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Old 10-01-2010, 02:24 AM
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Pontiac 400 vacuum outlet question

What is the vacuum outlet in the front of the intake (to the left of the thermostat when looking at the pictures), with several nipples supplying several vacuum hoses?What are those hoses supplying vacuum to? Both engines are pontiac 400s in a 68 gto, just like mine, but one gto doesnt have that vacuum outlet and the other does. I ask because my car has that outlet, but no hoses connected to it, yet everything seems to work fine. It has hideaway lights, but they are operated from a single vacuum outlet in the intak closer to the carb, at least thats what I was told, they are not working right now




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Old 10-01-2010, 03:44 AM
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The vacuum tree fitting next to the thermostat was a heat sensitive switching device, that ran the exhaust crossover flapper at the passenger side exhaust manifold outlet, possibly the vacuum block on the secondaries of the carb and/or the heated air door flapper in the original snorkel air cleaner. The exhaust flapper forced the exhaust to cross through the under-carb heat crossover for faster warm-ups, the vacuum block diaphram on the carb prevented secondary actuation when engine was cold.

On that vacuum tree sensor, one line comes from carb base or manifold full time vacuum, the other lines go to one of the vacuum operated devices mentioned, and the whole sensor is screwed into the water crossover, so that it senses when the engine has warmed up and shuts off vacuum to the exhaust crossover flapper and secondary lock-out.

I think it would have been present on all stock '68 Pontiacs, the one pic with it missing the owner may have removed it to clean up the engine bay/get rid of the hoses. Lots of folks lump it in with "that emissions crap" and get rid of it, but its purpose was to promote cold weather drivability sooner after start-up..
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Old 10-01-2010, 09:31 AM
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Ok, that makes sense. Until now, I thought the exhaust crossover was functioning full time, it just looked like an open passage throught the intake between the heads, I didnt know there was a flap that turned it 'off'. And because the car has a aftermarket carb, that would also help explain why mine is just there to plug up the hole in the intake
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Old 10-01-2010, 01:33 PM
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You're right, the crossover is open all the time. I may not have worded the description well and can't remember what the flapper bolted onto the outlet of the passenger side exhaust manifold outlet is actually called(exhaust diverter valve ), but the vacuum diaphram on the exhaust flapper pulled shut when the engine was cold, forcing nearly all of the passenger side exhaust to cross under the intake, not letting it go out the normal way out of the exhuast manifold, heating the intake and carb faster.
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Old 10-01-2010, 03:12 PM
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on my 72 firebird that same thing runs to something for my A/C or a vacuum pod.
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Old 10-01-2010, 03:54 PM
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Generally speaking, the the temperature actuated vacuum switch can turn ON vacuum when the engine coolant has reached a predetermined temperature to provide vacuum to the vacuum advance and EGR, or to turn OFF the vacuum- like to the exhaust heat riser (or "stove").

OT- Early heat risers were operated by a bi-metal thermostat coil that acted as a temperature sensitive spring and a counterweight. These would often "knock" when the thermostat spring failed- until the owner wired it open, that is.
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Old 10-16-2010, 01:04 PM
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where would I find a vacuum plug to replace this tree that isnt being used? I think it may be bleeding off vacuum
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Old 10-16-2010, 01:26 PM
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Quote:
Originally Posted by adam83
where would I find a vacuum plug to replace this tree that isnt being used? I think it may be bleeding off vacuum
At the hardware store or parts store either one will have it if not get the size and order it from jegs or summit. JMO Heres a pic of mine pluged off.
Put alittle thread tape or something on it and it should be fine.IMHO

Cole
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Old 10-16-2010, 01:35 PM
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sweet thanks
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