Fuel Pressure Low

Thanks RW,

I am unaware of any seasonal changes to fuel mixture, in the Sydney Basin, New South Wales, Australia.

My 912 ULS fuel plumbing is pretty much as per the manual, with a fuel return line (#35 restrictor jet?).

The fuel lines themselves are all insulated with orange fire resistant sleeves.

The two "red Cube" fuel flow sensors, located above the engine crankcase, are not insulated.

The home made fuel distribution manifold, is as above.

My engine bay temperatures are a consistent 10 C above ambient eg14C OAT - 24C EBay.

I am familiar with fuel vaporization but have never experienced it in flight, only on the ground, very hot day, always after a short engine shut down.

What I am seeing is counter intuitive - pressure drops, flow increases (where could it be going?)😈

Sean Griffin wrote:

Thanks RW,

I am unaware of any seasonal changes to fuel mixture, in the Sydney Basin, New South Wales, Australia.

My 912 ULS fuel plumbing is pretty much as per the manual, with a fuel return line (#35 restrictor jet?).

The fuel lines themselves are all insulated with orange fire resistant sleeves.

The two "red Cube" fuel flow sensors, located above the engine crankcase, are not insulated.

The home made fuel distribution manifold, is as above.

My engine bay temperatures are a consistent 10 C above ambient eg14C OAT - 24C EBay.

I am familiar with fuel vaporization but have never experienced it in flight, only on the ground, very hot day, always after a short engine shut down.

What I am seeing is counter intuitive - pressure drops, flow increases (where could it be going?)😈

Hi Sean,

A couple of things to consider. Fuel vapour could be spinning the flow sensor faster than liquid fuel would be one explanation, perhaps there is a restriction on the suction side of pump causing the vapour?. Did the symptoms start around the same time as your cooling system changes?

GuDay Murray,

Thought you may have been at Holbrook yesterday, with your YGBN mate, in the very nicely painted, bright yellow, Sonex (put mine to shame). 3 Sonex, the third, a Onex from West Wyalong.

"....vapour could be spinning the flow sensor faster than liquid fuel..."

There's a thought. I would have guessed slower, due to density/viscosity difference, but then my engineering guesses are always suspect.

Now that you mention it the random low pressures did start after cooling arrangement changes however part of changes has been much lower Ebay temps & higher airflows, which should mitigate against heating of fuel lines. Will look into it.😈

The transducer is not necessarily spinning faster, although it might be. (my engineering guesses also aren't all that reliable)

I believe that the EI FT-60 fuel flow transducer ("red cube" as also supplied by Dynon and Garmin) operates on the principle of counting the vanes of a rotor interrupting a light beam. When there are vapour bubbles in the fuel stream, these can refract the light beam enough to cause false pulses to be sent to the EMS.

(from EI: "The FT-60 is compatible with gasoline, Av gas, diesel, jet A, kerosene and any other fluid with similar optical characteristics.")

A sudden increase in fuel flow indication at the same time the pressure drops is a hallmark of a fuel vapor problem.