Airflow Performance pump failure

Well, actually not so much a pump failure, as a pump failing due to something else, but I still ended up with a dead pump.

I run 91E10 mogas in my system, and thought I had cleaned out all the natural rubber seals and components on everything to avoid problems with the fuel attacking the natural rubber seals. I missed ONE O-ring on one of my fuel filters, coming in from the right tank prior to the pump, and after 285 hours it finally degraded to the point where it was leaking. At first I noticed a very faint whiff of fuel smell, weak enough I could have dismissed it as a figment of my imagination. A couple flight hours later I noticed my fuel pressure dropping rapidly on the right pump (dedicated to my right tank). It normally runs 45 psig constant - now it was dropping into low thirties and high twenties. Friday of last week I took it for a post-maintenance test flight and the pump would only provide 11 psi on runup.

I called BS and went back to the hangar and sure enough, the pump is cavitating - and temps were cool enough that the fuel is not going to be vaporizing from heat. I checked my fuel filters for blockage that could be causing the cavitation, and that's when I found one of them with the natural-rubber O-ring that was badly deteriorated. I remembered some discussion prior from Ross that said these pumps were quite allergic to cavitation and it would destroy the pump quickly, so I rigged up a test set and fed it clean fuel with no restriction on the inlet, and sure enough it would only give about 15 psig output - so the pump is nerfed.

Anyway - just a datapoint - cavitation (or air ingestion) is bad juju for these EFI pumps and natural rubber O-rings are only good for a couple hundred hours of exposure to 91E10. I'm in the process now of raping out the entire fuel system and installing an Andair full-duplex valve plus the SDS pump module and fuel regulator.

Hi Greg,

Was the degraded o-ring allowing air into the suction line and thus causing the cavitation? Or... was the o-ring causing an obstruction somehow?

Thanks for sharing!

digidocs said:

Hi Greg,

Was the degraded o-ring allowing air into the suction line and thus causing the cavitation? Or... was the o-ring causing an obstruction somehow?

Thanks for sharing!

Click to expand...

No, the O-ring was pretty much trashed and was allowing air to enter the body of the fuel filter, as it was on the suction side of the pump. The pump was objecting strenuously to that. I'd seen a few symptoms of that over the previous 2-3 flight hours and didn't put it all together until the pump quit making good pressure, then when I found that O-ring the light bulb went on.

Greg, any idea what actually fails inside the pump? Impeller, bearings, motor or maybe a combination there of? Have you opened it up? If so, what do the components look like?

Great write up. Thank you for sharing your experience.

I have not opened it up - but I would suspect the impeller caught the brunt of air bubbles in the incoming fuel, cavitation can destroy even the hardest of materials on a pump impeller. Granted I was ingesting air bubbles and therefor the damage method may be a little different but I wouldn't think it would be a lot different.

I do have the pump still and may open it.

I could have just replaced the pump and kept going - but my current fuel system will not allow me to access the fuel in one wing tank if I lose that pump - so with the right pump dead I lose that. I had already made up my mind to change that to a dual-pump module like SDS sells, and an Andair full-duplex valve, such that I have a pair of pumps now for primary and backup, and can still access all fuel in either wing even with a pump failure. I had already started gathering materials for that purpose when this pump went Tango-Uniform, so I just pulled the trigger on the change now.