Page 11
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Technical description 200-9
Line pressure regulator
The line pressure regulator ensures that the
pressure in the circuit remains constant when
the fuel pump is in operation and also controls
the recirculation of fuel to the tank. When the
fuel pump is switched off, there is a rapid drop in
pressure to approx. 2.5 bar (36 psi), at which
pressure (the rest pressure) the regulator closes
the return line and the rest pressure is main-
tained by means of the ‘0’ ring seal and the quan-
tity of fuel contained in the fuel accumulator. The
purpose of the rest pressure is to prevent the fuel
from vaporizing in the circuit when the engine is
warm, making restarting difficult.
The line pressure regulator forms an integral unit
with a shut-off valve to which the return fuel line
from the control pressure regulator is con-
nected. When the fuel pump is operating, the
shut-off valve is actuated mechanically by the
liné pressure regulator, whereupon the return
fuel from a control pressure regulator by-passes
the shut-off valve to the return line.
$ 7372
Line pressure regulator with fuel pump in opera-
tion
$7373
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Line pressure regulator with engine off
MB iine pressure
Cc] Return, no pressure
Mi Rest pressure
tc $013
Line pressure regulator, fuel pump operating
1 Line pressure regulator
2 Shut-off valve
A Line pressure
B Return line
C Control pressure return
Saab 900
Page 12
200-10 Technical description
When the fuel pump stops operating and the line
pressure regulator valve is pressed into its seat-
ing, the shut-off valve is also pressed into its
seating, preventing the fuel flowing back
through the control pressure return.
Warm-up regulator
When the engine is warm, the warm-up regulator
(a spring-loaded diaphragm valve) maintains a
constant control pressure above the control
plunger. When the engine is cold and requires a
richer fuel/air mixture, the control pressure is de-
creased, allowing the control plunger in the fuel
distributor to rise, and more fue! to flow to the in-
jection valve.
When the engine is cold, a bi-metal strip reduces
the spring load of the diaphragm. This causes
the diaphragm to open and more fue! to flow
through the recirculation line to the fuel tank,
thus reducing the control pressure. When the en-
gine is running, current flows through the coil
which surrounds the bi-metal strip. As the bi-
metal strip heats up, it bends away from the
spring, and the pressure on the diaphragm, and
thus the control pressure, increases. When a
warm engine is started, there is no reduction in
the control pressure, as the bi-metal strip is then
affected by the engine temperature.
Line pressure regulator, fuel pump idle
A Line pressure
B Return line
C Control! pressure return
¢ 4
4 $ 4477
Warm-up regulator, cold engine
[35555 Control pressure
Return, no pressur
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1 Diaphragm
2 Push rod
3 Pressure spring
4 Bi-metal strip
5 Heating coil
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$ 4478
Warm-up regulator, warm engine
(253550 Line pressure
Return, no pressure
Saab 900
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Page 13
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Technical description 200-11
Regulation of control pressure
Fuel under control pressure is removed from the
system via an orifice (a) in the fuel distributor.
The control pressure is reduced to 3.7 bar (54
psi) and 0.5-3.7 bar (7-54 psi) during the warm-
up period due to the pressure drop across the
orifice (a), which is determined by the amount of
fuel flowing through the orifice and the warm-up
regulator. An additional orifice (b), located be-
tween the control pressure passage and the top
of the control plunger, is designed to eliminate
rapid oscillation of the air flow sensor lever.
Fuel booster (cold-start) valve
The cold-start valve, located inside the throttle
housing, is in communication with line pressure.
Actuated by a solenoid, the valve is controlled by
a thermostatic time switch that senses engine
temperature. The fuel booster valve is actuated
when the starter motor is running or the fuel
booster pressure switch contacts are energized.
At temperatures of -20°C (-4°F) or below, the
valve can inject fuel for a maximum period of
9.5s. At higher engine temperatures the injec-
tion time becomes progressively shorter and the
valve ceases to operate at a temperature of ap-
prox. 45°C (113 °F).
Thermostatic time switch
When the engine is cold, with a temperature
below 45°C (113°F), the switch should close al-
lowing current to flow for some time (depending
on the actual temperature) when the starter
motor is running.
Check that the switch closes when the starter
motor is running by connecting a test lamp
across the terminals of the unplugged connec-
tor.
Because it is not possible to make a more accu-
rate check of the time the switch is closed or of
the temperature, if in doubt, fit a new switch.
Control pressure regulation
GE Line pressure
a Line pressure -0,1 bar (-1.5 psi)
Hig Injection pressure
Control pressure
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Fuel booster (cold start) valve
Saab 900
Page 14
200-12 Technical description
Injection valves
The injection valves, fitted in the inlet manifold,
inject fuel continuously upstream of the engine —- .
inlet valves. A spring-loaded disc and needle .
valve inside each injection valve opens when the
fuel pressure reaches approx. 3.3 bar (47 psi). A.
Also incorporated inside the valve is a fuel mer
Strainer.
Injection valve
1 Fuel strainer
2 Valve disc
3 Needle valve
Air induction system
Air induction system
1 Aircleaner 4 Throttie housing
2 Air flow sensor 5 Inlet manifold
3 Rubber bellows 6 Auxiliary air valve
Saab 900
Page 15
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Technical description 200-13
Air cleaner
The air cleaner, which contains a paper element,
is fitted to a bracket on the front of the LH wheel
arch. The air flows through the inlet pipe into the
air cleaner, through the air filter and upwards to
the air flow sensor, which is bolted to the top of
the air cleaner.
Airflow sensor
The air flow sensor consists of an air venturi tube
inside which an air flow sensor plate moves. The
air flowing into the venturi from the air cleaner
lifts the air flow sensor plate, allowing the air to
flow through. The greater the flow of air, the
higher the sensor plate is raised.
The air flow sensor plate is fitted to a lever which
is compensated by a counter-weight. The lever
acts on the control plunger in the fuel d istributor,
which is pressed down by the control pressure, Air flow sensor
thus counteracting the lifting force of the air flow
sensor plate.
34483
The lever acts on the contro! plunger in the fuel
distributor by means of an adjustable link with a
needle bearing at the contact point. The basic
fuel setting, and thus the CO setting, is adjusted i
by means of the adjustment screw on the link. ‘
This adjustment is made by means of a special
tool and access to the screw can be gained |
through a hole in the air flow sensor between the i
air venturi and the fuel distributor.
sm
The air/fuel mixture varies with the load. The in-
clination of the venturi walls therefore varies in The venturi in the air flow sensor
Stages in order to provide a correct fuel/air mix-
ture at all loads. Thus, the mixture is enriched at
full load.
Rubber duct connector
The rubber duct connects the air flow sensor to
the throttle housing.
Page 16
am a at |
200-14 Technical description
Throttle housing
The throttle housing is connected to the inlet
manifold and, in addition to the throttle valve, it
contains the idling air passage and the idling ad-
justment screw, connections for the hoses to the
auxiliary air valve and for the cold start valve, and
the vacuum outlet for ignition advance.
Inlet manifold
The injection valves, mounted in the inlet man-
ifold, inject the fuel into the inlet passages at the
joint between the inlet manifold and the cylinder
head. A thermostatic time switch or simple ther-
mostatic switch is fitted inside the inlet man-
ifold, which also incorporates outlets for lines to
the brake servo unit and for crankcase ventila-
tion.
Auxiliary alr valve
The function of the auxiliary air valve, together
with the warm-up regulator, is to compensate for
losses due to friction and condensation in the
inlet manifold and combustion chamber on cold
starting, so that the required idling speed will be
obtained. The valve is located in a passage
which by-passes the throttle valve. The air flow-
ing through the auxiliary air valve has also flowed
through the air flow sensor, so a fuel quantity is
obtained which corresponds to the air flow.
The valve is actuated by a bi-metal strip which
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$4485
Auxiliary air valve, cold engine
1 Bi-metal strip with heating coil
2 Valve
3 Auxiliary air port
Saab 900
Page 17
7 a
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Technical description 200-15
opens the valve completely when the engine is
cold. When the engine is started, current flows
through a coil and heats up the bi-metal strip,
gradually closing the valve. When a hot engine is
to be started, the engine temperature acts on the
bi-metal! strip and the valve remains closed.
Auxiliary air valve, warm engine
Saab 900
Page 18
200-16 Technical description
Oxygen-sensor (Lambda)
regulated system
In addition to the standard components of the Cl
fuel-injection system, the oxygen-sensor-regu-
lated (Lambda) system incorporates the follow-
ing components:
1 Electronic control unit {ECU)
2 Throttle-position sensor
3 Oxygen sensor
4 Catalytic converter
5 Modulating valve
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Saab 900
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Page 19
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Technical description 200-17
The oxygen sensor, fitted in the exhaust man-
ifold, senses the amount of oxygen in the
exhaust gases and sends this information to the
ECU.
The ECU, via the modulating valve, controls the
pressure difference in the fuel distributor to keep
the air/fuel mixture within the Lambda 1 range.
Lambda 1 is the optimum air/fuel mixture for
complete combustion. Lambda, the Greek letter,
denotes the ratio between the actual (delivered)
and theoretical air/fuel mixture.
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13 14,5 16 kg air/kg fuel
$ 5708
Provided that the air/fuel mixture is kept within
the Lambda 1 range under ali conditions of en-
gine load, the harmful substances (CO, HC and
NOx) in the exhaust wil! undergo chemical reac-
tion in the catalytic converter, keeping emis-
sions inside the prescribed limits.
Oxygen sensor
The oxygen sensor, whose outward appearance
is reminiscent of a sparking plug, consists of a
primary cell and a solid electrolyte. The electro-
lyte is a ceramic material (zirconium oxide) and
has been temperature stabilized by the addition
of a small amount of yttrium oxide.
" Saab 900
Page 20
LSE g&@j{ ee
200-18 Technical description
It is tubular in shape, with one of the ends
blanked off, and its surface has a platinum coat-
ing to make it electrically conductive.
The outside of the electrolyte is in contact with
the exhaust gases and the inside in communica- ae a oe
tion with the ambient air. When there is a differ- aca EER SIRE
ence in the partial oxygen pressure between the ra =
inside and outside of the tube, the signal voltage
from the oxygen sensor rises.
When the sensor signals a high level of oxygen
(lean mixture) to the ECU, the ECU compensates
by changing the pulse ratio of the modulating
valve to enrich the mixture or vice versa when the
sensor signals a low level of oxygen (rich mix- 1 Electrolyte
ture). 2 Exhaust gases
3 Ambient air
The oxygen sensor does not become operative
until the working temperature exceeds 300°C
(572 °F). Because the accuracy of the sensor de-
clines as the sensor ages, the sensor must be re-
placed after 40,000 km/25,000 miles (this does
not apply to electrically heated sensors).
Catalytic converter
The final phase of exhaust emission control con-
sists of a catalytic converter, located in the front
section of the exhaust pipe. It consists of a
honeycomb ceramic insert, the walls of which
are coated with catalytic material (platinum and
rhodium),
Cars equipped with catalytic converters must
never be run on anything other than unleaded
petrol, because lead destroys the active con-
stituents of the catalyst and is also detrimental
to the oxygen sensor.
Catalytic converter
1 Ceramic insert
2 Passages
3 Catalytic coating
Saab 900