leaking gas cap

[Steve Gallegos]

Today I went to a cruise in, about a 30 mile drive. Got there everything fine. Left about 4 hours later, thought I'd take the surface streets back, air temp about 70. Gas tank little less than full. Got into some bumper to bumper traffic. A guy pulls along side of me and tells me there is gas coming out of my gas cap. I stop remove the cap clean the area and put it back on. About 6 miles later in B to B traffic I look back, the gas is coming out of the gas cap again. I stop do the same thing. Get on the freeway drive about 10 miles pull into my house, no leak. I have a Backdraft. Any ideas out there

[Three Peaks]

Check to see if your fuel tank vent is working. If you overfill your tank, you can expect some expansion and leakage . I usually stop at the first time the pump clicks off and haven't had a problem since.

Bob

[Ron61]

I agree with what Three Peaks said.Never top the tank off to the very top. I stop when the pump shuts off instead of trying to get that extra gallon in there. And if you have a vented type gas cap make sure that it is clear and working.

Ron

[Steve Gallegos]

How do I check if the vent tube is open? It goes from the neck of my tank, facing down, and connects to the bottom of the fuel tank. The gas cap is vented, that is where the gas is coming out and the tank was about 3/4 full.

[DanEC]

If your vent tube at the filler is connected back to gas tank then it doesn't function as an overflow or vent. So you have (or should have) a vented cap. You can verify by carefully cracking the cap open after a drive and listening for air movement due to pressure or vacuum in the tank. If you hear any then the vent in the cap may be closed up.

But from your description of overflow, I would say that a seal on the cap is leaking or you have over-filled the tank - or probably a combination of the two. If fuel is escaping you aren't building up any pressure or vacuum in your tank. I have a Mopar with a side mounted flip top cap (a 67 GTX) that is prone to dripping some fuel if I fill up the tank. I keep the seal coated with dielectric grease which both protects the gasket rubber to keep it soft and helps seal it. Might try that.

Do BDR fuel systems have an internal cap under the flip top - quick fill cap?

[spdbrake]

The BDRs have a second cap (vented) under the quick fill.
The problem the OP has is thermal expansion of the fuel and overfilling the tank.
The vent line and filler pipe pick off the tank at almost the same location on the side of the tank. The vent is connect to the filler pipe approx 4" from the top.

Since the vent line is positioned on the inboard side of the filler neck 4" down the issue is aggravated when you stand to the side of the car and point the fuel nozzle inboard. It can cause splashback and fills the vent line.
Safer to have the nozzle pointing forward.

When the car is overfilled, fuel will be in the fill pipe and vent line. After a few turns and stops and starts the tank will be burped of all air and the fill pipe and vent line will still have fuel in it. Then comes the thermal expansion of the fuel. Cold underground tanks and a warm fuel tank in your car.

Buying fuel from a busy station is less of an issue than a Mom & Pops station as far as getting cold fuel.
Buying cold gasoline
Expanding gasoline a heated issue - latimes

In aviation we don't fill our jets in the morning to the caps and leave them in the sun. Makes a huge mess. We use a "summer load", 3-4" down from the caps.

[keezling]

I'm puzzled that you drove 30 miles and on your way back (30+ miles now) your tank is puking at the filler? What did I miss? My car would be down 3+ gallons at that point. I know gas expands, but that much?
Congratulations on leaving a cruise in with your car intact by the way��

[Steve Gallegos]

Guys, thanks for the info. Spdbrake, when I lock the cap, there is still some play in the cap, it is not tight to the fitting. Is this right?

[spdbrake]

Yup that's fine. It needs to vent.