Gas Tank Pressure

[SirBB]

When removing the gas cap the tank has a huge amount of pressure on it, it really blows the cap off, this does not seem normal to me, anybody have an opinion?

[jopizz]

Make sure your gas cap is vented.

John

[DKheld]

Tried a couple of different vented caps. I finally had to drill a small (tiny) hole in the top of the filler neck outside the trunk basically just behind the cap (went back to the OEM cap). Not a good idea but the only thing I could make work.

The pressure in mine was so high after sitting that it would push fuel past the needle valve in the carb and you could really smell it. This only happens in the summer months when it's hot so probably an evaporation issue with all the ethanol in new fuels.

Just curious - do you have a new tank? (probably Canadian made by MQ Products - think they are the only ones re-producing them)

[Dakota Boy]

I have experienced this as well. The "vented" gas caps let air in but not out. Sorry I don't have any good solutions. I have a new tank as well.

[jopizz]

I believe the ethanol in the gas has something to do with it. The last Squarebird I had used to build up a lot of pressure during the hot months even with a vented cap. I don't remember that ever happening years ago. The suggestion of drilling a small hole is probably a good idea. I imagine you could drill a small hole in the cap if you didn't want to drill into the gas tank.

John

[simplyconnected]

Our '59 Galaxie has a small vent tube coming off the filler neck. It's really 3/16" steel brake line. The tube loops high into the trunk void and exits next to the license plate, inches from the fuel cap. It's hardly noticeable unless someone's looking for it.

This tube loops high in case the car climbs a steep grade. The idea is, the liquid level in a full tank will always be lower than the highest part of the tube. So, if the tube fills, the liquid never gets to the top. When the car flattens out all the liquid goes back where it came from.

When the fuel tank is vented it doesn't matter whether the cap is vented or not. We never smell gas apart from the filling station. - Dave

[DKheld]

Here's the hole I drilled on mine - looks bigger than it is - had to use a larger drill bit to create a starter area before I switched to the tiny bit to finish off the hole. Camera was tilted a bit but I guess the hole is at about the 2-3 O'clock position. Figured it was best not to drill at the top because water from washing etc might trickle in - doubt it would really matter though as small as the hole is.



Dave has mentioned before - probably not the best idea to have a hole in the filler neck (true) but all the caps I tried didn't work and like Greg said - they only let air in anyway. The two caps I tried had the thumb tabs that stuck out on the back and wouldn't let the license plate flap close right either. Not to mention they were a real pain to remove. They were just like these. ("borrowed" pic from the web...)


I switched back to my original cap after drilling the hole (first pic).

Here's a tube inside my tank. Think this is just so that if you overfill the tank a vapor lock will not occur and when filling it lets air out so that the gas can flow in properly. Thought about making the hole in the filler neck just above that tube but the big rubber grommet where the filler neck exits the body would cover the hole so went for the side.


I don't remember having the pressure problem with the old tank - even with modern fuels. My problem with the old tank was after I had it boiled and sealed. The sealer couldn't stand up to the ethanol and started peeling which blocked the pick-up.

No problems with pressure in the new tank since drilling the hole - even when I fill the tank. I don't let the gas get all the way up into the filler neck when filling but it gets close. Also eliminated the flooding situation when hot and the pressure pushed gas through the carb into the intake.

Now if we can just get an OEM style mechanical fuel pump that will last more than a couple of years.....

[SirBB]

Thanks for all the comments. Yes the tank is new, the old one had plenty of vent holes (thanks to rust). I also have had issues with the carb. bowl overfilling, possibly this is a partial contributor. I do also have a new fuel pump. From all my research the caps only vent to let air in, not out, the EPA would probably frown on someone making a cap to let vapors escape from a tank these days.

The original cap (or previous one) did not do this, but from what my daughter said (her daily driver) it mysteriously disappeared after one of her fill-ups, so she got a new one at the parts store.

I might try to drill a really tiny hole in the cap.

[StealthSRT10]

I have a new tank as well and didnt notice any issues till a hot day. Think i will be drilling into the cap as well.