1967 Fuel Tank Problem

[MikeLawson]

I am having a repeat problem with my 1967 DeVille every time I fill the fuel tank.  Today, I filled the tank but it was not overfilled, about 7/8 according to the gauge.. After driving the car about 10 miles, I returned home and shut the engine off. Almost immediately, I heard a gurgling sound from the rear of the car and saw there was gasoline on the floor. I looked under the car and found that the gas was coming from the vent pipe connected to the top of the fuel tank.  I removed the filler cap (non-vented type) and verified the fuel level was down inside the filler pipe and the tank was not overfilled.

Can anyone can tell me if there is a way for pressure to build up in the tank?  My only thought is that it has something to do with the return fuel line from the filter and I I'm considering blocking off this line and so what affect it has. Any other suggestions would be greatly appreciated.

[fishnjim]

Don't disconnect or plug off any lines.
Probably two things going on, "modern" ethanol fuel and the condition of the fuel system. 
I'd double check that a non-vented cap is correct or the cap is not correct and doesn't fit properly.   
The vent line maybe plugged or collapsed. 
Gauge could be wrong - overfill.   
These cars are low with low inlet and have a tendency to spill, if the fuel sloshes and foams up when it's close to full.   Try some non-ethanol gas, if available in your area.   Unfortunately, there's detergent additives for fuel injection these days.   
I'd suggest that the tank come down and you inspect/clean and replace all the rubber parts.
I started not filling up in the '58 since it seems to do that in hot weather or parked on incline.   And my tank was replaced as the vent line.   Modern fuel v old car.

[MikeLawson]

Thanks for your reply. Let me add some additional information. I have replaced all the rubber lines to the fuel tank and removed the metal vent pipe to verify that it is not blocked. I replaced the original fuel tank cap it was gas would leak out the cap when the tank was full. The original cap was non-vented. After replacing the cap, pressure now builds up and forces fuel out of the vent line. After the last fill up, I verified the tank was not over filled by removing the cap and seeing that the fuel level was a good ways down from the end of the fill pipe.

[richardbergquist]

It could be an aggregate thing. You may have some leakage from the vent pipe but the big leak may be from the gasket around the fuel pick-up on top of the fuel tank. I don't know if you've had the tank down and looked at that gasket. For taking down a gas tank it's a fairly easy procedure. I did it on my back on a driveway.

[MikeLawson]

Hi Richard,

There is no leak from the fuel sending unit gasket. I drove the car again this weekend and right after I shut off the engine, I heard the gurgling sound from the gas tank and fuel came out the vent pipe. I drove the car again the same amount of distance but this time, I removed the gas cap before shutting off the engine. No gurgling sound and no fuel vented. I did notice that the left front corner of the fuel tank was warm to the touch due to the proximity of the exhaust pipe and this heat is probably causing vapors to build but they should be constantly discharged through the tank vent pipe. I going to try drilling a small hole in a spare gas cap and install it on the car to see if that helps the situation.

[savemy67]

Hello Mike,

One of your posts indicated you removed the vent pipe to check for blockage, and found none. 
The photos below indicate the vent pipe has three pieces - the nipple pressed into the tank, a rubber hose section, and a metal tube bent so that air, but not water, can enter the tank.  Were all three sections checked for blockage?

A non-vented filler cap is correct for '67.  The only vent is the one at the front of the tank.  The purpose of the vent is not to vent fuel vapors, but to allow atmospheric pressure to be maintained as fuel is consumed.  The sending unit cover has two openings (air conditioning equipped cars only) - one for fuel to be drawn out of the tank by the fuel pump, and one for fuel/vapor to be returned to the tank from the fuel filter glass bowl.

I believe fuel from the tank to the pump is under negative pressure when the pump is on its inlet stroke, and under positive pressure from the pump to the filter when on its outlet stroke.  The fuel filter glass bowl is thus under positive pressure, as would be any excess fuel/vapor escaping the glass bowl via the return vapor line.  The return fuel/vapor might add some positive pressure to the tank, but not enough to force fuel out the vent.  The relatively tiny amount of fuel/vapor going into the tank will condense and/or be accounted for by the drop in fuel tank level as fuel is consumed, all else being equal.

You can check that your vapor return line is clear by disconnecting it at the glass bowl and putting a few pounds of compressed air through it while listening for a hiss at the gas tank.

If liquid fuel is coming out of your vent, then the liquid fuel and its vapors are expanding enough to overcome atmospheric pressure.  This would point to a very full tank, or parking on a down slope.  Is your garage/driveway level?

Heat will cause the liquid fuel and its vapors to expand.  You noted the tank was warm due to the proximity of the exhaust.  It seems likely that the exhaust has been replaced over the years, perhaps it is not located correctly.

You may want to double check that all three sections of the vent are clear, and that the fuel/vapor return line is also clear, and that the car is level when parked.  The gas gauge is not a precise indication of the fuel level in the tank.  Neither is an observation of the fuel level in the filler neck, unless the car is dead level.  Then, the level in the filler neck should equal the level in the tank.  If the tank is full, the level of the fuel in the filler neck should be detectable, as a portion of the filler neck is below the level of the fuel in the tank when the tank is full.

Respectfully submitted,

Christopher Winter

[MikeLawson]

Chris,

Thanks for your input.  I did verify that the vent tube, connecting hose and tank nipple are all clear. The car is parked on a level surface. As you suggested, I disconnected the fuel return line from the fuel filter and connected a rubber hose to the end of the line. I then blew into the hose (lung power only) and I heard gurgling in the tank and a small amount of fuel came out of the vent pipe. It did not take much effort to make this happen. I still cannot figure out how enough pressure is being developed in the fuel return line after the engine is shut off to push fuel out of the tank vent.  I did replace both the fuel pump and fuel filter since I got the car and I do have the correct filter element for air conditioned cars. There has got to be a solution for this problem somewhere.