Check On The Engine Idle Speed; Restriction Of The Maximum Engine Speed (Protection Outside Of Revs); Check On Combustion Using The Lambda Sensor - Fiat Bravo Service Manual

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Having reached this condition, the normal functions for idling and cut-off during deceleration are only
reactivated when the fuel cut-off threshold is exceeded to prevent the engine from not running s m o o t h -
ly. The levels for restoring the supply and the cut-off vary according to the engine temperature.
There is another fuel cut-off logic in the control unit w h i c h intervenes in partial deceleration conditions,
in other words when the engine load is lower.
The function is only activated if the new condition persists for a pre-set time and after the ignition a d -
vance angle has been adjusted for the new situation.

Check on the engine idle speed

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 being switched on (climate control and radiator fan)and operates
the butterfly valve motor to produce an idle speed of 7 0 0 ± 5 0 rpm.
The idle is also corrected by varying the ignition angle because it has a quicker effect.
The correct self-learning of the minimum and maximum butterfly opening positions is therefore essential
for the effective control of the idle speed; it should be carried out at least once in production and each
time the engine control unit or the motorized butterfly casing are replaced.
The self-adjustment function allows the adjustment of the idle, in time, to any variations in the c o n d i -
tions of the components which manage the idle.

Restriction of the maximum engine speed (protection outside of revs)

When the engine speed exceeds 6,800 rpm, the electronic control unit reduces the operating times of
the injectors to prevent the engine from being overloaded, also protecting it outside of revs.
If the engine idle speed exceeds 7,000 rpm, the control unit activates the fuel cut-off function, restoring
the operation of the injectors when the speed goes below a certain level.

Check on combustion using the Lambda sensor

The Lambda sensor informs the control unit of the quantity of oxygen present in the exhaust and there-
fore the correct metering of the air/fuel mixture by means of a voltage signal at t w o levels corresponding
to a lean mixture and a rich mixture, respectively.
To obtain an ideal mixture for the operation of the three-way catalyzer and keep emissions to a m i n i -
mum, the petrol injected must be kept to a pre-established (stoichimetric) ratio in relation to the intake
air, measured from the unit value of the Lambda parameter ( = 1 ) ; in the case of a lean mixture it is > 1 , in
the case of a richmixture it is < 1.
The Lambda sensor, in contact w i t h the exhaust gases, produces an electrical signal whose voltage de-
pends on the concentration of oxygen present in the actual gases:
- if the mixture is l e a n ( > 1 ) , the sensor provides a l o w voltage l e v e l ( < 2 0 0 mV) and the control unit
slightly increases the quantity of petrol injected;
- if the mixture is rich ( < 1 ) , the sensor provides a high voltage level ( > 8 0 0 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 suitable self-adjustment functions.
The control unit also manages the supply of the Lambda sensor heater. In effect, on the basis of the e n -
gine speed and load 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 needs depending
on the engine conditions.
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Fuel system
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