FUEL SYSTEM CONTINUED
Fuel Timing and Delivery
The engine ECM controls the injected fuel by varying the signals to the unit injectors. The unit injectors inject fuel
ONLY if the unit injector solenoid is energized. The engine ECM sends a 90-volt signal to the solenoid to energize
it. By controlling the timing of the 90-volt signal, the engine ECM controls injection timing. By controlling the
duration of the 90-volt signal, the engine ECM controls the amount of injected fuel.
Injection timing is determined by engine rpm and other engine data. The engine ECM senses the top center
position of cylinder number 1 from the signal that is provided by the engine timing sensor. The engine ECM
decides when the injection should occur relative to the top center position and provides the signal to the unit
injector at the desired time.
Injector Actuation System
The unit injector pressurizes the fuel. The correct amount of fuel is then injected into the cylinder block at precise
times. The engine ECM determines the injection timing and the amount of fuel that is delivered. The unit injector is
operated by a camshaft lobe and a rocker arm. The camshaft has three camshaft lobes for each cylinder. Two
lobes operate the inlet and exhaust valves, and the other lobe operates the unit injector mechanism. Force is
transferred from the unit injector lobe on the camshaft through the lifter to the pushrod (Figure 6, Item 4). The force
of the pushrod is transferred through rocker arm assembly (Figure 6, Item 3) and to the top of the unit injector
(Figure 6, Item 1). The adjusting nut (Figure 6, Item 2) allows adjustment of the unit injector.
Figure 6. Unit Injector Mechanism.