See Figure 1
The 1984 5.7L (VIN 8) engine is equipped with a Model 400 Throttle Body Injection (TBI) system, also referred to as the Cross-fire Injection system. This system consists of a pair of throttle body units which are mounted in front and rear positions on a single manifold. This arrangement lets each TBI unit supply the proper air/fuel mixture through a tuned crossover runner in the intake manifold to the bank of cylinders on the opposite side of the engine. This is the reason for the name "Crossfire injection". Each TBI unit feeds the cylinders on the opposite side. The front TBI gets the fuel from the fuel pump. The fuel then goes through a fuel pressure compensator, through a separate line to the rear TBI, then through the fuel pressure regulator then returns to the fuel tank.
Each TBI unit has its own fuel injector which are controlled individually by the ECM. Each TBI has its own IAC valve, which operate together. A throttle rod connects the throttle valves together. A single Throttle Position Sensor (TPS) is used on the rear TBI unit only. Also, a throttle bore tube is located under the manifold cover, below each throttle valve to help in mixture distribution. The throttle body portion of each TBI unit has several tubes which supply a manifold vacuum signal required for the operation of related engine and emission control systems. A vacuum port, located above the throttle valve is connected to the capped tubes used for on vehicle service air flow balancing, for final throttle valve synchronization. The amount of fuel delivered to the injectors is determined by an electronic signal supplied by the ECM. The TBI unit does not control, but is controlled by, the ECM. The ECM monitors several engine and vehicle conditions needed to calculate the fuel delivery time of the injectors. Delivery time may be modified by the ECM to reflect various operating conditions, such as cranking, cold starting, altitude, acceleration and deceleration. Each TBI unit has its own fuel injector which are controlled individually by the ECM.