Pre-Heat Tube and Thermoduct
This component is required on most vehicles with carbureted engines. Late model fuel injected vehicles do not have a pre-heat tube. The pre-heat tube is usual a flexible metal duct attached to the engine's air filter housing on one end and an exhaust manifold cover on the other.
The duct is used to deliver heat to the air being drawn into the engine. It is designed to promote faster carburetor and engine warm-ups. Once the vehicle is at proper operating temperature the passage door shuts, stopping any further warm air from entering the air filer assembly.
If your vehicle is equipped with a carburetor, the pre heat duct or tube will look similar to the image indicated in the diagram below.
Your vehicle's Underhood Emissions label can provide you with information regarding the requirement of this component.
Location: The metal pre heat duct will be attached to the engine's air filter housing on one end and an exhaust manifold cover on the other. If your vehicle needs to be equipped with one, this component will not be difficult to locate.
In case of Failure: Frequent removal and installation of the air cleaner housing often causes the pre-heat tube to disconnect and at times be misplaced or lost.
Should you notice the duct is missing, simply visit any auto part store and purchase a new one. Very often auto parts stores can sell you a new duct cut specifically to your vehicle's specifications.
For the Smog Test: During the visual portion of the smog check, the smog technician will be inspecting the heat duct for proper connection. During your at-home inspection you should ensure this component is present and in good condition. The duct should have no cracks and should be properly connected at both ends.