Every 30,000 miles, the heat riser valve should be checked for free operation and lubricated with penetrating oil. The valve is located on the exhaust manifold near the point that the exhaust pipe attaches to the manifold. Try to turn the valve counterweight by hand. If it's stuck, tap the end of the shaft a few times with a hammer. Apply penetrating oil to the shaft ends and work the valve back and forth a few times. If the valve is still stuck and can't be loosened with oil and/or heat, it will have to be replaced.