300Tdi Discovery
Tdi
5
DESCRIPTION AND OPERATION
Exhaust Gas Recirculation (EGR)
Operation of the EGR system is dependent on the
following:
a. Engine temperature - must be between 20
°
C
and 100
°
C approx.
b. Engine speed - must be between 630 and 2850
rev/min.
c. Engine load - calculated by throttle position
sensor.
d. EGR valve lift position.
e. Duration of engine idling.
Under varying engine speed and load conditions the
control unit sends a signal to open the vacuum
modulator which allows a vacuum to be applied above
the EGR valve diaphragm, the vacuum supply being
taken from a ’T’ connector in the brake servo hose.
This process is controlled by an engine speed/load
map stored in the EGR control unit memory.
Engine speed is measured by monitoring the
waveform present on one phase of the generator.
Throttle position is measured via a sensor mounted on
the fuel injection pump throttle lever. Closed loop
control is achieved by allowing the control unit to
continually monitor EGR valve lift via a position sensor
mounted on the valve; this valve lift is compared with
the actual valve lift required on the control unit map
and adjusted if necessary.
With coolant temperature between 20
°
C and 100
°
C;
the engine having just returned to idle, EGR will shut
off after 25-30 seconds idling.
