Fiat 1998 Marea Weekend, Service Manual

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Marea-Matea Weekend®*»
Engine
99 range
Fuel system
10.
Having reached this condition, the normal functions for idling and cut-off during overrun are only reacti-
vated when the fuel cut-off level is exceeded to ensure that the engine runs smoothly. The levels for re-
storing the supply and fuel cut-off vary according to the temperature of the engine.
Another fuel cut-off logic is developed in the control unit which intervenes in partial deceleration condi-
tions, i.e. when the engine load conditions decrease.
This function is only activated if the new condition persists for a pre-set time and after the ignition ad-
vance angle has been adapted to the new situation.
Engine idle speed control
The control unit recognizes the "idle" condition (accelerator pedal released) from the signal supplied by
the accelerator pedal potentiometer and according to the signals from the brake and clutch pedal switches
and the electrical consumers switched on (climate control and radiator fan) and operates the butterfly
valve motor to produce an idle speed of 700\26150 rpm.
The idle correction is also carried out through the variation of the ignition angle since it has a more
prompt effect.
The correct self-learning of the minimum and maximum butterfly opening positions is essential for the ef-
fective control of the idle speed; it should therefore be carried out at least once in production and every
time the engine control unit or the motorized butterfly casing is replaced.
The self-adjustment function makes it possible to adapt the idle adjustment to any variations in the idle
management components.
Restricting the maximum number of revs (protection outside of revs)
When the engine speed exceeds 6,800 rpm, the electronic control unit reduces the operating times for the
injectors to avoid overloading the engine also protecting it outside of revs.
If the engine speed exceeds 7,000 rpm, the control unit activates the "fuel cut-off" strategy, restoring the
operation of the injectors when the speed goes below a certain level.
Controlling combustion via the Lambda sensor
The Lambda sensor informs the control unit of the quantity of oxygen present at the exhaust and therefore
the correct air/fuel metering by means of a two stage voltage signal corresponding to a poor and rich
mixture.
To produce the ideal mixture for the operation of the three-way catalyzer with minimal emissions, the
petrol injected must correspond to a pre-set (stoichiometric ratio) with the intake air, measured by the
unitary value of the Lambda parameter (=1); if the mixture is lean it is > 1, if it is rich is it < 1.
The Lambda sensor, in contact with the exhaust gases, produces an electrical signal whose voltage de-
pends on the concentration of oxygen in the gases:
- if the mixture is lean (> 1), the sensor supplies a low voltage (< 200 mV) and the control unit slightly
increases the quantity of petrol injected;
- if the mixture is rich (< 1), the sensor supplies a high voltage (> 800 mV) and the control unit reduces
the quantity of petrol injected.
The closed-loop management of the Lambda sensor allows the control unit to keep the Lambda value very
close to the theoretical unitary value, also aided by appropriate self-adjustment functions.
The control unit also manages the supply of the Lambda sensor heater. In effect, on the basis of the en-
gine load and speed conditions, the control unit processes a forecast for the temperature of the exhaust
gases and consequently controls the heater supply current, restricting it to the actual requirements de-
pendent on the engine conditions.
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