Fuel Pressure Fluctuations

[Rfisher30]

I'm dealing with fuel pressure fluctuation on a Garmin G3X 4.0 to 6.2 every few seconds. Kitfox S7 with 912 ULS; return line installed. Engine RPMs are consistently stable and the measured psi is stable. I’ve read the SBs. Checked for fuel line leaks, loose connections and weak crimps - none. Re-crimped sensor connections - no joy. Considering replacing the Kavlico P4055-15G 0-15 psi sensor, but they’re no longer available. I checked the Garmin Installation Manual Table 21-1 for an alternate. In talking to Garmin about their 0-15 psi sensor I uncovered a part number error in their G3X Installation manual, but it still comes up with a different sensor with a much larger operational range.


Has anyone else experienced the problem? I'm looking for suggestions on how to proceed or an alternative part.

[jiott]

This may not apply to your fluctuation problem which you say happens every few seconds. The fluctuation problem I had occurred randomly and usually lasted for several minutes, but maybe what I learned might help someone else if not you.

One thing worth checking if you are having fuel pressure fluctuation problems, especially low pressure indications, is the fuel pressure sensor electrical connector boot. The Kavlico sensor, and probably others, have a ribbed sealing gasket to keep dirt and moisture out of the electrical connection pin area. The sensor measures pressure by comparing fuel pressure on the nose of the sensor against AMBIENT pressure, the difference being actual fuel pressure. The sensor senses AMBIENT pressure on the backside of the sensor in the electrical connection area. The problem occurs If that sealing gasket is too good and a higher than real ambient pressure gets trapped in that area. This often occurs when in a climb and the higher low altitude ambient gets trapped in there and cannot bleed off fast enough. As you can tell, this causes a lower differential pressure which then reads out on your panel as a false low fuel pressure. For the sensor to work properly, that "sealed" area must breath to always reflect true ambient pressure.
I had this problem for years on my Rotax engine, transient low pressure readings that always corrected themselves after a few minutes ( the gasket would eventually breath enough). Someone pointed out this potential cause to me and I made a very small cut in the gasket with a razor blade to allow better breathing and WALAH no more false low pressure readings. Try it!

[Rfisher30]

Thank you for the response.

I talked with Brandon at Kitfox who suggested cleaning the fuel filter which on inspection looked pretty clean. I cleaned it anyway, then demated/cleaned the fittings and reconnected the fuel lines. No joy. Lockwood Rotax did not have a solution. So I’m at in a quandary on how to proceed.

Any help appreciated.

[Rfisher30]

[QUOTE=Rfisher30;110953]Thank you for the response.

I talked with Brandon at Kitfox who suggested cleaning the fuel filter which on inspection looked pretty clean. I cleaned it anyway, then demated/cleaned the fittings and reconnected the fuel lines. No joy. Lockwood Rotax did not have a solution. So I’m at in a quandary on how to proceed.

Any help appreciated.

UPDATE: I ordered a replacement Garmin 0-15 PSIG Fuel Pressure Sensor but when I removed the suspect Kavlico sensor I observed foreign matter in the orifice of the sensor (see photo). The material appeared to be a soft solid blocking on top of a glutinous ‘goo’. I cleaned the orifice with swabs (photo), flushed it with carburetor cleaner and cleaned the opening where the sensor attaches. I also flushed the fuel line to ensure it was clean, reinstalled the sensor and after checking for leaks, ran the engine. Fuel pressure at idle was a steady 5.3 PSIG rising to 6.0 PSIG that stabilized at 5.8 PSIG at 4,500 RPM. I’ll confirm the suspected problem.

I suspect what I found was a combination of fine particulate that passed thru the fuel filter (dirty gas) and ethanol/water that you see when testing gas for ethanol. I generally fly with 100LL and occasionally with 95 Octane gasoline which contains up to ~8% ethanol but I cannot be sure.

See pics. I appreciate the help.

[Shadowrider]

Good find!