Bubbles in Return Fuel Stream
With 250 hours on the aircraft, I had my first experience with pump cavitation, likely from fuel vaporization. After climbing to 4500', I turned off the aux pump and the engine immediately lost power. Turning the aux pump back on kept the engine running, and within about 3 minutes the engine would again run on the main pump only. The plane had been sitting for about an hour in hot sun just before takeoff so the tanks were warm. However I was using an ethanol free fuel with an advertised RVP of 7. If the RVP of 7 is accurate, I should have had a lot of headroom against this happening.
Since this had never happened before, even in hotter weather at higher altitudes, I decided to do some fuel system tests. The aircraft is low wing, and the fuel pump is mounted low in a compartment below the pilot footwell. This is a cool space with aluminum 3/8” rigid fuel line from the pump to the selector valve and on to the fuel tanks. There is moderate gravity flow to the pumps and the inlet pressure at the pump is very close to zero gauge pressure when one pump is running. With both running the inlet pressure is approximately -.25 PSI (.5 Hg), a very slight vacuum that should not be enough contribute to vaporization.
I also did a return flow test, which showed showed 24 GPH with one pump, and 34 GPH with both pumps. While doing this test I ran the output line into a clear container and could see a steady stream of bubbles in the return flow stream. What I don't know is if this is the normal result of the fuel moving from a pressurized state to ambient pressure? Or, do I need to be looking for an air leak on the suction side?
