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. . . green thing in these pictures? It offends my sense of honor and decency that it has a fitting for another hose/line but that nothing is attached. All input is welcome and appreciated.
That is a heat controlled vacuum switch. When thermostat opens and heats that up, it opens the valve so to speak to let vacuum pass through. It is often used for things like EGR so the vacuum only gets passed to EGR valve to open it when the engine is at operating temp. If you have a/c it might be used to control vacuum for that too. If you no longer have the EGR valve, that might be what was connected to it. I believe they were also used for vacuum to the charcoal canister/EEC system on the later models. When it opens you suck the gas vapors from the charcoal canister, but only when at operating temp.
Regardless of it's purpose, if you don't find the line that is supposed to be connected, I would cap it so you don't have a vacuum leak when the switch does open.
Also thought of another use, I see you still have the heat riser from exhaust manifold to air cleaner housing. There is a valve inside the snorkel that is closed when it is cold. Once engine is warmed up that is supposed to open. It would be opened by a temp activated vacuum switch, so that might be the line that is missing. It appears the larger line goes to your carb and presumably manifold vacuum port on the carb, the smaller line currently connected might be going to your EGR valve, but can't tell with air cleaner on. The other line would go to a vacuum diaphragm located on the snorkel of the air cleaner.
Also thought of another use, I see you still have the heat riser from exhaust manifold to air cleaner housing. There is a valve inside the snorkel that is closed when it is cold. Once engine is warmed up that is supposed to open. It would be opened by a temp activated vacuum switch, so that might be the line that is missing. It appears the larger line goes to your carb and presumably manifold vacuum port on the carb, the smaller line currently connected might be going to your EGR valve, but can't tell with air cleaner on. The other line would go to a vacuum diaphragm located on the snorkel of the air cleaner.
This was a reasonable option, so I looked at it in detail by consulting both the shop manual and the parts catalog and looking at my engine. My truck does not have the thermactor emission control, and the breather does not have the valve to which you refer. That said, I'm continuing to pursue your prior theory and will report back to you on it.
That is a heat controlled vacuum switch. When thermostat opens and heats that up, it opens the valve so to speak to let vacuum pass through. It is often used for things like EGR so the vacuum only gets passed to EGR valve to open it when the engine is at operating temp. If you have a/c it might be used to control vacuum for that too. If you no longer have the EGR valve, that might be what was connected to it. I believe they were also used for vacuum to the charcoal canister/EEC system on the later models. When it opens you suck the gas vapors from the charcoal canister, but only when at operating temp.
Regardless of it's purpose, if you don't find the line that is supposed to be connected, I would cap it so you don't have a vacuum leak when the switch does open.
You, sir, nailed it. I found Scan 1 below in the '74 service manual. That confirms it is precisely what you said.
I also found Scan 2 below. I'm going to use this diagram to try to locate the other "thing" to which the top fitting should be connected and cure that deficiency.
Thank you for your help.
A.S.
Last edited by AppalachianSurvivor; Jan 14, 2025 at 10:06 AM.
Reason: To refer to attachments
Thank you, sir, for confirming the earlier opinion given. As you can see from my response to the first post, I found things in the '74 service manual that both confirmed your answer and that may provide me a way to properly hook the empty port up as intended.
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