[Richard L]

I watched both videos. The action of the HFS-3 is working as designed. There are no surprise there.

In the absensce 12Vfeed to the fuel injector(+), the HFS-3 will be seeing the injector(-) as in full-on condition. The IDC% detection circuitry is looking for a digital wave, ie +12v or 0V.

I can see a solution: Why don't you separate the +12V supply to the fuel pump and the fuel injector(+). Relocate the fuel injetcor(+) to the other side of the relay. You still retain the safety aspect of cutting off the fuel at the fuel pump.

It is unlikely the fuel injectors are held 100% full on in the event of an accident.

Question: have you tried driving the car on load and see if the HFS-3 triggers? Or link FDC to see what the H3 picked up.