Fuel Pressure Low

“I just completed my maiden flight in my Kitfox with a 912ULS. It is a new recent engine purchased in 2012 with the latest fuel pump installed. I also have the fuel bypass return option installed. The fuel pressure sensor is a Kavlico purchased with the Rotax 912 sensor kit from Dynon. My pressure readout is on my Dynon Skyview screen. The aircraft is of course a high wing with tanks in the wings and gravity flow to the inlet of the engine driven fuel pump. I do NOT have an auxilliary electric fuel pump.

Everything ran fine on the maiden flight, but during WOT climbs the fuel pressure would hover around the low pressure redline (2.2 psi). As soon as I leveled out in cruise it would be in the green at 4.0 to 5.0 psi. At idle to 2000 rpm it climbs to 6.5 to 7.2 psi. Should I be worried or is this range of pressures normal?”

I believe this is very normal. As you increase power, the demand for fuel reduces the fuel pressure, as you level off, the pressure returns to normal. In a descent, fuel pressure builds up as power is reduced. Level off, normal fuel pressure. Very normal for a carbureted engine.

Hi Harold,

What you describe above, might be called "normal" response to fuel supply (pump/gravity) and engine power demand.

For the most part, I don't think the low pressure warnings/indication experienced by the writers, follows the pattern you describe.

My Rotax 912ULS/Dynon Skyview Classic, will indicate low fuel pressure at almost any stage of flight, no indication of engine fuel starvation (hesitation/missing) and usually the pressure will return to normal within a few moments. There does not seem to be a pattern. This phenomena started at about Hobbs 300 hrs. If the low pressure persists, I use my Boost pump which instantly returns the pressure readings to normal.

I have;

 # Drained the Kavlico sensor fuel line.

 # Located te sensor as high as possible in the engine compartment.

 # Removed the sensor plug seal

All to no avail.

The next step will be to replace the Kavlico sensor.😈

Further to my statement above:

Today I did 1.5 hr trip/return (3hrs) - all went well on the outward bound leg. On the return leg I had the first of 2-3 low pressure warnings. 

As in the past, I waited to see if the pressure would correct itself - not this time, down to 1.5 psi.

Just before I switchen on the Boost, pump to correct the situation, I noticed, for the first time, my fuel flow indication had almost doubled from 15 L/hr to 25L/hr.

I switched on the pump, pressure went from 1.5 to 5 psi in an instant, fuel flow moved slower but returned to 15L/hr.

This may be what you might expect from say water in a hose - When the flow is reduced, pressure rises. but not what I would expect from a fuel system with the only fuel outlets being two carburettors & a fixed return line jet.

Anyone care to try for an explanation?😈

Sean, perhaps vapor in the system.  Just my guess.  See if you can get some Avgas to try a bit, it has far less RVP than auto fuel.  (Reid Vapor Pressure) 

here is a simple explanation on a classic car web...https://www.prestigemoto.com/hardcore-tech/blog/vapor-lock-and-how-to-avoid-it.html#gref

Cheers