Vented gas caps

Discussion in 'The Garage' started by dyeager535, May 10, 2004.

  1. dyeager535

    dyeager535 1 ton status Premium Member

    Dec 13, 2000
    Likes Received:
    Roy WA
    Anyone out there with the "new style" gas caps that were factory without EVAP?

    Friends '83 5.7L C30 is non-evap from the factory so he says (and I don't really doubt it) however I've noticed at least from the online catalogs, there is no difference between that applications gas cap, and say a 1986 5.7L K5.

    I know that has to be untrue, as the tanks without EVAP have no way to vent BUT the cap. I was able to find one in my dads garage that is obviously older, (metal cap) but threads onto the "hidden" fill neck setup. It has two valves that are visible, and you can both blow through the cap from the underside, as well as suck air through it. Obviously some resistance, but the EVAP caps won't let ANY escape to the atmostphere, only in.

    I plugged the EVAP fitting on my TBI sender in my truck, and put 5 gallons in it, and used the cap I'd been running with an EVAP line before engine swap. I came back maybe two hours later, and there was a puddle of gas around the front of the truck which shouldn't have been there (from a loose fuel line fitting) since it had to make its way "uphill" and through a non-functioning fuel pump. (although I think they will free-flow) Anyways, after tightening the line, I opened the gas cap to add more to the tank, and the pressure buildup was immediately obvious.

    I really doubt the non-EVAP rigs just ran a breather off of the tank sender, but since I don't own one, and don't have access to my friends now, I'm curious if anyone else has seen these caps, if they did in fact exist up into the '80's n certain apps. Even diesel caps are not vented to atmosphere, from the ones I've tested.
  2. 84_Chevy_K10

    84_Chevy_K10 Banned

    May 30, 2001
    Likes Received:
    IL, USA
    I have been leaving my gas cap loose for years. /forums/images/graemlins/rotfl.gif

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