[Ernie Martin]

Sorry, I missed that question. But, no, the selectors were not turned to OFF after the previous flight. I checked that, and I also checked that there was fuel in the tank by cross-feeding the other engine and running that engine's aux pump until I saw fuel dripping from that engine.

Ernie