I fitted a fuel pressure gauge between my fuel filter and metering head. It's proved very helpful in troubleshooting.

Sits on 80-90psi when running. Maintains about 30psi with the car off.

Not sure of the part your attempting to change. The black electronic bit on the back of the metering head? (Name escapes me) this regulates how much fuel gets through to injectors based on various information from sensors and fuel ecu.