FUEL FLOW FLUCTUATIONS
The 0-360 engine in my RV-8A is experiencing fuel flow fluctuations on the ground and in flight, as measured by the EI FP-5L flowmeter/totalizer. The changes in the flowrate correspond to expected changes in EGT. For example, on a recent flight the flowmeter read 11.7 gph on takeoff with throttle and mixture full (previously it would read 13-14). EGT is 15 or higher and was 13.5 previously. Leveling off at 2,000, the flow continues to read 11.7 +/- with RPM at 2400 and manifold pressure 24 but mixture full rich. Slowly over 3-5 minutes (with no change in any of the settings), the flow will creep up to the 12-13 range and EGT will decrease to 13 or sometimes less. After pulling the mixture back to 50 rich of peak, the flow will vary in the 11-12.5 range with no change in mixture or throttle. During periods when the flowmeter is showing low flow and the EGT is high with the mixture at full rich, the only way to reduce EGT is with the throttle.
The diagnostic steps that I have used are as follows: 1) At idle after start, fuel pressure is about 2# using only the mechanical fuel pump. With the boost pump on, it will go to 6#. After the engine has run for 5-10 minutes, the pressure will go to 8 or 8.5# with the boost pump on. In flight, the pressure is about 6# with or without the boost pump. Turning the boost pump on has no effect on the flow; 2) In spite of the fluctuations in the fuel flow rate, the totalizer is dead on the fuel used (within 3%) after a flight. If the flow meter was wrong, then I would expect the fuel used to be wrong and I would not be seeing EGT changes with fuel flow changes; 3) Fuel flow is not impacted by switching tanks; 4) The fuel filter was removed and found to be pristine; 4) the air filter was cleaned during the annual 2 months ago, but I have removed it and no obstructions were found; 5) the carburetor appeared to be original (17 years old) with no recorded maintenance in the logs. The carb has been replaced with a rebuilt carb and no outwardly evident defects were found with the old carb. The engine flow behavior was not improved by the replacement of the carb.
Is this problem caused by a slowly failing mechanical fuel pump? I have not had to replace a mechanical fuel pump before, but I am told that this pattern of fluctuating fuel flow is not consistent with a failing fuel pump. I have been told the fuel pumps generally fail all at once.