Overfill Limiting Valve?

1971 - 1974 Mopars

  1. TopBanana72

    TopBanana72 Well-Known Member

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    Greetings! I found this site and am now a new member. I have had my Charger for 2 years. It is:

    1972 Dodge Charger Hardtop
    318/904/8.25
    Matching numbers everything,
    Originally HT8 Dark Tan Metallic (read Brown)
    Color change on the 20 year-old restoration to FY1 Top Banana (fine with me!)
    Simple mods—4bbl, duals.

    I posted the question over on the 71-74 DodgeCharger site, but thought I would try here too... here goes:

    Here is a question and topic: Fuel EVAP system and gas problems.
    I have been trying to understand this issue that I am having and have read about it in other forums, but still can’t exactly get the answer for our cars.

    The story,
    When I picked up the car, I noticed that the gas cap was loosely fitting, so I stopped by an Autozone and got a new cap that fit tight.
    Later after some days and hot days, I heard the gas tank bang when the car was in the garage after a trip. I thought, huh? What the hell was that? I started reading about it and realized that the PO had removed the charcoal canaster and hoses, etc. during the resto and capped off the vent line up near the engine bay. This explains the PO’s use of the loosey goosy fuel cap. So, I realized that the tank needed vented and bought a vented gas cap. This has stopped the tank pressure banging problem, but a new problem has emerged….

    Now, when I have full or close to full tank, the gas will start expanding in the tank and force it’s way out the filler neck (slowly as the cap is pretty tight.). This causes a gas leak and strong smell. I couple of times, I had to siphon gas out to stop this dangerous condition. Now, I have been sure and not to fill up the tank until I figure this out. I do have the charcoal canister and some hoses, and understand the system and can probably restore it and have it back in place. The bracket is even right there in the engine bay. I understand doing this, I would return to a proper non-vented cap.

    So the questions…. Is restoring the system as designed really going to stop the gas expansion leak/full tank issue?

    Also, in the FSM, there is a diagram of the system mentioning an “overfill limiting valve”. I have not been able to find this valve. Where exactly is this valve? I have heard it is at the liquid vapor separator, or sometimes I hear it’s supposed to be up front near the charcoal canister. I have a Feb. 15th build date, so the liquid vapor separator is in the driver’s side rear wheel well and the vent lines come off the side of tank. Perhaps the overfill limiting valve not being there or not functioning is causing me to fill the tank up too much? I can’t imagine if the EVAP system was restored that all that vapor expansion would fit in the canister. I’m worried that the expansion issue would overwhelm the vent lines and fill up the canister with liquid fuel.

    I appreciate anyone’s advice, experience and thoughts. It is unsettling to have this potential gas leak when my tank is full or even near full. It’s hard to tell how much to fill up the tank to avoid the issue.

    upload_2019-2-25_21-58-43.png OFLV.jpg OFLV.jpg
     
  2. steve340

    steve340 Well-Known Member

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    In your post you mention proper non vented cap.
    Your diagrams show a vented cap in all scenarios. You need a vented cap.
    Also it mentions the overfill valve is not on all models. Perhaps you never had one?
    The basic idea is the cap is vented and the fuel expands into the liquid/vapor separator and the engine sucks away any smell/fumes. Any fuel that gets into the separator drains back to the tank through the lowest tube you can see in the diagrams.
    The engine is attached to the highest tube above the fuel in the separator.
    The cap is vented so the fuel tank does not expand as pressure builds or suck in and collapse while the engine is running. You need atmospheric pressure above the fuel in the tank for the fuel pump to operate correctly.
     
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    • Fran Blacker

      Fran Blacker FBBO Gold Member FBBO Gold Member

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      Don't know if a parts book would help but you could look up the part number and location on car. If it was removed, with a part number you can call or go to websites with a part number search. Tony's Parts you can type in the pn. You can download a Parts Manual at mymopar. Its a PITA to use one on a download because you find part in exploded view and page with the pn is 20 pages away.
      http://www.mymopar.com/index.php?pid=107
       
    • Photon440

      Photon440 FBBO Gold Member FBBO Gold Member

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      The three diagrams that you show all have the term "pressure/vacuum filler gas cap" which is not vented under normal situations.
      From Stant's site regarding post 1971 emmision caps:
      Emission Control type caps began in 1970 in California and 1971 nationwide. The fuel emission control system was designed to prevent evaporation of fuel into the atmosphere. The special fuel caps required are equipped with carefully designed and tested pressure and vacuum relief valves to maintain a specified amount of pressure or vacuum in the tank before relieving.
      They will vent in emergency, such as when the tank is involved in a collision.
      Full read: http://www.stant.com/index.php/english/products/consumer-products/caps/oe-equivalent-fuel-caps/
       
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      • 451Mopar

        451Mopar Well-Known Member

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        Just place the fuel tank vent line higher than tank level in the engine compartment. If your worried about dirt getting in the line, add a filter, maybe a fuel filter with one side connected to the vent line, and the other just open. in '71 the vent line just went to the crankcase vent cap.
        Make sure the hoses from the gas tank to the vapor seperator (behind the drivers side rear wheel) are not blocked off, along with the vent line.
        They could be clogged internally, or the previous owner may have intentionally inserted something in the lines to block them, not knowing that is how the tank is vented?
         
      • Badvert65

        Badvert65 Well-Known Member

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        I had the same problem with my 74 Satellite Sebring. The problem was somewhere in the charcoal canister. Stopped when I replaced the canister.
        My tank actually developed a crack from expanding and contracting so much before the problem was fixed. You could disconnect the vent line from the tank going to the canister to test this.
         
      • 72RoadrunnerGTX

        72RoadrunnerGTX FBBO Gold Member FBBO Gold Member

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        Which fuel tank vapor separator do you have on your ’72? Where is it located? Before or after April 1, 1972 production? Not all platforms use the in-line check valve. Those diagrams are general system layouts. The later design, smaller separator mounted forward of the fuel tank has this check valve built in. BTW, the caps and filler necks changed at about the same time. A later cap on an early neck will be lose and not seal.

        “Is restoring the system as designed really going to stop the gas expansion leak/full tank issue?”

        I’m running a restored late ’72 and up vapor recovery system on several cars, with the correct non-vented cap, and do not have the issues described. The system works as designed when fully intact.
         
        Last edited: Feb 26, 2019
      • TopBanana72

        TopBanana72 Well-Known Member

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        That's a good idea. I bought some stuff from him before.
         
      • TopBanana72

        TopBanana72 Well-Known Member

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        Thanks!, that's about the way I understand it.
         
      • TopBanana72

        TopBanana72 Well-Known Member

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        That's my understanding as well. I purchased a correct Stant pressure relief type from Rock Auto. It specified pre- April 1 type. I will use this when restoring the system,
         
      • TopBanana72

        TopBanana72 Well-Known Member

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        The canister is in pretty good shape. No cracks. I think I will try and re-use it as I have never been able to find one of these to buy for '72, either NOS or repop.
         
      • TopBanana72

        TopBanana72 Well-Known Member

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        I have the before - April 1 type vapor separator in the driver's rear well. I can see the rubber vent lines in there. I did order a pre-april 1 cap from Rock Auto.

        This is perfect, I was wanting to hear '72 systems are OK as designed. This firms up my plans. I'll post back some day about my results of my pre-April 72 system.

        Thanks to ALL! this is such an AWESME site!
         
      • 72RoadrunnerGTX

        72RoadrunnerGTX FBBO Gold Member FBBO Gold Member

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        The ’71 and early ’72 fuel tank vapor separator is also labeled as an expansion tank for the fuel tank. It was prone to rust holes due to its location in the wheel well. Road dirt/water gets trapped between the rock shield and tank, leading to external rust through. Pretty common now to find them leaking fuel or fuel vapors. Pull it out and look it over closely. Pressure test it if you need to.

        Another thing to look for on the ’72 only charcoal canister is pin holes in the purge control diaphragm. Lift the diaphragm cover to remove and inspect the diaphragm or vacuum test the diaphragm at cover nipple, it will hold a vacuum if it's good.
        IMAG0101.jpg
         
        Last edited: Feb 27, 2019
      • TopBanana72

        TopBanana72 Well-Known Member

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        That’s an excellent idea. I have never seen it. I know the rock shield because they sell a repop of the cover. Too bad they don’t make the expansion tank. On the canister, that purge valve stamped says don’t remove. You think it’s ok to though?
         
      • 72RoadrunnerGTX

        72RoadrunnerGTX FBBO Gold Member FBBO Gold Member

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        I just snaps on and off, just a diaphragm and a spring. I would assume it warns of running the system with it removed as it would expose stored fuel vapors. You can put a hand vacuum pump on the small nipple, pull it down a bit, see if it holds. If it does, no need to open it up.
        DSC06223r.jpg
         
        Last edited: Feb 27, 2019
      • TopBanana72

        TopBanana72 Well-Known Member

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        ah! good, I will do that.

        Also, I have an aftermarket Holley 4bbl on the 318 now with no vent connection and no intention of going back to the carter 2bbl. I guess I just cap the carb nipple on the canister? ... and run the rest of the hoses to their destinations.
         
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        • YY1

          YY1 Well-Known Member

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          Not sure about the 72 vent tube manifold (four into one) but the 73 version has a brass shuttle valve that is supposed to let vapor out but not back flow fuel.

          Mine stuck and caused the gas cap to forcefully blow off the filler neck every time it was removed.

          I performed surgery to remove the brass shuttle and reassembled the manifold.
          Problem solved.

          I am currently running the vent line directly into the valve atop the float bowl (without the charcoal canister). This does not seem to be a problem either. There must be enough free space in the line to allow for expansion, and I'm sure I'm getting at least some benefit of drawing in vapor when the bowl valve opens.

          No drips, no gas smell.
           
        • 72RoadrunnerGTX

          72RoadrunnerGTX FBBO Gold Member FBBO Gold Member

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          “benefit of drawing in vapor when the bowl valve opens”

          The fuel bowl vent valve is only open at idle and while the engine is off, at rest. The purpose of the charcoal canister is to store fuel vapors from the fuel tank and the carb bowl while at rest and consume the stored vapors through the purge line when the engine is running. I would expect hot start problems with the fuel tank evaporative vapors being collected/stored in the air cleaner.

          If not running a canister, better off running a ’71 three nipple breather, vapors are stored in the crankcase and purges through the PCV.
           
        • YY1

          YY1 Well-Known Member

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          I'm just saying my problem went away and no additional problems have materialized.

          No starting issues, no driving issues, none.
           
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