Pontiac Fiero 1988 Service Manual page 535

6E2-C2-4 DRIVEABILITY AND EMISSIONS - FUEL INJECTION (TBS)
Pressure R e g u l a t o r
The pressure regulator is a diaphragm-operated
relief valve, with injector pressure on one side and air
cleaner pressure on the other. The function of the
regulator is to maintain a constant fuel pressure at
the injector at all times, by controlling the flow in the
return line (by means of a calibrated bypass).
The pressure regulator is serviced as part of the
fuel meter assembly and can be disassembled.
If the pressure regulator in the TBI supplies
pressure which is too low (below 62 kPa or 9 psi), poor
performance could result. If the pressure is too high,
unpleasant exhaust odor may result.
Idle Air Control (IAC) Valve
The purpose of the idle air control (IAC) valve is to
control engine idle speed, while preventing stalls due
to changes in engine load (Figure C2-5).
The IAC valve, mounted on the throttle body,
controls bypass air around the throttle valve. By
moving a conical valve IN (to decrease air flow) or
OUT (to increase air flow), a controlled amount of air
can move around the throttle valve. If rpm is too low,
more air is bypassed around the throttle valve to
increase rpm. If rpm is too high, less air is bypassed
around the throttle valve to decrease rpm.
JL
ED
H
G O
EE
FUEL METER ASSEMBLY
THROTTLE
BODY
ASSEMBLY [ ¥ ] ELECTRICAL
IDLE AIR CONTROL VALVE
, N P U T S , G N A L
FILTERED AIR
iiJLET
PINTLE
VACUUM PORTS F O R ENGINE
O R EMISSION CONTROLS
THROTTLE VALVE
8P 0318-SY
3
[7]
TERMINAL PINS
B A L L BEARING A S S E M B L Y
STATOR A S S E M B L Y
ROTOR A S S E M B L Y
SPRING
PINTLE
GRIME S H I E L D
8 L E A D S C R E W
8P 0907-AS
Figure C2-5 - Idle A i r Control System
Figure
C2-6
- Idle Air
Control
(IAC)
Valve
The IAC valve moves in small steps called
"Counts", which can be monitored by a "Scan" tool
which plugs into the ALDL connector.
During idle, the proper position of the IAC valve is
calculated by the ECM, based on battery voltage,
coolant temperature, engine load, and engine rpm. If
the rpm drops below a specified rpm, the throttle valve
closes and the ECM senses a near stall condition, then
calculates a new IAC valve position to prevent stalls.
If the IAC valve is disconnected or connected with
the engine running, the idle rpm may be wrong. In
this case, the IAC valve may be reset by driving the
vehicle over 30 mph, or by raising the rpm over 2000
while in ALDL mode (test terminal grounded).
The IAC valve affects only the idle characteristics
of the engine. If it is open fully, too much air will be
allowed to the manifold and idle speed will be high. If
it is stuck closed, too little air will be allowed in the
manifold, and idle speed will be too low. If it is stuck
part way open, the idle may be rough, and will not
respond to engine load changes.
On TBI Model 700 applications, the IAC valve
(Figure C2-6) is flange-mounted. The IAC valve has a
dual taper, 10 mm diameter, pintle. For replacement,
use only an IAC valve with the correct part number
and appropriate pintle shape and diameter.
Throttle Position Sensor (TPS)
The throttle position sensor (TPS) is mounted on
the side of the throttle body opposite the throttle lever
assembly. Its function is to sense the throttle valve
position and relay that information to the ECM
(Figure C2-7). Knowledge of throttle position is
needed by the ECM to generate the required injector
control signals (base pulse). If the TPS senses a wide
open throttle, a voltage signal indicating this
condition is sent to the ECM. The ECM then widens