Kevin -

I have experienced a similar problem on occasion. It seems to happen after
the fuel in the wing tanks has become heat-soaked from sitting on a hot,
sunny ramp. I believe that the pre-warmed fuel is prone to vaporizing under
suction inside the fuel lines as the ambient pressure drops. Like you, the
problem goes away if I turn on low boost. I have an air-blast shroud going
to the engine-driven fuel pump but not the gascolator. I too can turn off
the boost pump upon reaching cruise altitude, but that might simply be
because the fuel in the tanks has cooled off by that time.

You might want to just wait and see if the problem goes away during the
cooler months. The only other reasonable explanation for fuel pressure
fluctuations would be an air leak somewhere in the system. The engine-driven
pump is a rotary type, so fuel pressure would drop as the vanes wear, but
it's hard to imagine a scenario where that pump would cause the symptoms you
describe.

Adam Molny
Legacy N181AM
300hrs on airframe, 550hrs on IO550-N

-----Original Message-----
From: Kevin Stallard [mailto:kevin@arilabs.net]
Sent: Monday, September 22, 2014 8:59 AM
Subject: Mechanical Fuel Pump Oddities

Hi All,

Recently (within the last couple of moths) as I climb past 8,500ft, my fuel
pressure becomes unstable...

Thanks
Kevin