Fuel Charging and Controls

Sequential Multiport Fuel Injection (SFI)
The sequential multiport fuel injection (SFI) system is classified as:
- multi-point.
- pulse time.
- mass air flow controlled.

Fuel is metered into each intake port in sequence with the ignition firing order in accordance with engine demand through the fuel injectors mounted on the tuned lower intake manifold.

Throttle Body

CAUTION: Cleaning will remove the sealant. Do not clean the throttle body plate and bore.

The throttle body functions as follows:
- The throttle body controls airflow to the engine.
- A cam lever and accelerator cable rotate the throttle shaft from idle to Wide Open Throttle (WOT) position.
- The plate position at idle is set by a lever contacting a throttle return control screw. The head of the throttle return control screw is removed to make the idle plate set permanent.
- The idle air flow is controlled by an idle Air Control (IAC) valve which is mounted on the upper intake manifold.
- The Wide Open Throttle (WOT) plate position is obtained when the lever contacts the body casting.
- At all throttle positions, the Throttle Position (TP) sensor produces an electrical signal which is sent to the powertrain control module.
- A sealant is applied to the downstream side of the throttle body plate and bore to reduce plate-to-bore leakage.

Intake Manifold
The intake manifold:
- has runner lengths which are tuned to optimize engine torque and power output.
- has pockets for the fuel injectors that are machined to prevent both air and fuel leakage into lower intake manifolds.
- has fuel injectors mounted in position to direct the fuel spray immediately in front of each engine intake valve.

Fuel Injectors
The eight fuel injectors are:
- electro-mechanical devices which both meter and atomize fuel delivered to the engine.
- mounted in the lower intake manifolds and are positioned so that their tips direct fuel just ahead of the engine intake valves.

The fuel injector bodies consist of:
- the solenoid.
- the needle and valve assembly.
- the director/metering plate.

An electrical control signal from the powertrain control module activates the injector solenoid, allowing fuel to flow. Since the fuel injector flow orifice is fixed and the fuel pressure drop across the injector tip is constant:
- fuel flow to the engine is regulated by the length of time the solenoid is energized.
- atomization is obtained at the outlet of the fuel injector.

Fuel Injection Supply Manifold
The fuel injection supply manifold delivers high-pressure fuel from the vehicle fuel supply line to the eight fuel injectors. The fuel injection supply manifold consists of the following:
- tubular fuel rail.
- eight fuel injector connectors.
- mounting flange for the fuel pressure sensor.
- mounting attachments which locate the fuel injection supply manifold and provide fuel injector retention.
- fuel inlet connections with push connect fittings.

Fuel Pump Relay
The fuel pump relay supplies power to the fuel pump when the powertrain control module grounds the relay during START and engine RUN conditions.

Fuel Pressure Sensor
The fuel pressure sensor senses the pressure difference between the fuel injection supply manifold and the intake manifold, and provides the powertrain control module with the current fuel injection supply manifold pressure. The return fuel line to the fuel tank has been deleted in this type of fuel system. The differential fuel intake manifold press together with inferred fuel temperature provides an indication of the fuel vapors in the fuel rail.

The powertrain control module uses this information to vary the duty cycle output to the Fuel Pump Driver Module (FPDM) which controls the speed of the fuel pump. The speed of the fuel pump sustains fuel injection supply manifold pressure which preserves fuel in its liquid state.

The dynamic range of the fuel injectors increase is due to the higher pressure, which allows injector pulse width to decrease.

A/C Clutch Control Relay
The A/C compressor clutch control relay improves idle smoothness after engine start-up and also improves acceleration performance. The A/C is cut off for approximately five seconds under the following conditions:
- just after the engine is started.
- when the throttle position sensor indicates a full throttle condition.