912ULS with 165 hours. A forum search found complaints of low fuel pressure but no one with my issue.

My fuel pressure has started to wildly flutuate, mostly 4 PSI plus and minus 1 PSI but occasionally down to almost zero for a second and then back to 4. The engine never reacts to any of this so I assume the carb bowls never get close to empty. Turning on the electric fuel pump does not stop the fluctuation. I cleaned the screen in the electric pump and the screen in the gascolator. All I found was just a very few specks. Nothing in large enough quantity or size to cause a blockage. I then replaced the fuel pressure sender (KAVLICO P4055-15G) with a direct reading gauge and did a ground run of the engine. During the ground test at various RPMs from idle to 5000, with the electric pump on or off, the pressure never went below 3 or above 5. I have Garmin G3X system but the sender is the same that Dynon uses. The Dyon website describes how some of the the Kavlico fuel senders were faulty so I replaced it with a new Kavlico unit. A test flight showed the same fluctuations. Then I watched a Rotax-Owner video on changing the mechanical fuel pump and learned Rotax cautions to not have the mechanical pump vent hose terminate in a high or low pressure area. Mine was positioned directly in the slipstream. So I shortened the hose. During my last flight during climb at 85 knots the fuel pressure fluctuated as bad as ever. However in level flight for an hour and descent for landing, the fuel pressure was steady as about 4.5 PSI.

I'm now thinking the mechanical fuel pump is the next thing to change.
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