Fiat 1998 Marea Weekend, Service Manual

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Marea-Marea Weekend®^
99 range
Engine
Fuel system
10.
These values are obtained experimentally by means of a long series of practical tests carried out on proto-
types at the engine test bench to identify the advances which give the best possible compromise between
the conflicting requirements of maximum power and minimum consumption and harmful exhaust emis-
sions.
The optimum advances have then been stored in the svstm control unit memory. Whilst the engine is run-
ning, the control unti is constantly informed of the (engine) speed and load conditions and, on this basis,
it "selects" the optimum advance from its memory for striking the spark at the spark plug for the cylinder in
the explosion stroke with the optimum advance.
In addition, the control unit corrects this value according to further factors such as the engine coolant
temperature, the intake air temperature, detonation and the butterfly valve position so that the ignition
point is always optimum.
The information required by the control unit for operating the single coils is transmitted by means of elec-
trical signals from the following sensors:
a. An rpm sensor which produces an alternating, single phase signal whose frequency indicates the en-
gine speed.
b. An air flow meter with a temperature sensor which, according to the quantity/temperature of the air
drawn in by the engine, transforms these values into electrical signals, sending them to the electronic
control unit.
c. Two detonation sensors which, located on the top part of the cylinder block/crankcase, one be-
tween cylinders 1 and 2 and the other between cylinders 4 and 5, allow the control unit to recognize
the cylinder where there is detonation (or the onset of detonation) and only correct the ignition ad-
vance for the spark plug of the cylinder concerned.
d. An accelerator pedal potentiometer which transforms the angular value of the actual pedal into an
electrical signal allowing the control unit to recognize minimum, partial and full load conditions.
Injection/ignition system functions
In addition to electronically controlling the moment of ignition and the air flow rate during idling, in order
to allow the engine to run smoothly as the ambient parameters and loads applied vary, the control unit
must control and manage the injection so that the stoichiometric (air/fuel) ratio is always within the opti-
mum value.
The electronic control unit establishes the "time" for operating the injectors using a relatively simple rule
which can be summarized as follows.
Taking the physical properties of the fuel (viscosity and density) and the difference in pressure between
the pressure of the fuel and the pressure in the inlet manifolds as constant, the quantity of fuel injected
only depends on the "opening time" of the injector.
The injection/ignition system basically performs the following functions:
- adjusting the injection times;
- adjusting the ignition advances;
- controlling cold starting;
- controlling enrichment during acceleration;
- fuel cut-off during overrun;
- controlling the engine idle speed;
- restricting the maximum number of revs;
- controlling combustion via the Lambda sensor;
- fuel vapour recovery;
- detonation control;
- phase transformer control;
- inlet manifold control;
- system self-adjustment;
- autodiagnosis;
- connection with engine immobilizer device
(Fiat CODE);
- management of the climate control system;
- management of the radiator fans.
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