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Friday, January 27, 2012

CMFI details for GMC vehicles?

This query is asked by my customer for GMC vehicles.

The details are as follows :----

See Figures 1, 2 and 3


Click image to see an enlarged view
Fig. Fig. 1: Cross-sectional view of CMFI assembly
The Central Multi-port Fuel Injection (CMFI) system, found on 1994-95 4.3L models, functions similarly to the TBI system in that an injection assembly (CMFI unit) is centrally mounted on the engine intake manifold. The major differences come in the incorporation of a split (upper and lower) intake manifold assembly with a variable tuned plenum (using an intake manifold tuning valve) and the CMFI unit's single fuel injector which feeds 6 poppet valves (1 for each individual cylinder).
The non-repairable CMFI assembly or injection unit consists of a fuel meter body, gasket seal, fuel pressure regulator, fuel injector and six poppet nozzles with fuel tubes. The assembly is housed in the lower intake manifold. Should a failure occur in the CMFI assembly, the entire component must be replaced as a unit.
As with other fuel injection systems, all injection and ignition functions are controlled by the computer control module. The module accepts inputs from various sensors and switches, calculates the optimum air/fuel mixture and operates the various output devices to provide peak performance within specific emissions limits. The module will attempt to maintain the ideal air/fuel mixture of 14.7:1 in order to optimize catalytic converter operation. If a system failure occurs that is not serious enough to stop the engine, the module will illuminate the SERVICE ENGINE SOON light and will continue to operate the engine, although it may need to operate in a backup or fail-safe mode.
Fuel is supplied to the injector through an electric fuel pump assembly which is mounted in the vehicle's fuel tank. The module provides a signal to operate the fuel pump though the fuel pump relay and oil pressure switch. The CMFI unit internal pressure regulator maintains a system pressure of approximately 58-64 psi (400-440 kPa). When the injector is energized by the control module, an armature lifts allowing pressurized fuel to travel down the 6 fuel tubes to the poppet valves. In the poppet valves, fuel pressure (working against the extension spring force) will cause the nozzle ball to open from its seat and fuel will flow from the nozzle. It takes approximately 51 psi (350 kPa) to force fuel from the poppet nozzle. Once the module de-energizes the injector, the armature will close, allowing fuel pressure in the tubes to drop and the spring force will close off fuel flow.


Click image to see an enlarged view
Fig. Fig. 2: CMFI air flow schematic


Click image to see an enlarged view
Fig. Fig. 3: Various CMFI engine components are mounted to the intake manifolds (the CMFI unit is located under the upper intake)
Other system components include a pressure regulator, an Idle Air Control (IAC) valve, a Throttle Position (TP) sensor, Intake Air Temperature (IAT) sensor, Engine Coolant Temperature (ECT) sensor, a Manifold Absolute Pressure (MAP) sensor and an oxygen sensor.
The idle air control valve is a stepper motor that controls the amount of air allowed to bypass the throttle plate. With this valve the computer control module can closely control idle speed even when the engine is cold or when there is a high engine load at idle.
The computer module used on CMFI vehicles has a learning capability which is used to provide corrections for a particular engine's condition. If the battery is disconnected to clear diagnostic codes, or for safety during a repair, the learning process must start all over again. A change may be noted in vehicle performance. In order to "teach'' the vehicle, make sure the vehicle is at normal operating temperature, then drive at part throttle, under moderate acceleration and idle conditions, until normal performance returns.

This will help.
Thanks.

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