SEQUENTIAL MULTIPORT FUEL INJECTION (SFI–V8)
A1
CIRCUIT OPERATION
2
REV: 09/2001
Mass Air Flow Sensor (X105)
The Mass Air Flow Sensor (X105) utilises a ‘hot film’
element contained in the intake air tube to monitor
the mass flow of the air stream drawn into the
engine. It contains two sensing elements: one
element is controlled at ambient temperature, while
the other is heated to 200
°
C/ 390
°
F above the
ambient temperature. The air flow passes the heated
element and cools it down therefore lowering the
resistance of the hot film element. In order to
maintain a constant temperature the circuit to the
heated element has to supply more current. The
measured air mass flow is used by the ECM (Z132)
to determine fuel quantity to be injected in order to
maintain a storchiometric air/fuel mixture for
optimum engine performance and low emissions.
Throttle Position Sensor (X171)
This sensor is a variable resistor which determines
throttle angle position and angular velocity. The
signal is used by the ECM (Z132) to calculate fuel
injection duration under various operating conditions.
The closed idle switch position is used for idle speed
control in conjunction with road speed. Failure of the
Throttle Position Sensor (X171) will result in poor
idle and lack of throttle response. If the Throttle
Position Sensor (X171) fails in the “closed” mode,
then the engine will only revolve up to 1740 rpm
when the ECM (Z132) will initiate “over run fuel
cut–off”.
Heated Oxygen Sensors (X139, X160, X289, X290)
The Heated Oxygen Sensor consists of a ceramic
body of zirconium and yttrium which is coated with
gas–permeable platinum. If the sensor reaches
sufficiently high temperatures (above 350
°
C/660
°
F)
it generates a voltage which is proportional to the
oxygen content in the exhaust stream in comparison
with the ambient oxygen content. From this value the
ECM (Z132) can adjust the injected fuel quantity that
as to achieve the correct air/fuel ratio. This reduces
the emissions of Carbon Monoxide (CO),
Hydrocarbons (HC) and oxides of Nitrogen (NOX) to
acceptable levels.
Presently, two heated oxygen sensors are used, one
in each exhaust down pipe just before the catalyst.
In the event of sensor failure, the system will default
to ”open loop”. Operation and fuelling will be
calculated using signals from the remaining ECM
inputs. The fault is indicated by illumination of the
malfunction indicator lamp (MIL). ECM diagnostics
also uses heated oxygen sensors to detect catalyst
damage, misfire and fuel system faults.
North American vehicles have two extra heated
oxygen sensors mounted one after each catalyst.
These are used to determine whether the catalysts
are operating efficiently.
Idle Air Control Valve (M112)
The Idle Air Control Valve (M112) controls the idle
speed of the engine by moving the plunger a set
distance, known as a step. Fully open is zero steps
and fully closed is 180 steps. The motor moves each
step by sequentially changing the polarity to each of
the two coils.