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Please explain how the fuel tank vent fitting works? 84 Colony Park

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    #1

    Please explain how the fuel tank vent fitting works? 84 Colony Park

    I'm trying to get to the bottom of my gas tank pressurizing. For the last few years I just left the cap slightly loose and no issues but occasionally when the tank is mostly full I can smell raw fuel driving down the road with the windows up. I have the oem tank vent hose connected to a small round air filter(like you would use for a valve cover breather) and positioned just above the top of the gas tank - no oem evap charcoal cannister or purge valve anymore. It's got a turbo LS motor so the factory stuff is long gone.

    There is no evidence of fuel leaking around any of the seals on the tank, so I tried to blow through the tank vent hose with the gas cap tightened all the way. I was able to build pressure in the tank, that released only when the gas cap was opened. I guess I either have a clogged tank vent fitting or I don't understand how it works? I would think that I wouldn't be able to build pressure in the tank because that hose should be open to flow both ways right?

    thanks for help!
    1984 Mercury Colony Park Wagon - 393 Dart block LS 91mm turbo with 4L80E and 9" 6.24@115 in the 1/8 mile (9's in the 1/4), 4650lbs w driver, AC, Drag n Drive rig 1300hp

    1988 Lincoln Town Car - stock for now
    Tags: None
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    #2
    The tank vent has rollover leak prevention in it (a ball that fills the hole to slow/stop fuel leaking out when upside down). Sometimes the ball gets gunked up and the vent gets clogged.

    Quick question though... are you sure it's pressure and not vacuum?

    The cap should allow air in but not out. The vent should allow pressure out but not in IIRC. That keeps the system one way only. But yeah, if the vent is plugged up... It will pressurize due to fuel evaporation and will make filling a PITA.

    Not sure when the cap changed, but maybe the older ones seal both ways and only the vent lets pressure out. But yeah, if you're running a turbo LS, you should probably make sure the cap lets air in.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

    Originally posted by gadget73
    ... and it should all work like magic and unicorns and stuff.

    Originally posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.
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      #3
      What he said. usually its something in that valve at the top not working. I've heard of people taking them apart and removing the check ball and slightly enlarging the hole to allow it to vent better.

      but yeah vapor out through the thing in the top of the tank which runs up to the carbon canister, air in through the cap.
      86 Lincoln Town Car (Galactica).
      5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

      91 Lincoln Mark VII LSC grandpa spec white and cranberry

      1984 Lincoln Continental TurboDiesel - rolls coal

      Originally posted by phayzer5
      I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

      Comment

        #4
        got it, thx guys. I'll drop the tank and check that valve. I have the original gas cap and it appears to seal completely but I'll look at it closely. I've been crutching the tank vent by just cracking the cap open so no pressure builds....it also works to allow more air into the tank when I'm racing because of the sheer volume of fuel I'm using (e85 at approx 1000 rwhp) is more than the stock setup was designed to accommodate in terms of vacuum in the tank from the huge fuel pumps I'm using. It would be nice to close that cap tight when I'm just cruising on the street knowing the vent valve will just do it's thing. And, if I still get smell I'll run that line all the way up to the motor through a homemade charcoal cannister
        1984 Mercury Colony Park Wagon - 393 Dart block LS 91mm turbo with 4L80E and 9" 6.24@115 in the 1/8 mile (9's in the 1/4), 4650lbs w driver, AC, Drag n Drive rig 1300hp

        1988 Lincoln Town Car - stock for now
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          #5
          OK, looking at replicating something similar to the stock charcoal cannister evap system on my drag n drive ls turbo 84 wagon, but I don't know exactly how it works.

          I have the tank, check valve at the top of the tank and hose running from that but I can;t remember how the system was set up from there. I assume tank check valve to charcoal cannister and then to the purge valve and then to the intake manifold?

          I know there was a "cannister purge valve" and a charcoal cannister, does anyone have a diagram of that system?

          Also, and this is a long shot, anyone know the logic the ecu used to activate the purge valve? My EVTM for this car and my 88 does not have info beyond the wiring diagram for the purge valve.
          1984 Mercury Colony Park Wagon - 393 Dart block LS 91mm turbo with 4L80E and 9" 6.24@115 in the 1/8 mile (9's in the 1/4), 4650lbs w driver, AC, Drag n Drive rig 1300hp

          1988 Lincoln Town Car - stock for now

          Comment

            #6
            Originally posted by drhill View Post
            I assume tank check valve to charcoal cannister and then to the purge valve and then to the intake manifold?
            Yes.

            As for the logic... not real sure about that.


            Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
            rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

            Originally posted by gadget73
            ... and it should all work like magic and unicorns and stuff.

            Originally posted by dmccaig
            Overhead, some poor bastards are flying in airplanes.
            • Likes 1

            Comment

              #7
              Originally posted by sly View Post

              Yes.

              As for the logic... not real sure about that.
              I read on a mustang forum that it only opens at light cruise range, I may be able to program something into my Holley ECU to match that. Unless someone happens to know otherwise I'll try it. I have so many holes in the floor area (roll cage etc) that smelling raw fuel in the car on long trips is getting old (and nauseating!)
              1984 Mercury Colony Park Wagon - 393 Dart block LS 91mm turbo with 4L80E and 9" 6.24@115 in the 1/8 mile (9's in the 1/4), 4650lbs w driver, AC, Drag n Drive rig 1300hp

              1988 Lincoln Town Car - stock for now
              • Likes 1

              Comment

                #8
                Originally posted by drhill View Post

                I read on a mustang forum that it only opens at light cruise range, I may be able to program something into my Holley ECU to match that. Unless someone happens to know otherwise I'll try it. I have so many holes in the floor area (roll cage etc) that smelling raw fuel in the car on long trips is getting old (and nauseating!)
                Can't plug up the holes in the floor to avoid gas fumes?
                What I Own: 1993 Mercury Grand Marquis GS
                What I Help Maintain: 1996 CV / 1988 CV / 1988 Tempo

                Comment

                  #9
                  Originally posted by drhill View Post

                  I read on a mustang forum that it only opens at light cruise range, I may be able to program something into my Holley ECU to match that. Unless someone happens to know otherwise I'll try it. I have so many holes in the floor area (roll cage etc) that smelling raw fuel in the car on long trips is getting old (and nauseating!)
                  Atleast run the vent line into the charcoal canister and leave the other line off the canister open. It'll atleast reduce the smells. Since I carb swapped my MGM, I just left the other port on the caniser open and haven't had the car smell like fuel, or not that I've noticed.
                  1985 Mercury Grand Marquis LS, "Maisa"
                  2005 Volvo V70 Bi-Fuel
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                  Comment

                    #10
                    you could probably just run a line from the charcoal canister to a port just behind the air filter. The slight vacuum from there would be enough to suck out any fumes.
                    86 Lincoln Town Car (Galactica).
                    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

                    91 Lincoln Mark VII LSC grandpa spec white and cranberry

                    1984 Lincoln Continental TurboDiesel - rolls coal

                    Originally posted by phayzer5
                    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

                    Comment

                      #11
                      good ideas thanks guys. I'm a little leery about running a hose to the air cleaner because once the turbo spools up it'll be sucking pretty hard on that evap line. I thought of having a check valve in line between the intake manifold and purge solenoid since that line would see vacuum and boost otherwise. but maybe just venting the cannister and forgetting about the purge solenoid and intake manifold connection is the answer....
                      1984 Mercury Colony Park Wagon - 393 Dart block LS 91mm turbo with 4L80E and 9" 6.24@115 in the 1/8 mile (9's in the 1/4), 4650lbs w driver, AC, Drag n Drive rig 1300hp

                      1988 Lincoln Town Car - stock for now

                      Comment

                        #12
                        Originally posted by drhill View Post

                        I read on a mustang forum that it only opens at light cruise range, I may be able to program something into my Holley ECU to match that. Unless someone happens to know otherwise I'll try it. I have so many holes in the floor area (roll cage etc) that smelling raw fuel in the car on long trips is getting old (and nauseating!)
                        I have seen this question asked on the Holley forums, not sure what the fix was though
                        ..

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