Fiat 1998 Marea Weekend, Service Manual

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Engine
Fuel system
10.
ENGINE EXHAUST ASSEMBLY DIAGRAM
Marea-Marea Weekend®^
1998 range
4. Silencers
P4F22IJ01
In the I.A.W. system, closed-loop mixture concentration control is activated by a lambda probe sensor.
This measures the amount of oxygen in exhaust gas upstream of the catalytic converter.
The lambda probe readings allow the electronic control unit to correct the concentration continually to
maintain the air/fuel ratio constant.
Harmful exhaust emissions are controlled in this way. This control is complemented by a trivalent cat-
alytic converter.
Efficient catalytic converter operation and the ability to contain exhaust gas toxicity are dependent on
the air/fuel ratio supplied to the engine.
The trivalent catalytic converter reduces the three pollutant gases present in exhaust gases simultane-
ously: uncombused hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx).
Two types of chemical reaction take place inside the catalytic converter:
- oxidation of CO and HC to carbon hydroxide (CO ) and water (H 0 )
- reduction of NOx, to nitrogen (N ).
The catalytic converter is put rapidly and irreparably out of action by the following:
- presence of lead in fuel. This lowers the level of conversion to levels that render the catalyst ineffec-
tive;
- presence of uncombusted fuel in the converter. A fuel flow lasting 30s in an environment at 800 °C
(temperature insde the catalytic converter) is sufficient to cause melting and breakdown of the cat-
alytic converter. It is absolutely necessary to ensure the ignition system is working properly. Therefore
the spark plugs should never be disconnected for for any reason with the engine running. When
tests are carried out, the catalytic converter should be replaced with an equivalent length of pip-
ing.
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Publication no. 506.763/09