1988 PONTIAC FIERO Service Repair Manual

6E3-A-38 5.OL (VIN F) & 5.7L (VIM 8) DRIVEABILITY AND EMISSIONS
MANIFOLD AIR
TEMPERATURE
MAT SENSOR SIGNAL
NSOR GROUND
CODE 25
MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT
(HIGH TEMPERATURE INDICATED)
5.OL (VIN F) & 5.7L (VIN 8) 'T" "SERIES (PORT)
Circuit Description:
The manifold air temperature sensor uses a thermistor to control the signal-voltage to the ECM. The ECM
applies a voltage (4-6) on CKT 472 to the sensor. When manifold air is cold, the sensor (Thermistor) resistance
is high, therefore, the
ECM will see a high signal voltage. As the air warms, the sensor resistance becomes less,
and the voltage drops.
Test Description: Numbers below refer to circled
numbers on the diagnostic chart.
1. Code
25 will set if:
@ Signal voltage indicates a manifold air
temperature greater than 150°C
(302" F) for 2
seconds.
@ Time since engine start is 1 minute or longer.
@ A vehicle speed is present.
Diagnostic Aids:
A "SCAN" tool reads temperature of the air
entering the engine and should read close to ambient
air temperature when engine is cold, and rise as
underhood temperature increases.
Check harness routing for possible short
to ground
in CKT 472.
Refer to "Intcrmittents" in Section "B".

DRIVEABILITY AND EMISSIONS 5.OL (VIN F) & 5.7L (VIN 8) 6E3-A-39
MANIFOLD AIR TEMPERATURE (MAT)
IF NO ADDITIONAL CODES WERE
"SCAN" SHOULD DISPLAY TEMPERATURE
BELOW
-30°C.
DIAGNOSTIC AID
MAT SENSOR
TEMPERATURE TO RESISTANCE VALUES
(APPROXIMATE)
CLEAR CODES AND CONFIRM "CLOSED LOOP" OPERATION AND NO "SERVICE ENGINE SOON" LIGHT.

CODE 32
EXHAUST GAS RECBRCULATION (EGR) CIRCUIT
%.OL (VIN F) & 5.7L (VIN 8) "F" SERIES (PORT)
Circuit Description:
The EGR valve vacuum is controlled by an ECM controlled solenoid. 'I'he ECM will turn the EGR "ON" and
"OFF" (Duty Cycle) by grounding CKT 435. 'I'he duty cycle is calculated by the ECM, based on information
from the coolant and mass air flow sensor and engine
RPMs. 'I'here should be (NO EGR) when in park or
neutral,
TPS input below a specified value or TPS indicating wide open throttle (WOT).
With the ignition "ON", engine stopped, the EGR solenoid is de-energized and, by grounding the diagnostic
terminal, the solenoid is energized.
Test Description: Numbers below refer to circled
numbers on the diagnostic chart.
Code 32 means that the
EGR diagnostic switch
was closed during start-up or that the switch was not
detected closed under the following conclilions:
@ Coolant temperature greater than 80°C
( 176°F).
@ EGR duty cycle commanded by the ISCM is
greater than
48%.
@ TPS less than wide open throttle (WO'I'), but
not at idle.
@ Codes 21,22,33,34 not present.
@ All conditions above must be met for about 4
minutes.
If the switch is detected closed during start-up, or
if the switch is detected open when the
,lbove
conditions are met, the "Service Engine Soon" light
will remain
"ON" unless the switch changes state.
1. This test will determine if the ECM set the code
due to CKT 935 being grounded on start-up.
If the
"Scan" does not indicate the switch is closed but
the customer complained of a "Service Engine
Soon" light after start-up, then this circuit should
be checked carefully for an intermittent grounded
condition.
2. If the "Scan" indicates the switch is no longer
closed after disconnecting it, be sure the switch is
not closed due to heat.
(EGR being "ON" prior to
test).
3. 'I'his test will check for
u possible open in CKT
935. 'I'he EXM supplies 9-12 volts to CKrl' 935 and
the "Scan" should indicate switch being closed
when CK'P 935 is grounded.
4. By grounding the diagnostic terminal, the EGR
solenoid should close, and allow vacuum to be
applied and the vacuum should hold.
5. This test will determine if the electrical control
part of the system is at fault or if the connector or
solenoid are at fault.
6. I3y plugging the EGR valve side and ungrounding
the diagnostic terminal, the solenoid valve should
open and allow vacuum to bleed off through the
vent.
7. With the engine not running and vacuum is
applied to the valve, the valve should
move to the
fully open position.
8. 'I'his engine uses a negative back pressure valve
and the valve should close when the engine is
cranked over.

DRIVEABILITY AND EMISSIONS 5.OL (WIN F) & 5.7L (VIN 8) 6E3-A-41
- -- - - @ BEFORE USING THIS CHART, CHECK VACUUM HOSES
FOR LEAKS, RESTRICTIONS, AND CHECK PORTED
VACUUM SOURCE TO EGR SOLENOID; SHOULD HAVE
AT LEAST
7" Hg VACUUM (23.7 kPa) AT 2000 RPM. @ USING A "SCAN" TOOL, CHECK EGR DIAGNOSTIC
SWITCH INPUT.
DOES "SCAN" INDICATE SWITCH
IS CLOSED?
EXHAUST GAS RECIRCULATION
(EGR) CIRCUIT
5.0L (VIM F) & 5.7L (VIN 8)
"F" "SERIES (PORT)
@ DISCONNECT EGR TEMP. DIAGNOSTIC SWITCH. @ JUMPER HARNESS TERMINAL TO GND. @ DOES "SCAN" INDICATE SWITCH IS CLOSED?
@ DISCONNECT EGR TEMP. DIAGNOSTIC
SWITCH CONNECTOR.
@ DOES "SCAN" INDICATE SWITCH IS CLOSED?
UNABLE TO GET 10" VACUUM
ABLE TO GET 10" VACUUM
.
@ DISCONNECT EGR SOLENOID. @ PLUG EGR SIDE OF SOLENOID. @ CONNECT TEST LIGHT BETWEEN @ UNGROUND DIAGNOSTIC TERMINAL.
HARNESS CONNECTOR TERMS. NOTE
VACUUM (SHOULD BLEED OFF 1.
EGR DIAPHRAGM WHILE
CONNECTED TO
SHOULD MOVE
HOLD VACUUM
T
20 SECONDS.
IS GOOD IF DIAPHRAGM
HAS MOVED TO SEATED

6E3-A-42 S.OL (VIN F) & 5.7L (VIN 8) DRIVEABILITY AND EMISSIONS
SYSTEM GND
SYSTEM GND
SYSTEM GND
ANALOG GND
BURN-OFF CONTROL CKT
-
CODE 33
MASS AIR FLOW (MAF) SENSOR CIRCUIT
(GMISEC HIGH)
5.OL (VIN F) & 5.7L (VIN 8) ""FY"IES (PORT)
Circuit Description:
The mass air flow (MAF) sensor measures the amount of air which passes through it. The ECM uses this
information to determine the operating condition of the engine to control fuel delivery.
For
a detailed
description of the MAF sensor operation refer to Section
"C".
The oil pressure switch or the ECM, through control of the fuel pump relay, will provide 12 volts for the
MAF power relay which provides the
12 volts needed by the MAF sensor.
The ECM provides
a current limiting 5V on the signal line (CKT 998). The MAF sensor then changes the
signal by dropping the voltage, so thai with low air flow the ECM sees
a low voltage and a high air flow will
cause the ECM to see near the
5 volt supply.
Test Description: Numbers below refer to circled 3. If a burn-off signal is present at the MAF sensor
numbers on the diagnostic chart.
with the engine running, a Code
33 will set. Be
Code
33 indicates: ECM has seen flow in excess of
sure no voltage is present on CKT 994 for the first
45 grams per second (above about 2.2 volts) for one 2 seconds after the ignition is turned "ON", or
second when: after the
2 second period.
@ Engine is first started 4. The ECM sources a voltage (4-6 volts) to the MAF
OR sensor on CKT 998. This test checks for that
@ TPS is less than 4 throttle. voltage. @ RPM is less than 2000.
Due to the 5 volt pull-up resistor in the ECM if Diagnostic Aids:
CKT 998 becomes open, the ECM will see a high
voltage signal and set
a Code 33. Intermittent:
By jumpering the fuel pump test
1. This test will determine if the conditions to set the terminal (G term, of ALDL) to 12 volts, the MAF
code still exist. sensor will stay powered up and the signal line should
2. With the ALDL terminal "G" jumpered to 12 volts,
see a low voltage, less than 250 mv or low grams per
there should be
12 volts at the sensor. If no second on a "SCAN" tool. By wiggling the related
voltage is present, make sure that the fuel pump
is wiring the intermittent may be detected. Also, an
running.
If not, repair fuel pump circuit. erratic signal with the engine running may indicate
fili~ltv wirinq or components.

DRIVEABILITY AND EMISSIONS 5.OL (VIN F) & 5.7L (VIM 8) 6E3-A-43
ADDITIONAL CODES WERE
@ IGNITION "OFF" @ CHECK FOR OPEN CKT 998.

6E3-A-44 5.0b (VIN F) & 5.7L (VIN 8) DRIVEABILITY AND EMISSIONS
SYSTEM GND
SYSTEM GND
BLUNVHT 450 SYSTEM GND
BURN-OFF CONTROL CUT
CODE 34
MASS AIR FLOW (MAF) SENSOR CIRCUIT
(GMISEC LOW)
5.OL (VIN F) & 5.7L (\/IN 8) "F"" SERIES (PORT)
Circuit Description:
The mass air flow (MAF) sensor measures the amount of air which passes through it. The ECM uses this
information to determine the operating condition of the engine, to control fuel delivery. For a detailed
description of the MAF sensor operation refer to Section C.
The oil pressure switch or the ECM, through control
of the fuel pump relay, will provide 12 volts for the
MAF power relay which provides the
12 volts needed by the MAF sensor.
The ECM provides a current limiting
5V on the signal line (CKT 998). The MAF sensor then changes the
signal by dropping the voltage so that with low air flow the ECM sees
a low voltage and a high air flow will
cause the ECM to see near the
5 volt supply.
Test Description: Numbers below refer to circled
numbers on the diagnostic chart.
Code
34 indicates: ECM has seen low air flow less
than
2.5 gmlsec. (low voltage) for one second when:
@ Engine is first started
OR
@ RPM above 600
@ TPS above 6%. To obtain 6%, the engine has
to be running at about
2300 rpm in neutral.
1. A Code 34 may be caused by an engine that
exhibits a low, rough, unstable or incorrect idle
problem. If this condition exists, disconnect the
MAF sensor. If the unstable idle still esists, refer
to Symptoms in Section
"R". (Rough, unstable,
incorrect idle, or stalling.) If the idle improved
with the sensor disconnected, replace it.
2. This test will determine if the conditions still exist
to set a code or if the problem is intermittent.
3. With the MAF sensor disconnected, the ECM
should see a high signal voltage and set a Code
33.
If a Code 34 resets then the wiring or the ECM is
at fault.
Diagnostic Aids:
A low, rough or unstable idle could result in a Code 34.
Also be sure air ducts are tight and not cracked.
Check CKT
998 for short to ground. Refer to
"Intermittents" in Section "R".

DRIVEABILITY AND EMISSIONS 5.OL (VIN F) & 5.7L (VIN 8) 6E3-A-45
0 SEE TEST DESCRIPTION #1 ON FACING PAGE.
OLTS ARE AVAILABLE TO MAF