GM 6.5L V8 – Fuel System – TIMING

The cam ring that controls the pressurizing of fuel in the pumping chamber of the rotor can be rotated in the pump housing to change fuel injection timing. The automatic advance mechanism for the injection pump uses a sliding piston connected to the cam ring with a pin. As the piston slides in its housing bore, the cam ring rotates to change injection timing (refer to Figures 7-41 and 7-42).

One of the outlet passages of the transfer pump connects to the piston of the automatic advance mechanism, following a passage in the head of the injection pump, through a hollow head locating screw and into a housing passage. Fuel under transfer pump pressure pushes the piston, causing the injection timing to advance automatically in relation to engine speed.

The throttle shaft has a face cam mounted on the outside of the injection pump that operates a servo valve inside the bore of the advance piston (refer to Figures 7-43 and 7-44). The face cam contacts a lever arm, which changes the spring force acting on the valve by pushing a plunger in contact with the spring. In this way, the timing of fuel delivery can be varied to correspond to engine requirements.

The mechanical action of the throttle shaft, face cam and rocker lever allows a more rapid advance of injection timing under light loads by permitting an unrestricted flow of fuel under transfer pump pressure to the advance end of the piston. Under heavier loads, the throttle shaft, face cam and rocker lever increase the servo valve spring force, resulting in a restriction of fuel flow to the advance piston.

During warm engine operation, fuel under housing pressure pushes on the retard end of the advance mechanism piston, providing lubrication and a fluid cushion (refer to Figure 7-45).

When the HPCA solenoid is ON (during cold engine operation), a drop in housing pressure causes transfer pump pressure to push the advance piston farther (refer to Figure 7-46). This results in smoother engine operation during warm-up and combines with the action of the fast idle solenoid to temporarily increase engine idle speed.

If the HPCA solenoid is OFF during starting in cold temperatures, white exhaust smoke may result. If the HPCA solenoid is ON during warm or hot engine operation, black exhaust during acceleration may result.


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