Gas Cap question

1971 - 1974 Mopars

  1. gary h

    gary h Well-Known Member

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    Although my '71 GTX has been running well, I noticed a "Swoosh" when i removed the gas cap today and noticed gasoline on the inside of the cap (Tank was half full). The temp today was mid Eighties.the cap may be the original and all lines are correct,including the Vapor tank in the wheel well. Is this Normal ? Should the Gas Cap be Vented ? Thanks
     
  2. Jerry Hall

    Jerry Hall FBBO Gold Member FBBO Gold Member

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    Either the vent is stopped up or the cap needs to be vented. I'm not sure what the setup is on a 71 GTX, but a friend had a problem on a 72 Charger and the vent lines were plugged with dirt dobber nests. His car would actually run for about 15 minutes and quit from lack of fuel. He would remove the gas cap and hear the same sound you are taking about. He could replace the gas cap and go for another 15 minutes.
     
  3. Bee1971

    Bee1971 Well-Known Member

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    Does your cap look like this ? 1971
    You can see the tiny hole inside the white plastic , spring operated pressure release
    BC3413C1-FBB7-4C56-A6A9-EC9B4D9388B6.jpeg C7EC3D48-CE0A-4950-BDE5-8DDF2CA295EF.jpeg

    The factory gas cap was pressure vacuum or so called


    The pressure vacuum gas cap is designed to vent small amounts of air into the car gas tank . The vented gas cap has a one-way release valve that is pressure-activated. When the pressure reaches a certain point (defined by so many pounds per square inch) the valve opens by a very small amount, relieving some of the pressure by allowing air into the gas tank , not out into the atmosphere
     
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    • WileERobby

      WileERobby Well-Known Member

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      Your vent system is probably clogged. Lines and/or canister.
       
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      • Billccm

        Billccm FBBO Gold Member FBBO Gold Member

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        Do you have a part number or type? Recently in Mopar Action Rick Ehrenberg recommended the 1971 gas cap for all mopars with it without emissions canisters.
         
      • Bee1971

        Bee1971 Well-Known Member

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        #3466825 Mopar NOS 1971-1972 B Body
        Certain months I think thru April 1971


        Today at your local jobber - Same vent principal

        #10806 Stant
         
        Last edited: Jun 16, 2021
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        • gary h

          gary h Well-Known Member

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          I replaced the rubber vent lines and Gas Tank a few years back.Today I cleaned the cap and it reads "Pressure Vacuum" So maybe this is a Normal thing.I think my cap is original so i might replace it anyway.So it is "Non Vented". I don't hear a "rattle" when i shake the cap ,like some do. IMG_20210616_113219937.jpg

          IMG_20210616_112834886.jpg IMG_20210616_112846770.jpg
           
        • Billccm

          Billccm FBBO Gold Member FBBO Gold Member

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          I believe the red plastic thing is a vent?
           
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          • Bee1971

            Bee1971 Well-Known Member

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            Yes , like in my description that i posted earlier

            Keep that - It looks original

            And see the pressed in rivet on the front side

            The raised area or the lip around the washer for rivet , you see two small holes like a C inbetween the turn flange

            Is your two holes that allows AIR TO ENTER through the gas cap into the gas tank , not vent into the atmosphere like you would think

            Complete opposite

            Then you have your vapor canister inside the drivers side rear wheel well

            And when that breathes , your vent line runs all the way up to the passenger side three nipple breather on your valve cover
             
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            • gary h

              gary h Well-Known Member

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              Correct Except on Mine the Return hose goes to the Filter/Separator at the Fuel pump (mounted on Eng.)Not to the Three Nipple Breather on Valve cover. Car was made in May '71 if that matters.
               
            • Bee1971

              Bee1971 Well-Known Member

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              From what i remember

              My 71 383 Superbee does not have the fuel pump seperator

              On your 71 with a 440 there is an 1/4" return line from the fuel pump separator back to the sending unit in addition to the 5/16 vapor line going from the 3 nipple breather cap on the right valve cover back to the vapor tank in the left rear fender well. Then there a four vapor lines from the vapor tank to the gas tank.

              So you should still have that vapor line off the three nipple breather running back to your drivers side rear wheel well tank vapor seperator

              Plus an additional 1/4" return line off the fuel pump seperator that runs back to your fuel sending unit

              With yours being a late build 1971 going into the production year 1972 I wonder if that changed ?
               
              Last edited: Jun 16, 2021
            • ODZKing

              ODZKing Well-Known Member

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              Fuel Cap part number is as follows:
              Up to 2/1972 - 3466825
              After 2/72 - 3549088 (Supersede To) 3780670
              1973/4 - 3906604

              Addition 6/18. Looking through my actual parts book I see there is a bulletin that was added since they missed 14-86-35 in th e71 book. IT states that 3466825 is "ALL" (w/vapor separator). FYI.
               
              Last edited: Jun 18, 2021
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              • gary h

                gary h Well-Known Member

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                Well here are some pics.Return line on sender Plugged.Two lines going to front of car,(3/8" line to Fuel pump).
                The other line 5/16 goes to the return nipple on the Vapor separator/filter mounted at fuel pump.The Metal 5/16 line ends at firewall below r.f fender.Probably supposed to go to the 3 Nipple Breather cap.Should i have 3 lines running on the frame rail ? Only see 2 lines.

                IMG_20210618_123128138.jpg IMG_20210618_123154026.jpg IMG_20210618_123728473.jpg
                 
              • Billccm

                Billccm FBBO Gold Member FBBO Gold Member

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                Okay I started with a STANT 10806 from Orielly and that did not fit. Tried a 10807 STANT and it fits REALLY SNUG, but it fits. The vent lets air in and not out (tested with a mitivac and some heatshrink tube).
                My 68 has a replacement gas tank and not sure of the supplier and type. I assume it is a "1971 and newer" type gas tank. My vent lines are not plugged, but I get a swoosh on occasion when taking off the cap, and on hot days I can't have a full tank as the vent will drip at the frame rails. (I am assuming this is the capillary effect, or the vapor's condensation)

                Like I said in this month's Mopar Action Rick Ehrenburg recommends at vented gas cap for ALL cars (old MOPARS) regardless of emissions, or not.

                For $6.99 it's a good experiment and my present gas cap has seen better days anyhow.
                 
              • ODZKing

                ODZKing Well-Known Member

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                I probably should have posted in my previous message that none of the Stant caps work. I've tried them too.
                 
              • 72RoadrunnerGTX

                72RoadrunnerGTX FBBO Gold Member FBBO Gold Member

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                Yes, that is normal for that car on warm days. The correct single action cap for ’71 has an over-under pressure relieve valves, it is not a vented, should not be a vented cap. The ’72 mid- year production change to double action caps and filler necks was April 1, 1972. I’ve had no issues with Stant caps when used on the correct application.
                DSC06385r.jpg
                DSC06386r.jpg
                 
                Last edited: Jun 20, 2021
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                • Billccm

                  Billccm FBBO Gold Member FBBO Gold Member

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                  Here is the mopar action response in the tech article about gas caps.

                  20210620_200327.jpg
                   
                • Bee1971

                  Bee1971 Well-Known Member

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                  The 10806 works just fine on my 71 SuperBee

                  Built December of 1970
                   
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                  • 72RoadrunnerGTX

                    72RoadrunnerGTX FBBO Gold Member FBBO Gold Member

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                    As stated, if the car was equipped with a fuel pump mounted vapor separator, a ’71 440 should have been, the 1/4” return fitting would have been plumped back to the sending unit via a third ¼” line. The 5/16” line from the fuel tank vapor separator/expansion tank needs to go to the 3-nipple breather, this effectively is the fuel tank vent.
                     
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                    • gary h

                      gary h Well-Known Member

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                      Got a new cap Stant, says Motorad on cap.Got swoosh and back of cap had gas on it(only 1/2 tank).Cap fits snug probably because of new rubber seal.So i need to get a 1/4'' return line.I am still wondering why i see no indication of the original return line.The other two lines are original.

                      IMG_20210621_122015446.jpg IMG_20210621_122043628.jpg
                       
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