Fiat Bravo Service Manual page 155

Engine Bravo-Brava @)i
6
v
Fuel system
10.
As a result:
- At very low temperatures the injector stays open longer Tj) graph (b) for a low air fuel ratio (rich mix­
tures);
- As engine temperature increases, injector opening time becomes shorter (tj) graph (a) and the air/fuel
ratio becomes correspondingly higher (lean mixture).
While the engine is warming up, the ECU also governs a step motor which is responsible for calculating
the amount of air necessary to ensure the engine does not stall.
Rpm is decreased as the temperature rises until the rated level is reached (850 ± 30 Rpm) when the en­
gine is warm. The ECU governs the step motor to maintain idle speed constant even if electrical and me­
chanical loads vary.
Full-load
Under full load conditions, base pulse constant must be increased to obtain maximum engine power
output.
The ECU detects a full load condition as a result of parameters supplied by the throttle position and ab­
solute pressure sensors.
On the basis of this information, the ECU implements the appropriate correction strategy by increasing
base pulse constant.
Over-run
Two strategies are superimposed during this stage:
1. Transitory negative strategy to reduce the amount of fuel required by the engine (lower emissions).
This stage is recognized by the ECU when the potentiometer signal changes from a higher voltage
level to a lower value.
2. A dash-pot strategy to lessen changes in torque delivery (lower engine brake).
This strategy is implemented when the potentiometer signal indicates that the throttle is closed and
rpm is high. The step motor gradually decreases the flow of air through the by-pass.
Connection to automatic transmission
The ECU adjusts engine idle speed on the basis of the load input when the selector lever is moved to a
given position and transmits information to the automatic transmission ECU on engine coolant tempera­
ture (pin 5) and throttle valve position (pin 44).
Barometric correction ^
Atmospheric pressure varies with altitude to bring about changes in volumetric efficiency of sufficient
entity to require a correction to baseline concentration (injection time).
Injection time is corrected according to changes in altitude and updated automatically by the ECU
whenever the engine is started up and under certain conditions of throttle position and rpm (dynamic
adjustment of barometric correction)
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Publication no 506.670