See Figure 1
This is a more effective form of heat riser which is vacuum actuated. It is used on all models built in 1975 and later. It heats the incoming mixture during engine warm-up, utilizing a ribbed heat exchanger of thin metal that is located in the intake manifold. This preheating allows the choke to open more rapidly, thus reducing emissions. Problems in this system might be indicated by poor engine operation during warm-up.
This valve should be checked initially at 6 months/7500 miles (12,000 km) and thereafter, at 18 month/22,500 mile (36,000 km)intervals.
Move the valve through its full stroke by hand, making sure that the linkage does not bind and is properly connected. If the valve sticks, free it with a solvent. Also check that all vacuum hoses are properly connected and free of cracks or breaks. Replace hoses or broken and bent linkage parts, as necessary. For details on the EFE system refer to Emission Controls in this information.
Every 30,000 miles (48,000 km), apply 10 in.Hg (68.95 kPa) of vacuum to the valve from an external source, or start the engine from an overnight cold condition. The valve should close all the way. If the valve binds and cannot be unstuck through application of solvent, it should close all the way. if the binds and cannot be unstuck, it should be replaced.