Throttle Body Assembly C2-5; Idle Air Control (Iac) Valve Assembly; Throttle Position Sensor (Tps) - Pontiac Fiero 1988 Service Manual

DRIVEABILITY AND EMISSIONS - FUEL INJECTION (PORT) 6E3-C2-5
[T] TERMINAL PINS
BALL BEARING A S S E M B L Y
STATOR A S S E M B L Y
fT] ROTOR A S S E M B L Y
PH SPRING
[ § ] PINTLE
7] LEAD SCREW
8P0933-AS
Figure C2-9 - Idle Air Control (IAC) Valve
The pressure regulator compensates for engine
load by by increasing fuel pressure as engine vacuum
drops. If the pressure is too low, poor performance
could result. If the pressure is too high, excessive odor
may result. CHART A-7 has information on
diagnosing fuel pressure conditions.
THROTTLE BODY ASSEMBLY
The throttle body assembly has a throttle valve to
control the amount of air delivered to the engine. The
throttle position sensor (TPS) and idle air control
(IAC) valve are also mounted on the throttle body.
The throttle body contains vacuum ports located
at, above, or below the throttle valve. These ports
generate the vacuum signals needed by various
components. Engine coolant is directed through the
throttle body to warm the throttle body and to prevent
icing.
The system operates in an acceptable pressure
range of 234 - 325 kPa (41 - 47 psi). If the pressure is
too low, poor performance could result. If the pressure
is too high, excessive odor and a Code 45 may result.
CHART A-7 has information on diagnosing fuel
pressure conditions.
Idle Air Control (IAC) Valve Assembly
The purpose of the idle air control (IAC) valve
(shown in Figure C2-9) is to control engine idle speed,
while preventing stalls due to changes in engine load.
The IAC valve, mounted in 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 plate. 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.
I D L E AIR C O N T R O L ( I A C ) VALVE A S S E M B L Y
IAC VALVE P I N T L E
PH
IDLE A I R HOUSING
T H R O T T L E V A L V E
T H R O T T L E B O D Y A S S E M B L Y
E L E C T R I C A L INPUT S I G N A L
flTI
AIR I N L E T
8PQ331-SY
Figure C2-10- IAC Valve Air Flow Diagram (Typical)
The IAC valve moves in small steps called
"counts," which can be measured by some test
equipment which plugs into the assembly line data
link (ALDL) connector (see Figure C2-10).
During idle, the proper position of the IAC valve is
calculated by the electronic control module (ECM)
based on battery voltage, coolant temperature, and
engine rpm. If the rpm drops below a specified rpm,
and the throttle plate is closed, the ECM senses a near
stall condition, and then calculates a new valve
position to prevent stalls.
If the IAC valve is disconnected and reconnected
with the engine running, the idle rpm may be wrong.
In this case, the IAC valve can be reset by starting the
engine momentarily and then turning the ignition
"OFF".
Different designs are used for the IAC valve. Be
sure to use the correct design when replacement is
required.
The IAC valve affects the idle characteristics of
the vehicle. If it is open too far, too much air will be
allowed into the manifold and idle speed will be high.
If it is not open far enough, too little air will be
allowed in the manifold, and idle speed will be too low.
If it is stuck part of the way open, the idle speed may
be incorrect, and will not respond to engine load
changes.
Throttle Position Sensor (TPS)
The TPS is mounted on the side of the throttle
body opposite the throttle lever. Its function is to
sense the throttle valve angle and relay that
information to the ECM. Knowledge of throttle angle
is needed by the ECM to generate the required injector
control signals (base pulses). If the TPS voltage
indicates a wide open throttle, the ECM then
increases the injector base pulse width, permitting
increased fuel flow, (see Figure C2-11)