Closed Loop

When O2 sensor reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, ECM operates in closed loop. In closed loop, ECM controls air/fuel ratio based upon O2 sensor signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If O2 sensor cools off (due to excessive idling) or a fault occurs in O2 sensor circuit, vehicle once again enters open loop mode.

On 4.3L engines, oxygen sensor is equipped with an internal heating element. This element enables system to reach "closed loop" sooner and maintain closed loop even during periods of extended idle.

Central Port Injection (CPI) - CPI is one of 3 fuel systems used on 4.3L engines. The non-repairable injector assembly consists of a fuel meter body, fuel pressure regulator, fuel injector and 6 poppet nozzles with fuel tubes. CPI assembly is housed in the lower manifold assembly. Fuel pump and pressure regulator maintain fuel pressure at 54-64 psi (3.8-4.5 kg/cm² ) under all operating modes.

When injector is energized, pressurized fuel passes down fuel distribution tubes to poppet nozzles located at rear of intake valves. Fuel pressure forces poppet valves open, spraying fuel into cylinders when intake valves are open. As fuel pressure drops (due to all poppets opening or injector de-energizing), poppet nozzle spring pressure closes poppet nozzle until pressure again builds high enough to overcome poppet nozzle spring pressure. Excess fuel is returned to the fuel tank via the fuel return line.