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?

The '68 Diagnostic manual has some testing procedures along with the operation discussion. It is for the single snorkel so I don't know what difference the second makes.

I know that when mine failed once - a simple problem when the heat riser tube came off. The car ran bad when it was cold.

Thanks Jim, but I have that same info in various FSM's. They all say the same thing, that it should draw hot air when cold, and modulate once the proper incoming air temp is reached. It even shows the 'vent' function venting into the air cleaner. Makes sense, as that lets the flap open. But it should also shut off the manifold vacuum. Mine vents at all times. It never shuts the hot air flap. Both sides just act like a huge vacuum leak. No response from Firebird Central, I think a phone call is in order.

So, Firebird Central was great. They are shipping out a replacement, no drama. I get a lot of stuff from them and they have always been good.

The new sensor from Firebird Cental worked as it should. If you test with a Mityvac you get between 5 and 10 inches of vacuum at the sensor before it vents into the air cleaner. The bad one vented at 1 inch and became a serious vacuum leak. The replacement operates the hot air flap correctly. They sent me a return label in the box so I could send the bad one back. An excellent company.

Getting ready to get my 69 out of the paint booth and on the road. Got MANY parts from them and never had an issue. Called them with a couple of questions and the people on the phone sounded very knowledgeable on the parts. A nice surprise for these days.