Certain early models may have been equipped with exhaust control valves (heat risers) which would be located near the head pipe connection in the exhaust manifold. The valves aid initial warm-up during particularly cold weather by slightly restricting the gas flow. The resultant heat generated by this restriction is transferred to the intake manifold where it results in improved fuel vaporization.
The operation of this valve should be checked every 6 months or 6,000 miles (9,600 km). If the valve appears to be sticking, lubricate the shaft bushings lightly with penetrating oil and then operate the valve manually a few times to work in the lubricant. If this is no help, replace the entire valve.