Vacuum advance TVS question?

Silver State 77

Silver State 77

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Im connecting vacuum hoses on a 1977 6.6 and im seeing the strangest thing, it looks like the vacuum advance is getting ported vacuum until the engine warms up, and then it gets manifold vacuum. is that right?
Why would they attach the manifold vacuum line to that Thermal switch?
Is that why they want the timing set so far advanced at 20 deg.?
 

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Silver State 77 said:
Im connecting vacuum hoses on a 1977 6.6 and im seeing the strangest thing, it looks like the vacuum advance is getting ported vacuum until the engine warms up, and then it gets manifold vacuum. is that right?
Why would they attach the manifold vacuum line to that Thermal switch?
Is that why they want the timing set so far advanced at 20 deg.?
Click to expand...
Looking at the diagram there is only one source of vacuum coming from the carburetor and it is used for both the distributor and EGR valve once the TVS opens.
 
I agree that there is only 1 source of vacuum for the Distributor on that diagram. As to why they would do it that way, they had to meet cold start emissions using the technology they had at that time. The limit the vacuum advance until the engine comes up to operating temperature. And remember the different divisions engines all had slightly different emissions controls. I am assuming you have a 403 Olds engine in your 77 GP, and not the 400 Pontiac version.

If you really are interested in how the system was supposed to operate get a copy of the 1977 GM emissions manual like the one on this link
 
melsg5 said:
Looking at the diagram there is only one source of vacuum coming from the carburetor and it is used for both the distributor and EGR valve once the TVS opens.
Click to expand...
Here is a better picture showing the 3rd vacuum line going under the PCV hose, it is connected to Manifold Vacuum.
The sticker under the hood shows the same thing, but my question is, How can your vacuum advance work in reverse, once the TVC opens?
______________________________
 

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cammerjeff said:
I agree that there is only 1 source of vacuum for the Distributor on that diagram. As to why they would do it that way, they had to meet cold start emissions using the technology they had at that time. The limit the vacuum advance until the engine comes up to operating temperature. And remember the different divisions engines all had slightly different emissions controls. I am assuming you have a 403 Olds engine in your 77 GP, and not the 400 Pontiac version.

If you really are interested in how the system was supposed to operate get a copy of the 1977 GM emissions manual like the one on this link
Click to expand...
I have a Pontiac 403 engine in my 77 Grand Prix, same as a Trans Am. It was first sold new in Utah, so it does not have a smog pump or cat converter.
Its shaking at all RPMs at first I thought it might be the carburetor, but now im thinking its an ignition problem.
That is what i get for changing too many parts at the same time. Now I have to put old stuff back on to see if it runs better. Lol;
 
Silver State 77 said:
Here is a better picture showing the 3rd vacuum line going under the PCV hose, it is connected to Manifold Vacuum.
The sticker under the hood shows the same thing, but my question is, How can your vacuum advance work in reverse, once the TVC opens?
Click to expand...
Sorry dont see how that second diagram is showing any other vacuum source to the distributor. You have one hose connected to distributor going to the TVS valve. How many ports does your TVS have?
 
melsg5 said:
Sorry dont see how that second diagram is showing any other vacuum source to the distributor. You have one hose connected to distributor going to the TVS valve. How many ports does your TVS have?
Click to expand...
Sorry, Its not a solid fat line, like the other ported vacuum lines are shown,
I used my paint, to color the 3rd line in Green, and left the original document lines to show its validity.
______________________________
 

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Its a mystery why they think all the smog devices installed on engines to burn cleaner, when all they really seem to do is, make it cost more money, use up more gas, and eventually destroy an engine with its own exhaust and back pressure.
I believe, these older classic cars that are still running today, its because they were made in the days before smog device.
I have a 1974 el Camino that my dad gave to me over 23 years ago, and it has over 500000 miles on the original engine, its very reliable transportation, with great gas millage, and its smog exempt. THANKS dad
 
Silver State 77 said:
Sorry, Its not a solid fat line, like the other ported vacuum lines are shown,
I used my paint, to color the 3rd line in Green, and left the original document lines to show its validity.
Click to expand...
How many ports on the TVS?
Are these lines missing currently that you need to refer to a diagram?
 
Yeah White 70's Pontiac's on a trailer! Kinda spooky, I just managed to get that Catalina moving again under its own power last month. It runs strong for a 50+ year old car.
 

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