Vented -vs- Unvented Gas Cap 72 Scamp w 318

Fuel and Air Systems

  1. mopar155

    mopar155 Well-Known Member

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    Hoping someone here has an answer for me. My buddy owns a 1972 Plymouth Scamp with a 318. The car has a chrome gas cap. He was at the Mopar Nationals and was wanting to purchase a replacement one and was told by 2 different vendors that they would not work on a 72 ? It currently has a chrome cap on it now? Does this year car/engine size need a vented or unvented cap and could some one please explain the difference to me? Thanks in advance, Ron
     
  2. 67Dart273

    67Dart273 FABO Gold Member FABO Gold Member

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    The 72 should have (unless it was removed) the emissions control vent system, (Evaporative Control System) known by many as the "carbon can" These used a "pressure vacuum" cap which had a pressure release at very low pressure in BOTH directions, IE when the tank develops "so much" positive pressure the cap relieves, and the same is true for vacuum

    In NORMAL operation, the carbon can vent system should take care of venting the tank and the pressure vacuum cap should not need to relieve. Look it all up in the shop manual:

    http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual,+download

    The 72 manual

    [ame="http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf"]http://www.abodyjoe.com/pictures/Mis...Serv%20Man.pdf[/ame]

    Earlier cars, like my 67, have a SEALED cap. The vent line on those is welded/ brazed into very near the top of the filler tube, goes up high in the rear fender in an upside down U, and then travels back down through the floor of the trunk, where the open end simply ends up inside the rear frame rail.
     

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    • slantsixdan

      slantsixdan =..=

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      A-bodies do not use vented caps. The car needs what is called a "pressure-vacuum" fuel cap. It is vented, in a manner of speaking, but it's not the same as what is commonly referred to as a "vented" fuel cap. A vented cap has a passageway for air to flow in and fuel vapours to flow out. A pressure-vacuum cap also does, but has two spring-loaded check valves -- one very low-tension one controlling inward flow, and one high-tension one controlling outward flow. Pressure-vacuum caps were first used in '70 on cars sold in California, then in '71 throughout the US and Canada. The design of the cap was changed in the middle of 1971 production. The cap for your friend's '72 Scamp is a late-'71 through '76 item. The '70-'71 cap will not fit correctly. A pre-1971 (pre-1970 California) cap is unvented and will not fit or work correctly.
       
    • 67Dart273

      67Dart273 FABO Gold Member FABO Gold Member

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      Dan must be gettin pretty sparce for material.
       
    • mopar155

      mopar155 Well-Known Member

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      Thanks for all the help guys !
       
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