The fuel flow system on a MU2 is as follows:

1). An engine mounted transmitter that counts the revolutions of a free spinning wheel producing a frequency signal.

2) A center wing mounted signal conditioner that takes the two frequencies (R&L) and converts them to voltages for the fuel flow gauges to read. It also produces a combined output for the totalizer

3). The Fuel flow gauges and totalizer

If a fuel computer is installed, its input is the same transmitter as the stock system. The wires are spliced into the wires between the transmitters and the signal conditioner. It then converts the frequency to a fuel flow number and sends it to the avionics through a serial connection.

In the case of Hilgards plane where the FF on his GPS navigator is correct but the gauge reads incorrectly, the problem is either the signal conditioner or downstream in the wiring to the gauges

In Will’s case the problem is most likely at the engine, either in the transmitter or wiring

As long as you are gathering information to present to the service center, look to see if activating the ignition or the unfeather pump affect the fuel flow readings

Unfortunately, the manufacturer of the system Foxburo is now,another company and a signal conditioner with a new circuit board is $$$$