See Figure 1
The heat control valve (heat riser) is a thermostatically operated valve used on some gasoline engines (in the exhaust manifold). It is used only on 1975-77 trucks. It opens when the engine is warming up, to direct hot exhaust gases to the intake manifold. The gases preheat the incoming air/fuel mixture. If it sticks shut, the result will be frequent stalling during warm-up, especially in cold or damp weather. If it sticks open, the result will be a rough idle after the engine is warm.
The heat control valve should be checked for free operation every six months or 6000 miles (9600 km). Simply give the counterweight a twirl (engine cold) to ascertain that no binding exists. If the valve sticks, apply a heat control solvent specially formulated for the purpose to the ends of the shaft. This solvent is available at most auto parts stores. Sometimes lightly rapping the end of the shaft with a plastic mallet (engine hot) will break it loose. If this fails, the components will have to be removed from the truck for further repairs.