Housing Pressure Cold Advance (HPCA)

The HPCA circuit is used to improve cold starting and aid in emission control. On light duty emissions (LH6 models), an ECM signal is used to activate the HPCA circuit. On heavy duty emissions (LL4 models), the HPCA circuit is controlled by a coolant temperature switch located on rear of right hand cylinder head. The circuit advances injection timing about 4 degrees when the engine is cold.

When engine temperature is below 80°F (27° C), the circuit decreases housing pressure from 10 psi to zero (.7 to zero kg/cm² ). At the same time, the fast idle solenoid is activated. When the temperature switch opens, the HPCA circuit is de-energized and housing pressure rises, retarding pump timing. The temperature switch will close again when engine temperature falls below 85°F (30°C).