Fiat 1998 Marea Weekend, Service Manual

Page: 238 / 846

Engine
Fuel system
Marea- Marea Weekend 0
16v
10.
mV
1000
800
600¬
400 -
200
0,995 *- < — A. 1,005
t
1
1
1 T
1 1 ' I
0,7 0,8 0,9 1,0 1,1 1,2 1,3
A,= 1 ideal mixture (stoichiometric)
X
1 Lean mixture
Excess air; the CO values tend to be low
X
1 Rich mixture
Lack of air; the CO values tend to be high
99 range
LAMBDA SENSOR
The Lambda sensor measures the oxygen
content of the exhaust gases: it is fitted on the
exhaust pipe upstream of the catalytic silencer.
The sensor output signal is sent to the control
unit for feed back correction of the mixture
strength.
When the sensor provides a low signal (volt-
age below 200 mV) the control unit recognizes
a weak mixture and increases the injection
time; later, when the sensor signal is high
(voltage above 800 mV), the control unit rec-
ognizes a rich mixture and decreases the injec-
tion time.
This sequence of interventions is repeated with
a frequency in the order of tens of Hertz so
that the engine operates with a mixture
strength constantly fluctuating around the stoi-
chiometric ratio.
At temperatures below 300 °C the ceramic
material is not activated, therefore the sensor
does not send reliable signals: to ensure rapid
heating during starting and maintain its tem-
perature during idling, the sensor is equipped
with a heater with the electrical resistance on
permanently.
The sensor can be rapidly put out ol
action by the presence of even slight
amounts of lead in the fuel.
The sensor heating resistance can be measured
by disconnecting the connector and connect-
ing an ohmmeter as illustrated in the figure.
Connector wiring
Resistance: 4.5+0.5 ohm at 20 °C
24
Print n° 506.763/14