Fuel Injectors


The fuel injectors are controlled by the ECU to supply fuel to the engine.  Throttle body injection (TBI) or single-point injection, uses a single, large injector in the throttle body to supply fuel.  The injector has 6 spray orifices that deflect off of a sharp nozzle to deliver a fine, 45^ cone-shaped spray pattern.  This system is also called pseudo-carb since it functions very similarly to a carburetor, except that it uses a computer-controlled injector to atomize and regulate fuel flow.  The injector cycles on and off at a rate based upon engine speed.  The amount of time the injector is on during each cycle (the duty cycle), determines how much fuel will enter the engine.  As the demand for fuel increases, the logic module will increase the amount of time the injector is on.

All turbocharged engines and some later model naturally aspirated (non-turbo) engines use multi-port injection (MPI).  In this system, a smaller fuel injector is located on the intake manifold, just in front of each intake valve.  The injector has one, small orifice to generate a cone-shaped spray pattern.  Fuel is injected directly into each cylinder and the logic module turns on the appropriate injector, depending which intake valve is open.  On all engines except those equipped with the SBEC, the injectors are actually fired in pairs (batch-fired) instead of individually.  Therefore, the batch-fired systems actually inject fuel behind the intake valve on the intake and power strokes.

The fuel injectors are supplied with 12V by the ASD relay.  The power module supplies them with ground to turn them on, when instructed to by the logic module.  1984 model power modules use a signal from the logic module to determine how long to leave the injector on, but use the fuel injector sync and ignition reference sensors to determine which injector to fire.  Later model power modules rely completely on the logic module for signals on which injector to fire and when.

Fault Codes

For 1984 models, when the logic module is turning on or off an injector, it momentarily checks the output of the injector driver transistor to see if it is responding the way it should.  If it does not see about 0V when on (open circuit) or 12V when off (short circuit), a fault code 42 is stored.  For later models with turbocharged engines, a fault code 26 is stored for injectors 1 and 2, or a fault code 27 is stored for injectors 3 and 4.  For single-point injected engines, a fault code 27 is stored for an open/shorted circuit, and if the final injector driver output current is not high enough, then a fault code 26 is stored.

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Updated 12/18/2003.

Copyright © 1996-2003 Russ W. Knize.