BMW DME Injector Cutoff Circuit Strategy
BMW has used the injector cutoff strategy in various forms for many years now.
The importance of this is that a technician may approach an engine with non pulsing injectors
and make it seems as if the DME itself is defective, then the issue is not the DME,
but an ulterior motive that it sees, causing injector cutoff.
Injector cutoff comes in two flavors, selective and complete.
Injector cutoff is a DME programming strategy.
It is programmed into the DME unit and may also render the injector completely inoperative.
Here are the conditions where the DME may use injector cutoff.
Condition number 1.
Engine or vehicle speed limiter.
For engine or vehicle speed limitation, the DME will deactivate injection for individual
cylinders, allowing a smoother limitation transition.
This prevents overreving, when the engine reaches maximum RPM, under acceleration, and
limits top vehicle speed to approximately 128 mph.
Condition number 2.
Idle speed actuation.
If a fault is detected with the idle speed actuator, the DME will initiate fail-safe
measures and selectively cut pulsation to the injectors, depending on the effect of
the fault, as in increased air flow or decreased air flow.
If there is a fault in the idle speed actuator circuit, the MDK will compensate to maintain
idle speed.
The check enigine lamp will be illuminated to inform the driver of a fault.
If the fault causes increased air flow, as in actuator failed to open, VANOS, and Knock
Control are deactivated which noticeably reduces engine performance.
Condition number 3.
Fault to the throttle controller.
In the event of an MDK or throttle drive by wire unit malfunction, the following actions
occur.
Activation of the check engine warning lamp.
MDK is deactivated, the throttle valve is opened mechanically by the springs.
To maintain vehicle control, the MDK opening is compensated for by closing the idle speed
actuator and retarding the ignition for engine power reduction.
Engine power is further limited by the DME using fuel injector cut off.
Condition number 4.
Clear flood mode.
Clear-flood mode is activated strictly by a wide-open throttle or high TPS input.
For this mode to be active, the DME must read a low engine speed value of under 600 RPM
and a throttle opening of 85% or higher, usually wide open throttle.
At this point, the DME reduces injector pulse to an A/F ratio of 20.0:1 or higher.
In most cases, the DME cuts injector pulsation completely.
All this is done in order to purge the cylinders of excess fuel and correct a flooding condition.
This mode is not used on all fuel control system, which is why some systems will flood
under heavy cranking.
A condition relating to this mode is an engine that will not start due to a high TPS voltage
and no injector pulse.
In such case, the DME will perceive the high TPS voltage as a W.O.T.
condition and activate the clear flood mode, thereby, reducing or cutting injector pulse
altogether.
The same can also happen if the TPS ground wire develops an open circuit, which would
cause a high TPS voltage input signal as well.
Condition number 5.
Selective injector cut off.
Selective injector cut-off mode is activated whenever there is a dangerous condition affecting
the engine.
This is a fairly new feature in fuel control systems, designed not only to protect the
engine, but the catalytic converter as well.
New regulations require manufacturers to guarantee the converter for up to 8 years or 80 thousand
miles.
This makes selective injector cut-off a prime feature for catalytic converter protection
in case of a misfire.
The selective injector cutoff mode is also used in torque management situations where
torque reduction is needed.
An example is the point when the transmission is shifting gears or the traction control
system is activated.
In these two examples, torque reduction is needed in order to protect the power train
components from sudden high torque conditions.
Another need for this operational mode is during high engine temperature conditions,
whereby, the fuel injection pulse is cut-off intermittently to four cylinders at a time
to prevent the engine from overheating damage.
By reducing the cylinder combustion intermittently, the engine generates less heat, saving it
from damage.
Condition number 6.
Deceleration fuel cut off.
When your vehicle is moving forward, with zero throttle and is in gear or engine braking,
the DME cuts off the fuel flow to the injectors.
This means you are experiencing forward movement with no fuel cost.
How long does it take the engine to enter and leave the deceleration cut off mode?
The transition takes between 500 and 750 milliseconds, depending on several other DME measurements.
What will make the DME leave deceleration mode?
Touching the accelerator, Dropping below 1,000 RPM and Putting the transmission in neutral
and disengaging the clutch will render deceleration cut off useless.