90-823225--1 10965E-24 - ELECTRONIC FUEL INJECTION (MULTI-PORT AND THROTTLE BODY)
By moving a conical valve known as a pintle, IN, to-
ward the seat (to decrease air flow), or OUT, away
from the seat (to increase air flow), a controlled
amount of air moves around the throttle valve. If RPM
is too low, more air is bypassed around the throttle
valve to increase it. If RPM is too high, less air is by-
passed around the throttle valve to decrease it.
The ECM moves the IAC valve in small steps, called
counts. These can be measured by scan tool test
equipment, which plugs into the DLC.
During idle, the proper position of the IAC valve is
based on engine RPM. If the RPM drops below speci-
fication and the throttle valve is closed, the ECM
senses a near stall condition and calculates a new
valve position to prevent stalling.
• Engine idle speed is a function of total air flow into
the engine based on IAC valve pintle position +
throttle valve stop screws and PCV.
• “Controlled” idle speed is programmed into the
ECM, which determines the correct IAC valve
pintle position to maintain the desired idle speed
for all engine operating conditions and loads.
• The minimum idle air rate is set at the factory with
stop screws. This setting allows enough air flow
by the throttle valves to cause the IAC valve pintle
to be positioned a calibrated number of steps
(counts) from the seat during “controlled” idle op-
eration.
• If the IAC valve is disconnected and reconnected
with the engine running, the idle speed may be
wrong. In this case, the IAC valve can be reset by
doing the following: turn off engine, wait ten se-
conds, and restart engine.
Multi-Port Injection Components
FUEL PUMP ELECTRICAL CIRCUIT
When the ignition switch is turned to the RUN posi-
tion, the ECM will turn ON the fuel pump relay for two
(2) seconds.
When the ignition switch is turned to the crank posi-
tion, the ECM turns the fuel pump relay ON causing
the fuel pump to start.
If the ECM does not receive ignition reference pulses
(engine cranking or running), it shuts OFF the fuel
pump relay, causing the fuel pump to stop.
FUEL RAIL/INTAKE MANIFOLD ASSEMBLY
The fuel rail performs several functions. It positions
the injectors in the intake manifold, distributes fuel
evenly to the injectors, and integrates the fuel pres-
sure regulator into the fuel metering system.
a
c
b
a - Fuel Rail
b - Fuel Injector
c - Intake Manifold
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