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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: MY1978 on May 01, 2021, 10:53:17 am
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I have a question about the damper position for the Thermac. I have tested the Thermac with a vacuum tester and it closes with vacuum. So, i need clarification as to the damper position when the engine is started and cold - Is damper open or closed? Mine is open on cold start, so taking air from the stove pipe and outside. Once it idles down, the damper is still in open position. Is this correct? I do not have the air filter in place and the cover is being powder coated. Should I have these in place to verify if working?
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Following is a description of the Thermac system and its normal operation:
The Thermac decreases throttle plate icing and improves fuel atomization and cold drivability by modulating the closed-open position of a temperature door (aka, damper) located in the air cleaner snorkel whenever the temperature of the air entering the air cleaner housing is cooler than about 130° F.
Below ~85° F, the Thermac damper in the air cleaner snorkel completely closes to block unheated ambient air and opens to air that is artificially heated by the exhaust manifold heat stove. Above ~130° F the opposite is true; the Thermac damper completely opens to unheated ambient air and closes to block air artificially heated by the exhaust manifold heat stove. Between ~85° F and 130° F, the damper rotates through midway positions allowing a blend of ambient and artificially heated air to enter the carburetor. With the engine OFF (not running), the damper door defaults to the fully OPEN position.
The Thermac system consists of the movable temperature damper door and its vacuum actuator (aka, vacuum motor), a vented bimetal temperature sensor located in the floor of the air cleaner housing on the filtered air side of the air cleaner element, two vacuum hoses that connect the damper vacuum motor to the temperature sensor and the temperature sensor to an intake manifold vacuum port, the enclosed air cleaner housing itself, and the various shields and pipe that comprise the exhaust manifold heat stove and duct connecting the heat stove to the air cleaner snorkel. The damper door vacuum motor is spring-loaded to force the damper into the default OPEN position. Vacuum applied to the vacuum motor overcomes the internal spring pressure of the motor to CLOSE the door. The temperature sensor controls the Thermac damper position by partially venting the vacuum signal passed to the vacuum motor in direct relation to the temperature of the air entering the air cleaner assembly.
Now, to your question. In application, the Thermac is controlled by temperature and engine vacuum working to overcome spring pressure:
- With the engine OFF (zero vacuum), the Thermac damper should default OPEN as you peer into the end of the air cleaner snorkel.
- With the engine RUNNING and the ambient air temperature >130° F, the Thermac damper should appear completely OPEN.
- With the engine RUNNING and the ambient air temperature <85° F, the Thermac damper should appear completely CLOSED.
- With the engine RUNNING and the ambient air temperature between 85° F and 130° F, the Thermac damper should appear midway between completely OPEN and completely CLOSED.
An exception occurs when/if instantaneous full-throttling drops manifold vacuum exceedingly low, in which case the vacuum signal to the Thermac motor momentarily diminishes and the damper proportionally opens if otherwise closed. Finally, for effective temperature sensing and air control, the Thermac air cleaner assembly must be fully assembled and installed as originally designed.
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Thank you again bd, you are most helpful. :)