Fuel Pressure Fluctuations

Could it be the return line related?

Thanks! Will look into that tomorrow and reply.

All return lines are good with zero kinks and zero leaks at any fittings.

We also messed around with the fuel selector positioning.

The only thing left is the Rotax fuel pumps, except strangely, there is no fluctuation when running on the right main with either pump on.   

Jason,

Is there any difference in the way the fuel pumps sound when running on one tank or the other?  Often you can hear pump cavitation if it’s happening. Typically cavitation will make the pump seem louder.  You can have an air leak on the suction side that won’t leak fuel but will draw in air bubbles.  Also, any restriction is the feed line to the pump can cause a pressure drop in front of the pump and cause the fuel to vaporize (boil) on the suction side.

You could install a temporary fuel pressure/vacuum gauge just before the pump inlet and see if the pressure drop in front of the of the pump is the same on both tanks.  Even a temporary section of clear fuel line before the pumps may allow you to see if any bubbles are forming when using the subject tank. I’m not suggesting flying the aircraft with these installed, it would be only for a pump test on the ground.  Also, make sure the fuel tank vent is clear. 

I really appreciate the information and will check on your ideas. The fuel pumps do sound slightly louder when on the right main, which is the side that doesn't fluctuate actually.. We just had our 100-hour inspection, and they, too, haven't figured it out and just believe it's the Rotax pumps. If it were the Rotax pumps, it is my understanding that the same fluctuation would occur whether in the right or left main.  We also considered changing out the sensor, but again, there is no fluctuation when on the right main.