Fuel Cut Defender Explained


Cautionary note

Increasing the power and performance of any car should be considered carefully. Modifications to your car should only be carried out within and in accordance with the manufacturers safe operating tolerances. Standard suspension and braking systems can often become compromised if the factory engine power output is exceeded or driving characteristics place constant and heavy load on these areas. In addition modifications to your car should be notified to your insurance company otherwise you may invalidate your insurance policy.

The use of a fuel cut defending device (FCD) such as this allows excessive boost levels to be achieved which places additional stress on the engine, turbocharger and drive-train of your car and could cause serious damage to any of these components.

The author accepts no responsibility whatsoever for any direct or indirect or consequential loss, injury or damage to any persons, equipment or property arising from the use or fitment of this FCD.

Be careful and please read ALL of these instructions.

What is a fuel cut defender ?

A fuel cut defender or FCD is a small electrical device which modifies the TPS (turbocharging pressure sensor) signal received by the ECU. The modification of this signal means that the ECU no longer sees what it considers to be excessive boost levels. As a result the ECU does not 'cut fuel' and the turbo boost levels can be raised to levels in excess of the recommended factory limits.

This picture shows the FCD  Fuel Cut Defender (FCD)

How does the ECU cut fuel ?

The boost level is sent to the ECU via the TPS signal wire which has a range between 0 and 5 volts. The voltage level is directly proportional to the boost level so as boost rises so does the voltage - in a linear fashion. Depending on the ECU settings, determined by car model and year, fuel cut will occur once a set voltage level has been reached, corresponding to the maximum boost level 'programmed' into the ECU.

Check out this page for a great explanation of fuel cut - including some additional details.

How do I know if I have hit fuel cut ?

Quite simply - you WILL know !

The engine will cut out when the boost limit is reached. The harshness of this action is dependent on how hard the car is being driven at the time. If you are driving hard on full boost ALL power will be lost causing severe deceleration and possible jolting of the engine. The ECU will allow the car to be driven off boost but as soon as boost is registered acceleration will be prevented. The engine 'check' light will normally be illuminated and the ECU will surpress turbo boost until it is either reset or the ignition is switched off and back on.

NOTE: During the adjustment procedure fuel cut is to be expected and to experience it is a valuable lesson. Fuel cut during overtaking can be an unnerving, not to mention dangerous, experience, particularly when it isn't expected.

How does the FCD work ?

The FCD simply modifies the TPS voltage signal 'fooling' the ECU into thinking that boost levels are lower than they actually are, thus preventing fuel cut. The FCD monitors the voltage signal and, once it has reached the level set by the adjuster, the voltage is 'clamped' so that it does not rise any higher. Providing the FCD is adjusted to a voltage level slightly below the ECU fuel cut point, the FCD will prevent the ECU from ever cutting fuel. This will allow the boost level to be increased to any desired level - subject to fuelling, turbocharger and engine limits.

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