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I am working on installing a Holley Sniper EFI system and Classic Instruments gauge cluster at the same time on my 1983 Chevrolet Silverado K20 (with the 350/5.7L V8), and as such I am working out which coolant temperature sensors to run to which device.

While investigating this, I went to check what I thought was a coolant temperature sensor on the top of the thermostat housing to see where it ran to, but upon closer inspection it appears not to be an electrical sensor (see picture). If I recall correctly, when I removed the air filter housing, two hoses were running from this coolant temperature sensor-like device to the air filter housing.

What is this device, what does it do, and where does it run to? Is it a coolant temperature sensor/switch, or something else? Do some models have a coolant temperature sensor here?

In case it's relevant, my vehicle does have the usual coolant temperature on the left side of the engine which runs to my old gauge cluster. thermostat housing

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I'm pretty sure there's no temp sensor there, at least one which would give you a reading on a gauge or to an ECU. Basically, it doesn't make sense to run that type of temperature sensor there because it would sit above the thermostat. It would heat/cool as the thermostat opens/closes and wouldn't give a true temperature for any device whether on the dash or as something for a computer.

I believe one side (of the two barbed ends) is hooked to vacuum off the carburetor/intake manifold while the other side is hooked to a flapper on the horn which is attached to the air filter housing. When the engine is cold, the part on the thermostat housing allows vacuum to be pulled through it. The flapper on the air intake horn opens up and provides warm air into the engine (there's a heater tube which runs down onto the exhaust manifold). This allows it to run a little better under really cold conditions while the engine is warming up.

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