My 1968 Firebird came with the THERMAC system on the air cleaner. I am currently using the early 70's dual snorkel air cleaner. It also has the THERMAC system. The sensor was bad, staying in HOT AIR mode all the time. I bought a new sensor from Firebird Central,[1968 to 1981], but it doesn't work right. It just vents to atmosphere all the time. It does not close the hot air flap at all. The air cleaner is totally stock, with everything hooked up correctly.
I understand how the system works, but nowhere does it explain exactly how and what the two ports do. One is hooked to manifold vacuum, the other to the hot air flap. If cold, it sends hot air to the carb. Once approximately 100° is reached, it blends the hot and cold air to maintain that incoming air temperature. The flap motor must vent, and the manifold vacuum must modulate to do this mixing. But my new one just vents all the time, and causes a huge vacuum leak. The engine leans out and runs like crap. It doesn't matter which port I use, it just vents. Plugging the vacuum line is the only fix right now. Has anyone ever tested the sensor ports and if so, what did you find?
