CARBURETED MODELS
In order to meet Federal emission standards at all altitudes, all vehicles using feedback carburetors are equipped with a high altitude compensation system.
The system consists of a High Altitude Compensator (HAC), passages for additional bleed air and additional jet air and a vacuum delay valve (vehicles with automatic transmission). As altitude increases, air pressure decreases and the air/fuel mixture becomes richer. The HAC system supplies additional air to the primary main well.
FUEL INJECTED MODELS
In a similar manner to carbureted models, fuel injection vehicles are equipped with a pressure sensor and a solenoid valve. This system acts as the high altitude compensator.
At high altitudes, the control valve is opened by the HAC aneroid bellows. This allows air to bleed through the valve into the primary main well. At low altitudes, the control valve remains closed and no additional bleed air flows to main well.
When ignition switch is turned to "ON" position and thereafter once every specified period, the solenoid valve is energized by the ECU for the specified time. This allows the pressure sensor to detect the ambient barometric pressure. Fuel delivery time is determined by the ECU in response to ambient barometric pressure, so that air/fuel ratios at high altitudes can be maintained to the same degree as those at sea level.
