Pressure in fuel tank

Chris Beauchamp

Low Mileage
Looking for some input on an issue i have noted with my 84. When removing the gas cap there is always a considerable amount of pressure built up in the fuel tank. I'm traveling with work this week, but would like to dig into this when i get home. This car did originally have a blocked return line and the tank had a section removed to allow cleaning out. It was when i was recently under the car replacing the park brake cables that i noted the bottom of the tank is kinda wet - assuming brazing has failed with tank pressurizing. Anyway, i know there is a charcoal canister and valves and my guess would be an issue here. Anyone been down this path with a fuel tank pressurizing. Thanks.
 
The vent tubes at the top of your tank look something like what you see in this picture of a carburetor tank for a 78 X. I am pretty sure the thin vent tubes are identical between carb and FI tanks. Perhaps one or both of the vent tubes got crimped during re-installation of the tank? Or plugged with crud during the repair? There are check valves in the vent hoses, you could check to see if they are reversed.

The pressure could be forcing fuel to weep at the hose connections. Good luck.
carb_tank_9.JPG
 
Some pressure in the tank is normal. The tank pressure is controlled by the 3-way valve thing in the diagram below. Port (a) is to the gas tank via the liquid/vapor separators (B).
  • When the car is parked, the pressure builds up and the top spring is over-come (Position 1) and the vapor escapes to the charcoal canister through port (b). When the pressure in the tank is too low to over-come the spring, the tank is sealed until you take the gas cap off and release it. Make sense? A stronger top spring would create more residual pressure in the tank and a weaker spring, less.
  • When the car is running, the valve is in Position 2 and air is sucked into the tank through a check-valve (c) to make up for the fuel being pumped out so you don't draw a vacuum and collapse the fuel tank.
  • Position 3 is if something goes wrong in the charcoal canister and the tank can't vent to it. It blows off the stiffer bottom spring and the vapors escape into the atmosphere. Sort of a pressure relief valve.

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I think having that valve might differ for various years? Depending on if its a carb or FI and what emissions equipment is installed? But I'm not sure. Regardless the answer is a good one in regards to 'normal tank pressures'. There needs to be some way to allow air into the tank as fuel is consumed from it. And some way to allow air out of the tank as temperatures rise and the fuel expands. But that is often captured to prevent vapor from escaping into the atmosphere. So whatever the method of tank ventilation controls you have on yours, perhaps it is not working correctly (stuck valve, clogged line, etc).
 
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