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
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
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,