Was woking on my 79' Corvette Coupe and had the air filter off and when putting it back together I realized that the vacuum diaphram is unconnected to vacuum connection. There are two of these diaphragms on the air filter assembly and the two hoses from the diaphragms run to a T-Hose connector and off of that T runs a single hose that is not hooked up to anything.

Reading through the shop manual it states that the diaphragm should "Run to the base of the carburetor (Source of intake manifold vacuum)." After attempting to find this, the only available connection is at the base, but it has a rubber plug on the end.

I don't know if the previous owner decided to disconnect this for some reason (I'm not a car guy) but wonder if I should remove the plug and connect up the air filter assembly to this.

I have a picture of the plugged tube and wonder if anyone knows if this is something I should hook back up or leave the air filter assembly wide open.

See the photo below, I have the plugged connection circled in red:



Thx for any and all help


Paul

 

Sorry, I made a mistake and deleted my message.

 

That port is a "manifold vacuum" connection. The air cleaner has a vacuum operated damper in the air inlet tube. The vacuum line should have run from the carb port to a temperature switch, and then to the air cleaner. When the engine is cold, the vacuum would NOT pass through the temp switch and so the baffle would keep the inlet tube closed, forcing it to get air from the exhaust manifold area. This would allow heated air into the carb so there were less hydrocabons produced [that if it were ingesting cold air]. When the switch opened up with engine heating, then the baffle would open so that cooler air could be taken in by the air cleaner.

If you want to minimize excess hydrocarbons, hook it up. You may be required to keep that equipment on the car for State emissions control requirements--if you have to get your car inspected for that. If not, it's up to you whether you want to keep it or 'ditch' it. The engine will run just fine that way it is set up right now.

 

On my 1980, when the temp sensor switch is cold it is OPEN and allows vacuum to pass to the flapper (closing it) And yes it comes from the manifold port that you have caped off. Mine originates from the manifold goes to a tee. One hose off of the tee goes to the sensor and the other leads to the center port of the other temp operated valve on the outside of the air cleaner.

 

I just picked up a 76 and the vacuum lines were all messed up. After some digging for info, starting here, I wound up hooking my air cleaner sensor (small valve unit on the inside base of the air cleaner with 2 vacuum line hookups) to that exact manifold vacuum fitting. With the engine cold, the sensor is closed, not allowing vacuum to get to the inlet duct diaphram (diaphram in the inlet duct), which without vacuum is open, allowing warm air from the heat hose (metal hose hooked to the cover on the left side exaust manifold) to go into the air cleaner inlet. As the engine warms, the sensor will open, allowing vacuum to pass to the duct diaphram and will close it, blocking off the inlet of warm air off the manifold and allowing cool air to flow from the cool air intake. After hooking into that vacuum inlet (originally I had no vacuum line going to the bottom of the air cleaner) and with the engine running for 3 to 4 (ok, maybe 5) minutes, the duct diaphram closed (only took a few secs), closing off the warm air and supplying cool... boy... do I love it when 30 something year old stuff actually works. I think the rearward side of the sensor went to that fitting with the foward side going to the duct diaphram (I only have one duct diaphram... sounds like you might have a L-82 (?) with two diaphrams). To make it short... I agree with the other responses you have gotten. If you need the diagram of the vacuum lines, I have it here somewhere. Post the request or e-mail me.

 

Spoke too soon... dgmartucci is correct. When the sensor is cold it allows the vacuum to pass and close off fresh cool air and opens the inlet to allow the warmed air to enter. Once warm it cuts the vacuum, the flapper closes allowing fresh cool air in and blocks the warm air off the manifold. No other sensors involved. Should have looked before I spoke... and shouldn't be posting at 12:30am after a few.

 

I just wish mine worked. I have replaced the switch 3 times trying to get it to work. I guess it just doesnt get warm enough in my air cleaner to close the switch.

 

How's the condition of the "stove pipe" cover over the exhaust manifold. Mine is in good enough condition (front and back cover) to get enough heat to the air filter assembly and even with that I picked up a really clean one to replace it on an auction site. I would imagine that if enough of it isn't there you may not be getting enough heat to trip the sensor.

 

The stove pipe and the heat shroud are both in good shape. The only thing I can see causing it not to get warm enough is that both flappers (I have the "y" type) don't completely close off the cold air coming into the snorkels. They close and seal on the bottom and sides but there is a 1/4 " gap across the top of the flapper. And there is also air coming in from the air pump. Although I am assured by the auto parts stores that I have the right switch, I think I still might have the wrong one.