Simply, the EGR is opened by ported vacuum when you step on the gas, and allows a small portion of exhaust to recirculate back into the intake manifold, keeping combustion temperatures down and reducing emissions. The CTO and TVS are in the circuit to block vacuum and keep the EGR from working when the engine is cold. The TVS should be open above a nominal 40° to 55° air temperature inside the air cleaner, and the CTO should be open above a nominal 115° engine temperature.

Incidentally, the CTO also blocks the purge signal from the Evaporation Canister when the engine is cold.

One nipple on the CTO was plugged from the factory, as was the vacuum nipple on the front left corner of the carburetor.