October 22, 2021

Fuel Cut FAQ

Turbochargers are positive feedback devices – the faster they spin, the more air they push into the engine. The more air they push into the engine, the more exhaust gases are produced. The more exhaust gases that are produced, and the faster the turbo spins. The faster they spin….

See the problem? The faster they go, the faster they go! Eventually something would have to break. To prevent this situation from occuring at all, turbos have a device on them called a wastegate that limits the turbo speeds to manageable levels. This is a pressure-activated valve that opens at a preset pressure and vents additional intake pressure either to the atmosphere or back into the exhaust stream.

Modern turbo design notwithstanding, it is still very possible for the wastegate on the turbo to malfunction, leading to exactly the ever-faster turbo behavior described above. Recognizing this, the designers of the DSM ECU built a fail-safe limiting mechanism into the DSM ECU modules. And it goes like this:

Should the ECU ever see an intake air mass greater than a certain preset level, it will stop fuel delivery and spark to the engine cylinders. This ‘critical mass’ depends on air volume, temperature and pressure, as measured by the MAS sensors. The act of the ECU cutting off fuel delivery to the engine is known as the infamous ‘fuel cut’.

Why’d they do this?? Well, should the wastegate ever malfunction in such a way as to allow the turbo to spin out of control, the ECU will stop firing the cylinders to save the turbo from exploding. This is not the only method the ECU has to limit the turbo operation, but it is the last and most desperate. The mass air limit at which the ECU will fuel cut was based off of the maximum amount of fuel that the stock fuel pump and injectors could deliver to the engine cylinders.

Unfortunately, those owners who purposefully modify their engines to provide greater power are deliberately forcing more air through their engines in order to produce more power. Recognizing the limitations of the stock fuel delivery system, serious modders usually upgrade the fuel pump and/or injectors in order to provide enough fuel capacity for the hopped-up engine.

However, the fuel cut limit, based on the stock fuel system, is pre-programmed into the ECU and cannot be changed. There is no method by which the ECU can be made aware of the improvements made to the fuel system, so it will blithely cease fuel delivery once the mass air intake reaches a predetermined level regardless of how much fuel capacity is really available. This characteristic of the DSM ECUs, while understandable, has been a source of nearly endless frustration for power-hungry owners.

There are solutions to this problem. Aside from reprogramming the ECU (which is sometimes possible) to eliminate the fuel cut, all of these solutions utilize the same basic principle – fool the ECU into believing there is less air entering the engine than, in fact, there is. This is often accomplished by altering one or more of the input signals sent to the ECU by the various air measurement sensors present in the MAS.

For example, a volume of air at higher temperature contains a smaller mass of air than an equivalent volume at lower temperature. The ECU determines the intake air temperature from a sensor in the MAS. Electrically altering this signal can make the ECU read a higher air temperature than actually exists. Based on this erroneous temperature, the ECU will calculate the mass air intake as being less than the actual amount.

Even expensive fuel control systems utilize this technique to keep the ECU from activating fuel cut. These systems, however, often intercept more than one ECU signal, and can provide their own monitoring equipment to allow the operator to properly observe the condition of the operating engine. This allows the owner to ‘fine-tune’ the car to achieve the desired characteristics while keeping the engine safe from harm.

Obviously, doing this type of modification is a relatively dangerous process. By changing the ECU inputs, the operator runs the risk of altering both open and closed-loop ECU operation to the point where the engine does not receive enough fuel. As always, it is up to the operator to monitor the engine operation to ensure that a damaging situation does not occur.