Does your old computer's flow sensor have an optical eye that counts the rotations of a small ball.
It might be possible that in the return line the fuel contains also bubbles that are counted also. If The fuel gets heated before returning it can start boiling and creating bubbles. This is what I found out with my old Zemco fuel computer and a mechanical fuel pump.
I ordered my mpguino-kit from fundamental logic about 8 weeks ago and still haven't received my shipment.
