Computers and Control Systems: Description and Operation

WARNING: DO NOT SMOKE, CARRY LIGHTED TOBACCO OR OPEN FLAME OF ANY TYPE WHEN WORKING ON OR NEAR ANY FUEL-RELATED COMPONENT. HIGHLY FLAMMABLE MIXTURES ARE ALWAYS PRESENT AND MAY BE IGNITED, RESULTING IN POSSIBLE PERSONAL INJURY.

The Sequential multiport Fuel Injection (SFI) system is classified as a multi-point, pulse time, mass air flow fuel injection system.

To maintain the required exhaust emission levels, the fuel metering system must be kept in good operating condition and adjusted to specifications.

Additional engine performance checks are required to keep the exhaust emissions at the specified minimum pollutant level.

Always refer to the Master Parts Catalog for parts usage and interchangeability before replacing a throttle body or a component part of a throttle body.

The Powertrain Control Module (PCM):
- accepts information from various engine sensors to compute the required fuel flow rate necessary to maintain a prescribed air/fuel ratio throughout the entire engine operation range.
- sends out commands to the fuel injectors to meter the appropriate quantity of fuel.
- determines and compensates for the age of the vehicle and its uniqueness.
- automatically senses and compensates for changes in altitude (for example, from sea level to mountains).

The Throttle Position sensor (TP sensor):
- is a potentiometer that provides a signal to the powertrain control module that is directly proportional to the throttle plate position.
- is mounted to the throttle body and engages on the throttle plate shaft.
- measures throttle plate shaft position.

The Mass Air Flow sensor (MAF sensor):
- is located between the Air Cleaner element (ACL element) and the air cleaner outlet tube.
- is integral to its conical housing.
- uses a hot wire sensing element to measure the amount of air entering the engine. Air passing over the hot wire causes it to cool.
- sends out an analog voltage signal to the powertrain control module to determine the intake air mass.
- the powertrain control module will then calculate the required fuel injector pulse width to provide the desired air/fuel ratio.

The mass air flow sensor hot wire sensing element and housing are calibrated as a unit and must be repaired as a complete assembly.

The Intake Air Temperature sensor (IAT sensor):
- changes resistance in response to intake air temperature.
- decreases resistance as the surrounding air temperature increases, providing a signal to the powertrain control module, indicating temperature of the incoming intake air.
- is located on the air cleaner outlet tube.

The Idle Air Control valve (IAC valve):
- is mounted directly to the intake manifold to the right of the throttle body.
- is an electro-mechanical device controlled by the powertrain control module.
- incorporates a linear actuator which positions a variable area metering valve.

A bypass channel controls both cold and warm engine idle airflow control as regulated by the idle air control valve.

The Engine Coolant Temperature sensor (ECT sensor):
- changes resistance in response to the engine coolant temperature.
- decreases in resistance as the surrounding temperature increases providing a signal to the powertrain control module that indicates the engine coolant temperature.
- is located in the water bypass tube.

There are four Heated Oxygen Sensors (HO2S):
- two are located in the exhaust pipes below the exhaust manifold before the catalytic converters.
- two are used as catalyst monitors behind the catalytic converters.
- these sensors react with the oxygen in the exhaust gases and generate a voltage based on this reaction.
- a low voltage indicates too much oxygen or a lean condition.
- a high voltage indicates not enough oxygen or a rich condition.

The Camshaft Position sensor (CMP sensor) is
- a variable reluctance sensor which is triggered by the high-point mark on the camshaft sprocket.
- It is mounted on the LH of the engine front cover.

The Crankshaft Position sensor (CKP sensor):
- sends the powertrain control module a signal indicating crankshaft position.
- senses a missing tooth on the crankshaft ring.

The Knock Sensor (KS).
- is located in the block valley below intake manifold.
- is used to detect engine detonation (spark knock). As a result, a voltage signal is sent to the powertrain control module, which retards the ignition timing as necessary.