altitude. You should make a habit of knowing what
the average pressure and corresponding barometric
pressure is for your area. Always use the Diagnostic
Test Procedures Manual for MAP sensor testing.
REMOVAL
The manifold absolute pressure sensor is mounted
on the drivers side of the intake manifold (Fig. 15).
(1) Disconnect the electrical connector from the
MAP sensor.
(2) Remove MAP sensor mounting screws.
INSTALLATION
(1) Install the MAP sensor to the intake manifold.
(2) Install MAP sensor mounting screws and
tighten mounting screws to 4 N·m (35 in. lbs.).
(3) Connect the electrical connector to the MAP
sensor.
O2 SENSOR
DESCRIPTION
The upstream O2 sensors are in the exhaust sys-
tem in the exhaust manifold collertor area before the
upstream catalytic convertor (Fig. 16).
The downstream O2 sensors are in the exhaust
system in between the catalytic convertors (Fig. 16).
OPERATION
A single sensor ground is used for all O2 sensors (4
sensors, 2 upstream and 2 downstream of the catay-
tic covertor).
As vehicles accumulate mileage, the catalytic con-
vertor deteriorates. The deterioration results in a
less efficient catalyst. To monitor catalytic convertor
deterioration, the fuel injection system uses two
heated oxygen sensors. One sensor upstream of the
catalytic convertor, one downstream of the convertor.
The PCM compares the reading from the sensors to
calculate the catalytic convertor oxygen storage
capacity and converter efficiency. Also, the PCM uses
the upstream heated oxygen sensor input when
adjusting injector pulse width.
When the catalytic converter efficiency drops below
emission standards, the PCM stores a diagnostic
trouble code and illuminates the malfunction indica-
tor lamp (MIL).
The O2 sensors produce voltages from 0 to 1 volt,
depending upon the oxygen content of the exhaust
gas. When a large amount of oxygen is present
(caused by a lean air/fuel mixture, can be caused by
misfire and exhaust leaks), the sensors produces a
low voltage. When there is a lesser amount of oxygen
present (caused by a rich air/fuel mixture, can be
caused by internal engine problems) it produces a
higher voltage. By monitoring the oxygen content
and converting it to electrical voltage, the sensors act
as a rich-lean switch.
The oxygen sensors are equipped with a heating
element that keeps the sensors at proper operating
temperature during all operating modes. Maintaining
correct sensor temperature at all times allows the
system to enter into closed loop operation sooner.
Also, it allows the system to remain in closed loop
operation during periods of extended idle.
In Closed Loop operation the PCM monitors the O2
sensors input (along with other inputs) and adjusts
the injector pulse width accordingly. During Open
Loop operation the PCM ignores the O2 sensor input.
The PCM adjusts injector pulse width based on pre-
programmed (fixed) values and inputs from other
sensors.
The Automatic Shutdown (ASD) relay supplies bat-
tery voltage to the upstream heated oxygen sensors.
The oxygen sensors are equipped with a heating ele-
ment. The heating elements reduce the time required
for the sensors to reach operating temperature. The
PCM uses pulse width modulation to control the
ground side of the heater to regulate the temperature
on 4 cyl. upstream O2 heater only. The downstream
O2 heaters are controlled by a relay.
Fig. 16 2/1, 2/2, O2 SENSORS
1 - Upstream O2 Sensor
2 - Downstream O2 Sensor
ZB FUEL INJECTION 14 - 37
MAP SENSOR (Continued)
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