See Figures 1 and 2
When the engine is cold, the heat control valve improves fuel vaporization for better driveability by quickly heating the intake manifold. Once the engine has warmed up, it helps keep the intake manifold at proper temperature.
With the engine cold, the bimetal spring positions the heat control valve to direct some of the engine's hot exhaust gases under the intake manifold which quickly bring it to the proper operating temperature.
When the engine is hot, the bimetal spring contracts, moving the position of the heat control valve to direct most of the exhaust under the valve and away from direct contact with the intake manifold.
The valve within the exhaust system has a counterweight on the outside of the pipe. This counterweight is viewed most easily from under the car. With the engine cold, check that the counterweight is in the upper position. After the engine has been warmed up, check that the weight has moved to the lower position.