GM Prizm/Nova 1985-1993 Repair Guide

Hot Idle Compensation (HIC) System



See Figures 1, 2 and 3

The Hot Idle Compensation (HIC) System allows the air controlled by the HIC valve to enter the intake manifold, maintaining proper air/fuel mixture during idle at high temperatures. When the air cleaner temperature is below 72°F (22°C), the HIC valve's atmospheric port is closed. This allows supplies intake manifold vacuum to the hot air intake (HAI) valve, allowing heated air to enter the air cleaner.

As the temperature in the air cleaner rises, the HIC valve will increase its opening to the atmosphere which, in turn, increases pressure on the diaphragm. As the pressure increases the HAC valve will close more, pulling cooler (more dense) air into the carburetor.

When air cleaner temperatures are between 84°F (29°C) and 126°F (52°C), the HIC valve's atmospheric port opens slightly. This begins to increase the pressure on the HAI diaphragm and its air control valve begins to close.

Click image to see an enlarged view

Fig. Fig. 1: HIC valve

Click image to see an enlarged view

Fig. Fig. 2: HIC system operation

When intake air temperatures reach 138°F (59°C), both the atmospheric port and the vacuum port of the HIC valve open further. This action steadily increases the pressure on HAI diaphragm and continues to close the air control valve, pulling more cool air into the carburetor. At the same time, the opening of the vacuum port allows outside air to bleed into the intake manifold to maintain the proper air/fuel mixture during high temperature at idle.

Finally at temperatures above 192°F (89°C), the HIC valve's atmospheric and vacuum ports open fully, applying maximum pressure to the HAI diaphragm and closing the air control valve completely. This allows the coolest possible air to enter the carburetor.

Click image to see an enlarged view

Fig. Fig. 3: HIC system operation chart


  1. Check that air flows from the HAI diaphragm side to the carburetor side while closing the atmospheric port.
  3. Check that air does not flow from the carburetor side to the HAI diaphragm side.
  5. Below 72°F (22°C), check that air does NOT flow from the HAI diaphragm side to the atmosphere port while closing the intake manifold side.
  7. Heat the HIC valve to above 84°F (29°C).
  9. Check that air flows from the HAI diaphragm side to the atmospheric port while closing the carburetor side.