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MR2 Turbo Fuel Cut

You're having a great time boosting onto the freeway. You shift into third and floor it and that's when it happens--you feel like you've hit a wall and the check engine light is on. "What's wrong with my baby?" you cry out! Worse yet, now every time you give it anything but a light amount of throttle, you seem to feel a hesitation from the car. Something is blown for sure? No, it's called fuel cut and it was built into your MR2 Turbo on purpose.

The stock CT-26 turbo that comes with the Turbo MR2 is physically capable of producing over 20 pounds of boost pressure above normal atmospheric pressure inside your intake manifold at 3200-3500 RPMs. This much pressure causes much more air and fuel than would otherwise be possible to enter your cylinders each time the intake valves open. This is what gives a 4 cylinder engine 6 or 8 cylinder power. Too much of a good thing can be bad however and to keep the engine running reliably for over 200K miles, Toyota added a boost control system to limit the boost pressure to about 12 pounds maximum. This system works by allowing the boost pressure to build up against a diaphragm which at a certain pressure will start to give and open a wastegate which lets exhaust from the engine bypass the turbocharger turbine wheel so that the turbocharger stops receiving energy to produce even more boost. On cool days when the air is dense or if the car has been modified with a performance exhaust system or boost controller to allow more exhaust energy reach the turbine wheel, the boost control system may have a hard time keeping the boost in check and the boost may creep above the maximum amount set by the Toyota engineers. To prevent this situation from persisting, the engineers built in a completely separate failsafe mechanism known as fuel cut.

The fuel cut mechanism works by using a pressure sensor that senses the amount of pressure in the intake manifold. This sensor works by producing an electrical signal that varies proportionally with the amount of pressure in the manifold. This signal is fed to two places. The first is the stock boost pressure gauge on your dash. This causes the little needle to go up when you  boost. The second is a silver box called the ECU (Engine Control Unit). The ECU is a computer and it controls your engine. It takes input signals from various sensors such as the manifold pressure sensor, makes quick calculations of what is happening, and then sends output signals to the fuel injectors, ignition and various other engine systems to command them to do the right things to keep the engine operating properly. One of the jobs of the ECU is to watch the signal voltage from the manifold pressure sensor and if it reaches a certain predetermined point enter fuel cut mode. When in fuel cut mode, the ECU stops telling the fuel injectors to inject fuel into the engine whenever the manifold pressure is above normal atmospheric pressure. This causes the engine to quickly lose power and steals away the exhaust gases that the turbine needs to produce more boost. It feels a lot like hitting a wall if you enter this mode while you were in full boost and like there a giant rubber band holding you back if you try to boost from an unboosted state. The ECU will stay in fuel cut mode until you turn the engine off. After that, the engine will be allowed to boost again but the ECU will have stored a code (code 34) in its memory that will not be forgotten until you disconnect the battery or pull the EFI fuse out for at least 15 seconds.

The fuel cut mechanism can be modified in several ways. The simplest is to cut the air hose that goes to the manifold pressure sensor and plug it up at both ends so that air can't get in or out. This approach has two big disadvantages. The first is that your stock boost pressure gauge will stop working because the sensor detects only nominal atmospheric pressure. The second is that the failsafe protection mechanism is completely disabled and you can boost to any pressure without the ECU preventing you. The first disadvantage can be eliminated with a slightly more sophisticated approach in which you either install a aftermarket fuel cut eliminator (GReddy makes one of these) or a special diode (called a zener) on your ECU or engine harness. These devices allow the pressure signal to reach the gauge and the ECU but prevent it from ever going above the voltage which puts the ECU into fuel cut mode. A more sophisticated approach is to increase the boost pressure at which fuel cut happens while not entirely eliminating it. This retains the extra layer of protection that Toyota designed into the car but puts it at a point that is more interesting to you. The HKS FCD and the very simple circuit described here both do this. The simple circuit is what I recommend because it costs 1/10 as much as the HKS gadget and is 10 times more adjustable. Both of these devices work by lowering the level of the manifold pressure sensor signal reaching the ECU. This will require the manifold pressure to reach a higher point before the ECU takes corrective action. Keep in mind, however, that the manifold pressure sensor can only measure up to 18 pounds of boost pressure, so there is no way that these devices can allow you to keep the fuel cut but raise it above 18 pounds.

Frequently asked questions:

At what boost pressure does fuel cut happen?

Approximately 12.8psi (pounds per square inch) at sea level which corresponds to a 4.2 volt signal on the manifold pressure sensor wire. Sometime late in the 1993 model year, the fuel cut was raised to 16psi.

I have a stock MR2 except for a boost controller and I've been able to boost over 12.8psi, why is that?

The manifold pressure sensor senses absolute pressure so the real fuel cut point is 14.7psi (normal sea level atmospheric pressure) plus 12.8psi which is 27.5psi absolute. If you live above sea level, the atmospheric pressure is lower and the fuel cut point will rise accordingly. In Denver, for example, you can boost to 15psi on a stock MR2 turbo without hitting fuel cut. Also, sometime late in the 1993 model year the fuel cut was raised to 16psi.

I have a boost controller, won't that eliminate fuel cut?

No. A boost controller lets you raise the point at which the stock wastegate will open and start to limit your boost, but it has no impact on the fuel cut system.

I have read that you have to disconnect the battery to reset the ECU after you hit fuel cut. Is this true?

No. Just stopping the engine clears the ECU fuel cut mode. You need to cut power to the ECU only to clear the code 34 that it has stored in its memory.

I had never experienced fuel cut until quite recently and now it seems to happen often. What's up?

Fuel cut will happen even on a stock MR2 turbo if conditions are right. Usually cooler days or mornings when the air is denser will provide the engine with enough extra power to creep above the stock boost settings and initiate the fuel cut mode. Upgrading your exhaust system or intercooler can also increase the efficiency of the turbo system enough to cause fuel cut to happen more often. If you moved from higher elevations closer to sea level, that will also increase the likelihood of fuel cut.

Someone told me that unplugging the manifold pressure sensor hose will cause problems because the ECU won't know when you are boosting and will not give the engine enough fuel to keep it safe. Is this true?

No, it is not. The ECU on MR2 Turbos that have an AFM (Air Flow Meter), which is that funny box that sits between the air filter and the turbo, uses the AFM's signal exclusively to determine how much fuel to use. No detrimental effects from unplugging the manifold pressure sensor hose and plugging it up properly have ever been validated.

Will fuel cut cause detonation since it will make the engine run lean from a brief moment?

No. Since the ECU cuts all fuel to the engine when it engages fuel cut, there is no fuel in the air fuel mixture to detonate. Pure air is just as safe for your engine as a properly tuned air fuel mixture is (more safe, actually).