Vacuum Delay, Lets try again

[GWJeeper]

I did a poor job of asking my question so lets try again. On our '91 there is a gray plastic vacuum delay switch mounted in the air cleaner, it is temperature controlled and is plumbed into the vacuum line that opens the inner "vapor" door in the air cleaner snorkel.
I understand that manifold vacuum opens the door and there is a time delay switch to buffer the door operation during periods of low vacuum. What I don't understand is why the vacuum signal is also controlled by this temperature delay switch. To my thinking you would always want the "vapor" door open when the engine is running regardless of the temperature. Having the door closed when the engine is running would be like an additional choke mechanism.

[letank]

I did a poor job of asking my question so lets try again. On our '91 there is a gray plastic vacuum delay switch mounted in the air cleaner, it is temperature controlled and is plumbed into the vacuum line that opens the inner "vapor" door in the air cleaner snorkel.
I understand that manifold vacuum opens the door and there is a time delay switch to buffer the door operation during periods of low vacuum. What I don't understand is why the vacuum signal is also controlled by this temperature delay switch. To my thinking you would always want the "vapor" door open when the engine is running regardless of the temperature. Having the door closed when the engine is running would be like an additional choke mechanism.

In fact there are 2 flappers and 2 vacuum bellow mechanism, one for the temp controlling the heater stove with the sensing probe in the air filter housing and the other for the flapper to control/prevent gasoline vapors from being let go in the air when the engine is off... so they are not really delay control valve.... the delay valves are little plastic gizmos with a color ring


here is the one for the vapor flapper, pictured with the delay valve for the heater stove

[Tripwire]

I always wondered about the "outside" flapper - the heat stove one i understand.... perhaps its to keep Mice out LOl

[Bob Barry]

The outer flapper is the heat-stove, which is normally open without vacuum; you want that closed by manifold vacuum until the incoming air is above a certain temperature. That's why its vacuum source is run through the thermostatic switch in the bottom of the air-cleaner housing.

The inner-flapper is normally closed to prevent the fuel-vapor from the carburetor escaping; it should open as soon as the engine is running and stay open.

The delays keep those flappers from snapping open and shut as you drive the truck and the manifold vacuum rises and falls.

The gray plastic switch on the side of the air-cleaner is for the EGR circuit, to prevent the EGR from engaging when the incoming air is too cold. The snorkel-flappers should not be run through that switch.

I explain a number of these things in the second video in my vacuum rebuild series, toward the end at about the 17:58 mark:

https://youtu.be/z8Lg-GsbYDk?t=1078

[GWJeeper]

In fact there are 2 flappers and 2 vacuum bellow mechanism, one for the temp controlling the heater stove with the sensing probe in the air filter housing and the other for the flapper to control/prevent gasoline vapors from being let go in the air when the engine is off... so they are not really delay control valve.... the delay valves are little plastic gizmos with a color ring


here is the one for the vapor flapper, pictured with the delay valve for the heater stove

Michel, I agree with everything you're saying but the piece I'm talking about is different.

[GWJeeper]

The outer flapper is the heat-stove, which is normally open without vacuum; you want that closed by manifold vacuum until the incoming air is above a certain temperature. That's why its vacuum source is run through the thermostatic switch in the bottom of the air-cleaner housing.

The inner-flapper is normally closed to prevent the fuel-vapor from the carburetor escaping; it should open as soon as the engine is running and stay open.

The delays keep those flappers from snapping open and shut as you drive the truck and the manifold vacuum rises and falls.

The gray plastic switch on the side of the air-cleaner is for the EGR circuit, to prevent the EGR from engaging when the incoming air is too cold. The snorkel-flappers should not be run through that switch.

I explain a number of these things in the second video in my vacuum rebuild series, toward the end at about the 17:58 mark:

https://youtu.be/z8Lg-GsbYDk?t=1078

Bob, I agree with everything you've said, perhaps my GW has the vacuum hoses routed incorrectly (hard to imagine, isn't it LoL) I've studied the FSM and the diagrams on OlJeeps pages and can't find anything that looks like it represents the gray plastic switch. I'll check out your video and see if I can figure out the correct routing

[GWJeeper]

Bob, your comments spurred me to greater heights of glory. I kept staring at the FSM vacuum diagrams until it finally began to make sense. I now see where the gray plastic piece is in the pix. My GW is so stock that I made the mistake of thinking that what I was seeing on the engine was correct.
I believe that the gray piece is probably in good working order but the hose connections have broken off. I haven't been able to find the exact part number of what is on the car but I've ordered the closest I could find and will gut the old one and put those internals in the new "case". It's just a couple of pieces of thing sheetmetal that I assume work on expansion and contraction.
thanks for the information.