Fuel Pressure Low

“Where is your fuel flow sender mounted, on the pressure side of the pump? Everything on the pressure side should be less prone to vapour bubbles. I have mine mounted on the pressure side and after the return line.”

While it’s true that the low pressure lines are initially more susceptible to vapor, the bubbles can start there and once they reach the pump it will then lower the pressure downstream of the pump also. This can create a fairly sudden drop in pressure downstream of the pump, and that pressure drop itself can cause additional bubbles to form in the line after the pump.  In other words, a reduction in pressure in itself can worsen the problem. This would explain why the boost pump effectively solves the condition. 

An extreme example of this is clearly demonstrated with the high pressure fuel system of the injected engines.  As the fuel exits the pressure regulator it goes from about 43 PSI to 1 or 2 PSI and a constant stream of bubbles is formed in the return line, and ultimately vented via the fuel tank. This is why the return line on the injected engines must enter the fuel tank above the fuel suction line, so that these bubbles are not ingested back into the fuel system. 

All good comments Jeff,

I have doubts about your hypothesis, in my context.

As stated earlier/example - Just returned from a 3 hr round trip - Outbound neary a problem - all systems good. Return leg - in Cruise (no Boost pump), about three low pressure indications . That's when I first noticed a rise in fuel flow. Application of Boost pump "fixed" /brought the indications back to normal immediately.

Other times, I have seen the pressure drop and the slow rise back to normal, without Boost pump.

I would expect that low fuel pressure would be most likly in the TO/Climb Out (Boost on) phase of flight - not so. Not even in Cruise step climbing (no Boost). Most indications have been in Cruise / level flight. I think I may have had one or two indications on descent.

Not once in many hours has the engine delivered anything less than smooth power.

I suspect an electrical problem.

About to do an interim (50 hr) service - will inspect the fuel reticulation system AND the pressure sensors, Dynon modules for good electrical connection😈 

This has been mentioned before, but bears repeating: The Kavlico (and possibly other brands) fuel pressure sensor references fuel pressure to ambient pressure. The ambient sensor port is inside the electrical plug area, which is sealed by a soft rubber gasket. This can cause a false "ambient" pressure to become temporarily trapped in that area. This is especially critical when you are climbing or descending and the ambient pressure is changing quite a bit. As you climb (when most of us have seen the problem) a higher pressure from the lower altitudes become trapped behind this gasket, causing the "gauge" pressure that you see on your display to read too low. After a short while it usually leaks past the gasket enough to restore normal readings. 

The answer to this for many of us has been to either remove the rubber gasket, or as I did just cut a very small notch in it, so it can breath freely. Once I did this, I had no more low readings, compared to low readings on a regular basis before. This may or may not solve the problem for everyone, but I sure would not waste my time on other solution attempts without doing this simple thing first. 

Thanks James,

I modified first and ultimately removed entirely, the Kavlico fuel pressure sensor seal/rubber gasket - no change😈

As RW mentioned earlier, try one tank of Aviation fuel and see what happens.  Simple test and you have to buy fuel anyway.