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Ported Vacuum Switch

Discussion in 'The Garage' started by Mantruck-k5, Jul 2, 2006.

  1. Mantruck-k5

    Mantruck-k5 Registered Member

    Feb 16, 2005
    Likes Received:
    Fairfield CA
    I've been a lurker here for a while and I do plan on buying a membership soon. I like this site and it's time I started paying up and also I get tired of not being able to see the pictures...
    My Blazer is a 1986 with California emissions. Since I'm just living here in California because I'm in the Air Force I could register my Blazer back in Texas and just rip all of the emissions stuff out but I'm so annoyed with all of my 57 vacuum hoses that it's now my personal goal to pass smog.

    The previous owner had saftey wired my heat riser on the manifold to the open position because the the two port vacuum switch had failed. I ordered a new one from Kragen but when it came in I found that the replacement switch stayed open until 115 degrees and then closed. My ever reliable Haynes manual says that the switch should be closed when cold and then open to allow vacuum to pull the heat riser open. Which is it? I don't feel like pulling my tail pipe from the manifold to see which way is open and I couldn't find one a pic-n pull-that was exposed... I oreder another one through Napa just now...

    Oh yeah, I hope everyone here can appreciate fender flares. Check out this guys truck for sale.
  2. diesel4me

    diesel4me 1 ton status Premium Member

    Jul 24, 2003
    Likes Received:
    how mine is hooked up...

    The "EFE" valve(heat riser) on my 86 305 in my 79 C10 is vacuum operated,and it has a spring built into the diaphram unit, that returns the butterfly to the open position(no exhaust directed under the intake) when NO vacuum is present!--so that means the ported vacuum switch allows vacuum to go to the EFE valve when a cold motor is first started ,and is DENIED after the motor warms up to whatever temp it is the switch is calibrated for..

    In an effort to stretch gas mileage,I tried hooking my EFE valve to manifold vacuum,so it will close,and send exhaust under the intake all the time--my 305 seems very "cold blodded",like many other small blocks I've had--it runs best with the carb toasty warm,its Edelbrock performer with an adapter to the stock Q-jet manifold is usually ice cold even when the motor is fully warmed up.driving it on cool rainy days is challenging,it ices up and stalls quite often if the EFE valve isn't closed..one of theese days I'll find a new PVS switch and water outlet so I can hook it up stock again..for now since I dont go far in it anyway,I'll just leave it as it is...

    ...my truck only goes 3-5 miles at a time lately,a lot of short trips..I've noticed it seems to get better mileage this way,though I bet I'll have to hook it up stock if I drove on the highway for 20+ miles,to avoid pinging..but the EFE valve would open anyway under a hard pull or hill climb,since it would lose vacuum when the throttle is opened wide...so far it runs great this way,maybe with the Edelbrock setup,it needs more "heat" to vaporize the gas better...???it seems much peppier with it set up that way too..:crazy:
  3. Big GMC truck

    Big GMC truck Registered Member

    Aug 23, 2005
    Likes Received:
    I went through the whole vacuum switch ordeal with my 1984 GMC k1500 305 about 3 months ago. My truck was purchased in South Carolina, but has all the smog stuff and passes smog in California. I'm pretty sure that the switch actually sends vacuum to the heat riser when the engine is cold in order to shut the internal plate. After the engine is up to temp, vacuum is no longer sent to the heat riser, which allows the internal plate to open in order to allow exhaust gases to exit.

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