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Old 11-19-2019, 03:22 PM
Starlifter Starlifter is offline
Join Date: Aug 2016
Location: Granbury, TX
Posts: 29
Default Fuel / Pressure Issue

Equipment: RV-7A / Superior IO-360 / Airflow Performance FI / LW 15473 Dual Diaphragm Fuel Pump

Symptoms: Often, in-flight, I’ll notice a drop in fuel pressure into the yellow range around 17 – 19 psi. I turn on the aux pump and pressure returns to the normal range, and after turning the pump back off, all will be well for an indefinite period, but normally anywhere from several minutes to well over an hour.

On a few occasions, if I have not “caught” the low reading, it has dropped as low as 8 psi and the engine has sputtered. Again, turning on the aux pump returns pressure to normal for another indefinite period.

The second issue, not as unsettling as the above, is after starting a warm or hot engine, often after topping the tanks, the engine RPM will surge cyclically, at an even rate, by around 100 – 200 RPM, at idle or very slightly above. Once out of the near-idle range, there is no issue, ever, and in-flight operation is normal.

Occasionally, I have seen a drop or two of fuel on or below the "witness tube" on the pump which indicates a loss of integrity of the diaphragm, but this is not consistent, and the pressure issue now happens most every flight.

As I did not build this airplane, I am hesitant to do any serious troubleshooting of the fuel system without a more thorough understanding of its operation.

Speaking with Tempest, it is recommended to replace the fuel pump after 10 years of operation. Mine is 13 years old so I will replace it. I've heard it can be tricky so any tips or tricks would be appreciated.

I appreciate any insights you may be able to provide.
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Old 11-19-2019, 07:05 PM
AlexPeterson's Avatar
AlexPeterson AlexPeterson is offline
Join Date: Sep 2005
Location: Maple Grove, MN
Posts: 2,198

Here is a good thread on replacing the fuel pump.

If your diaphragm is leaking fuel, you should not be flying it. You are having classic symptoms of a bad fuel pump.

Be aware (I believe it is discussed in the above thread) that there are dangers to not getting the pump installed correctly (lever inside the engine can be damaged...).

I've replaced mine again since that thread was started, and it was quite a bit easier than the first time! Biggest pain is getting/modifying an allen wrench so that it will go into the screw heads without a side bending load. I ground one down on one side where it interferes with the pump casting. A Bondhus brand ball driver would not go into the sockets on the bolts on my plane, just too snug.
Alex Peterson
RV6A N66AP 1600+ hours
Maple Grove, MN
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