What is this vacuum thing?

[TubeTruck]

So I was reading another thread and didn't want to hijack that one so I started this one. I have this vacuum thing on my truck that seems to control the vacuum advance on my distributor, but it's in the thermostat housing. Is it some sort of temp controlled vacuum modulator or something? I'll put up 2 pics, one original and the other with lines drawn for the hoses.

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edit: posted the right colored drawing

 

[Frankenchevy]

My guess is it adds a couple degrees when cold or pulls a couple when it’s hot? Probably wrong, but efi systems will add or pull timing based on temp parameters.

 

[1987 GMC Jimmy]

That’s called a thermal vacuum switch. They’re typically there for the EFE valve in the passenger exhaust manifold, but they can have them for canister purge, EGR, etc. Trucks did have distributor delay back in the spaghetti years, and you’ve got it going to the choke pull off and thermac for some reason. I guess it kinda makes sense, but if you’re debating deleting extraneous emissions parts, I wouldn’t lose sleep over it.

 

[C10MixMaster]

So I was reading another thread and didn't want to hijack that one so I started this one. I have this vacuum thing on my truck that seems to control the vacuum advance on my distributor, but it's in the thermostat housing. Is it some sort of temp controlled vacuum modulator or something? I'll put up 2 pics, one original and the other with lines drawn for the hoses.

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it's a thermal vacuum switch that switches between manifold and ported vacuum for the vacuum advance. To the best of my understanding on this, in theory you run on ported causing the engine to run hotter (late spark at idle) as the engine reaches operating temp or possibly over operating temp it switches to manifold vacuum (earlier spark at idle) allowing it to run cooler. Seems to be found on trucks with A/C.

Im not sure but i think yours is hooked up wrong. looks like the dist line should be in the middle position.

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[TubeTruck]

Thanks for all the replies guys! My truck doesn't have much power. I figured being a 350 in a heavy Suburban it wouldn't move that well but I think it should be moving better than it is. @C10MixMaster Thanks for the schematic! I believe you are right about my distributor being ported wrong. I'll move it later and let you know how it turns out.

I'm kind of wondering what happens if one of those valves were to fail also. Does it suck coolant into the motor? lol

 

[TubeTruck]

Nope, I followed the lines wrong lol This is what it looks like

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[AuroraGirl]

A rebuild or a new carburetor will spruce up your life.. believe me haha. I removed all that emission stuff that.. lets be honest.. isnt going to be working properly in 2019.

 

[TubeTruck]

A rebuild or a new carburetor will spruce up your life.. believe me haha. I removed all that emission stuff that.. lets be honest.. isnt going to be working properly in 2019.

Already removed/bypassed it, lol. Got a new air cleaner, plumbed the vac adv into the carb and capped the other. I have a Holley 650 sitting on a shelf that need a rebuild, along with an Edelbrock performer intake. I'll probably do that over the winter. I don't want to sink too much into this motor because a big block will hopefully be going in next summer, unless an axle grenades, then I'll be doing those first.

 

[AuroraGirl]

For me... if the truck performs valiantly its getting the floorboard fixed and made road worthy. For this winter she will serve me well plowing.