Aircraft RV-12, 912ULS, mogas, mechanical fuel pump about 200 hrs since new.
Can anyone explain this?
Today I performed a test that I have not tried before under these same conditions. I was at 6,800’ with a DA of 9,700’, OAT 82f. I was at 5400 rpm and turned off the electric fuel pump, after about 15 seconds the fuel pressure stabilized at .8 psi and the flow was 2.2 gph. At about 30 seconds into the test the engine started stumbling from fuel starvation. It never quit running but lost RPM and was very rough. I could smooth it out by dropping the RPM to 4900, (less fuel demand), or turning on the electric fuel pump.
Once I was back at my home airport with a generous altitude I tested again, same altitude and DA as before. The results were the same.
I have on occasion performed this test on cooler days at lower altitudes and never had a problem, engine runs fine and maybe a drop in fuel pressure to 3.5 psi.
Keeping the previous good test in mind I dropped to 4,000’ with a DA of 6,700, the OAT was 89f, engine RPM 5400.This time it was normal, no engine starvation, fuel pressure drop but not serious.
So my question is why does altitude have an effect on the pressure output of the fuel pump. This is clearly not an instrumentation issue since the engine was starved at 10,500 DA and not at 6,700.
