The first production Tuned Port Injection (TPI) systems appeared on General Motors’ vehicles in 1985. The GM vehicles built with these systems were the Corvette, Pontiac Firebird, Pontiac Trans AM, and the Chevrolet Camaro. Upon their introduction, these systems achieved a 35 % improvement over carbureted systems and a 20% improvement over available forms of fuel injection in horsepower, torque and economy.
The 1985-1988 TPI system utilized the following sensors and devices to control the engine: Mass Air Flow Sensor and Module, Manifold Air Temperature Sensor, Coolant Temperature Sensor, Oxygen Sensor, Throttle Position Sensor, Cold Start Switch, Cold Start Injector, Fuel Injectors, Idle Air Control Valve, Vehicle Speed Sensor, Electronic Spark Timing Sensor and Module, and Knock Sensor.
When the key is in the “on” position the Electronic Control Module (ECM), the main computer that controls all sensors and engine functions, powers up and readies the engine for start-up. When the starter is engaged and the coolant temperature is less than 100 degrees Fahrenheit, determined by the Coolant Temperature Sensor (CTS), the Cold Start Injector provides a spray of fuel to each cylinder via an air distribution system built into the intake manifold. If the engine temperature is greater than 100 degrees Fahrenheit, the Cold Start Injector is disabled by the cold start switch. Upon startup the ECM utilizes information in the Erasable Programmable Read Only Memory (EPROM) to establish the initial pulse rate for the fuel injectors. The ECM pulses, or opens and closes, the remaining 8 fuel injectors in sequence and the engine starts. During this, the Idle Air Control (IAC) valve is completely open to allow as much air as possible to enter the engine to prevent it from dying out. At this point, the engine is operating in open loop mode and will continue to do so until the engine warms up. After the warm up period the ECM scans the sensors, if all sensors are operating and within their proper range, the engine then goes into closed loop operation. This means that the sensors are dynamically controlling the engine. The IAC valve then begins to close, reducing the amount of air entering the engine and thus slowing idle to the value specified in the EPROM. In the event that the information received from a sensor is higher or lower than the normal range, a code will set in the ECM, and the Service Engine Soon light will turn on.
The ECM receives information on airflow, from the Mass Air Flow Sensor (MAFS) or from the Manifold Absolute Pressure Sensor (MAPS), engine temperature, from the Coolant Temperature Sensor (CTS), air temperature, from the Manifold Air Temperature Sensor (MATS), exhaust gas oxygen content, from the Oxygen Sensor, throttle position, from the Throttle Position Sensor (TPS), and vehicle speed, from the Vehicle Speed Sensor (VSS).
The Mass Air Flow Sensor is located in-line with the throttle body and air filter box. It is a hollow cylinder with a 4-wire connector. The blue wire carries a reference voltage of 8-volts, the gray wire returns the variable resistance to the ECM, the black wire grounds the sensor, and the red wire is for the burn-off or cleaning stage. At any RPM above idle, such as during driving or revving, the MAFS begins to function. It directly measures the amount of air entering the engine at any speed. The MAFS accomplishes this with a thin wire strung across a calibrated tube. This wire is fed a reference voltage that causes it to heat up. When the wire has reached operating temperature, the ECM begins to measure the return voltage. As air flows through the MAFS and into the engine, the wire begins to cool and thus changes its resistance. More airflow causes a cooler wire, which results in a lower the resistance, and vise-versa. The ECM then cross-references the return voltage to a table stored in the EPROM to determine the exact amount of air entering the engine. When the engine is shut down, the thin wire is then reheated to “burn-off” any debris that is still on the wire. This sensor was used on the earlier TPI systems from 1985-89.
The Manifold Absolute Pressure Sensor (MAPS) reads engine manifold pressure and translates this reading into a voltage signal that the computer uses to monitor