What is an FCD?
Fuel Cut Defencer. This stops fuel cut from occurring at the factory determined boost level.
What is the boost level fuel cut occurs at?
The level varies from car to car. "Allegedly" it is meant to be between 12-14psi but I have found this to be quite variable, my car ~11psi.
What's the idea behind Fuel Cut?
Quite simple really! It is to protect the engine from damage. As the boost pressure from your turbo rises and you start smiling, several things happen, more power, more heat and more chance of engine failure under certain conditions, particularly detonation. If something goes wrong and the boost pressure created by the turbo is deemed too high then the ECU will stop fuel from flowing out of the injectors, saving your poor old pistons from getting a hole in them, or worse...
In summary - to stop boost pressure from rising above a certain point, thus protecting the engine.
How does the ECU know what boost pressure the turbo is producing?
Just right of the middle of the engine bay on the firewall is the Turbo Pressure Sensor, (TPS). Manifold pressure is fed up from a hose attached to the bottom right of the manifold up to the TPS sensor. As the manifold pressure changes this change is felt by the TPS. The TPS converts the manifold pressure into a voltage signal which is sent down to the ECU.
How does the ECU cut the fuel?
The signal from the TPS is compared to a preset voltage in the ECU, (4.4V). As the boost pressure from the turbo rises the TPS voltage also rises. The ECU constantly monitors this voltage. If the boost was to keep rising the TPS voltage would reach such as level as to cross the 4.4V threshold. The ECU will then disable the signal to fire the fuel injectors - thus cutting the fuel.
How do I know if I ever have fuel cut?
You will accelerate, boost will go up, the car will roar forward, if fuel cut then happens you will have to peel your face of the windscreen! The first time it happens to you it will feel very severe, you'll be quite alarmed, but you'll get used to it. The engine check light will also illuminate. Error code 34 will be shown. To see how to check your ECU for error codes click here.
How do I get around fuel cut?
There are several ways. Personally I recommend under no circumstance whatsoever do you disable or remove this feature. The best and safest way to raise the fuel cut level is by using a couple of resistors. This is not the only method, there are several ways you can overcome the fuel cut:
My 2p - if you disable the fuel cut and your wastegate gets stuck, a hose parts, or something goes weird and you make 35psi boost; what is protecting your engine? Answer - God. Make your own FCD with a couple of resistors just as I have. It's cheap, reliable, safe, looks crap but works very well.
If I wanted to install one how would I fit it?
Check out my How to make and fit an FCD article
The fitting /installing an FCD article will only deal with the safe, cheap resistor method. However, some of the article may well be relevant to one of the other methods. Please e-mail me if you know better!