What is a FCD?

FCD - Fuel-Cut Defencer

In In Diamond Star applications, this comes with the PFC. In most cases, it appears that you can't buy it separately. The operating principle of the device is that of a frequency limiter. It is connected to the output of the Air-Mass sensor pin 2 on turbo TLE's - pin 1 on Galant VR-4's). The output frequency from the FCD tracks the input frequency until it reaches the limit setting... the output frequency then stays the same even if the input frequency (coming from the AMS) continues to increase. By fooling the ECU into believing that Mass Airflow never exceeds a predetermined amount, fuel cutoff is avoided. Unfortunately, fooling the ECU into believing the engine is consuming less air than it really is, results in a leaned-out mixture once limiting has begun. Because the turbo DSM's are set up to run quite rich on the top end, no harm is done expect possibly in extreme circumstances.

(NOTE: Other "hacks" have been discovered that will serve the same basic purpose: Removing the lower honeycomb section, enlarging the lower AMS inlet, and adding resistors/pots to the baro sensor and thermistor circuits are all somewhat effective techniques.)

Important: Do not use any device to eliminate fuel cut. Fuel cut is there for a reason. If you are hitting fuel cut the ECU believes that you are moving more air than the fuel system can safely deliver. If you are having to eliminate fuel cut with some sort of defender either you have an inadequate fuel system, inadequate tuning tools, or a leak somewhere.

DO NOT decrease air counts without adding more fuel per air count. this will cause you to just run lean all the time and cause exactly what fuel cut tries to prevent. so....
DO NOT put in a fuel cut defender.
DO NOT run a 2g maf on a 1g without a way to balance it out.
DO NOT just lean out your afc to unsafe afrs.
DO NOT turn down your gm maft to unsafe afrs.
DO NOT try and put in larger injectors without fuel management.
DO NOT try to up the fuel pressure without fuel management.