Isuzu Cars and Trucks 1981-1991

High Altitude Emission Control System

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I-Mark

This system consists of the altitude sensing switch, solenoid valve, air cleaner, 3 carburetor metering orifices and high altitude idle-up solenoid valve.

To avoid an excessively rich carburetor mixture at high altitudes due to decreased air density, the system functions to supply additional air through the carburetor metering orifices.

In addition, in order to stabilize the idle speed during high altitude operation, the system adjusts the fuel/air mixture according to signals from the idle-up solenoid valve.

At a certain altitude, the altitude sensing switch will close the circuit between the battery and the idle-up solenoid valve. The solenoid valve will open the passage between the air cleaner and the carburetor. Fresh air is then admitted through the 3 metering orifices and is supplied to the idle, off idle, primary and secondary main metering systems. Also, the idle up solenoid valve will open the passage to the port downstream of the throttle valve, providing additional fuel/air mixtures for stable idle speed at high altitude.

Inspection: Disconnect the three rubber hoses at the carburetor, blow air from inlet pipe A and supply electricity to the valve. Check to see that the valve is open. If the valve is open, air will be felt from all three pipes. Disconnect the electric supply and check to see that the valve is closed.



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Fig. High altitude emission control system RWD I-Mark



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Fig. High altitude emission control system FWD I-Mark



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Fig. Check high altitude solenoid valve FWD I-Mark

Amigo and P'up

On the 1988 Pick-Up, the major component of the system is the altitude switch which is installed on the right side of the dash panel. This switch senses altitude to the height of 2000 meters. The switch is connected to the ECM and makes the "System Malfunction Indicator" lamp inoperable in case of a "Rich Oxygen Sensor Error" at or above the designated altitude. There is no interaction between the altitude switch and the emission control system and the system does not require adjustment.

On 1989 and 1990 Amigo and Pick-Up, a MAP sensor is used to measure atmospheric pressure though a vacuum switching valve. The system prevents the vehicle's self-diagnostic system from generating a false "Rich Fuel Metering Error" at altitudes higher than the system's set point.



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Fig. High altitude emission control system circuit 1989 and 1990 Amigo and P-up

 
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