Housing Pressure Cold Advance (HPCA)

The HPCA circuit is used to improve cold starting and aid emission control. The circuit is controlled by a temperature switch located on right rear head bolt. The circuit advances injection timing about 4° when the engine is cold.

When engine temperature is below 125°F (45°C), the circuit decreases housing pressure from 10 psi to zero. 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 95°F (35°C).