![]() The injector driver circuit uses the peak-and-hold method, providing 4 amps to open the injector and about 1 amp to keep it open. The PCM varies the on-time or dwell of the MAXI injector to control the air/fuel mixture. Fuel delivery is controlled by pulse width modulation. The powertrain control module (PCM) estimates air flow using inputs from the MAP sensor, throttle position sensor, temperature sensor and engine speed. The CPI system is a speed-density system, so there is no airflow sensor. This allows the system to provide sequential fuel injection for better emissions, performance and fuel economy. In the second generation CSFI system, the injectors are controlled individually and fire only once every other revolution of the crankshaft. In the first generation CPI system, all the nozzles spray simultaneously when the MAXI injector opens (three times per crankshaft revolution). ![]() When the pressure inside the lines reaches the opening pressure of the poppet valves (43 psi), fuel sprays out of the nozzles into the engine’s intake ports. The MAXI injector is mounted in the center of the intake manifold on the 4.3L engine, and is hidden inside the split plenum manifold.Instead of spraying fuel directly into the manifold like a throttle-body injector, the MAXI injector routes fuel into six nylon fuel lines that have poppet-style spray nozzles on the end. Fuel injectors are expensive, so using one injector instead of six seemed like a good idea at the time. Some bean counter at GM probably came up with the idea as a way to reduce costs. Though the CPI system supplies fuel to each of the engine’s intake ports like other multi-port fuel injection systems, it has only one centrally located fuel injector, called the MAXI injector. The system was also added to 5.0L and 5.7L Vortec V8 engines. The system was used on 4.3L V6 Vortec engines through 1995, and was redesigned in 1996 and renamed “Central Sequential Fuel Injection” (CSFI). Back in 1992, General Motors introduced a new type of fuel injection system known as “Central Port Injection” (CPI), which GM also refers to as “Central Multi-point Fuel Injection” (CMFI).
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