Fiat 1998 Marea Weekend, Service Manual

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Marea- Marea Weekend 9
Engine
99 range
Fuel system
10.
Controlling the mixture strength (feed back control)
NOTE The mixture ratio is defined and denoted by the Greek fetter a (a/fa).
quantity of air drawn in by the engine
quantity of fuel injected
This is defined as the stoichiometric ratio and is denoted by a st.
theoretical quantity of air required to burn all the fuel injected
Quantity of fuel injected
This is defined as the mixture strength and is denoted by the Greek letter a (Lambda):
quantity of air drawn in by the engine
theoretical quantity of air required to burn all the fuel injected
It can easily be deduced that a st=
X .
The stoichiometric ratio depends on the type of fuel: for current unleaded petrol this value is around 14.7 -
14.8 which corresponds to a Lambda of 1:
A mixture is called rich when the quantity of air is lower than the stoichiometric value and in this case the
Lambda
X <
1:
A mixture is called poor (or lean) when the quantity of air is higher than the stoichiometric value and in
this case the Lambda
X
>1.
The strategy is designed to correctthe "basic" injection times so that the mixture strength continuously
fluctuates at a high Hertz frequency:
NOTE 7 Hz = 7 oscillation per second
In the following conditions:
- cut-off,
or
- engine fully laden,
or
- engine cold (coolant and air temperature below about 20 °C)
the strategy is disabled.
Self-adjustment
The control unit is equipped with a self-adjustment function which has the task of memorizing any differ-
ences between the basic map and corrections made by the Lambda sensor which may occur persistently
during operation. These differences (due to the ageing of the system and engine components) are perma-
nently memorized allowing the adaptation of the system operation to the gradual alterations in the engine
and components compared with when they were new.
The strategy is disabled whilt the charcoal filter solenoid valve is open.
If the control unit is replaced, a road test must be carried out where the engine is allowed to reach oper-
ating temperature and the control unit intervenes in the self-adjustment mode (especially during idling
stops).
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