Gas tank on '70 makes "woosh" sound when removing cap. Normal?

[Alan S.]

[Deane M.]

Re: Gas tank on '70 makes "woosh" sound when removing cap. Normal?

FWIW, I currently own a 70L46 car (second one). I have 2 correct "sealed" gas caps that make that sound during the removal for fueling pretty much every time. Been doing it for 10 years. My memory is the first one did it as well. Owned that one way back in the 80's.........
Seems normal to me.

[Mark E.]

Re: Gas tank on '70 makes "woosh" sound when removing cap. Normal?

FWIW, I currently own a 70L46 car (second one). I have 2 correct "sealed" gas caps that make that sound during the removal for fueling pretty much every time. Been doing it for 10 years. My memory is the first one did it as well. Owned that one way back in the 80's.........Seems normal to me.

So some folks have a '70 that wooshes, and some have one that doesn't.

Any fuel system pressure experts out there?

My theory: The vent valve opens at a set vacuum level in the tank.

This means:
- If the fuel cap is removed when vacuum exists but is less than the vent valve's set level ==> Woosh occurs.
- If the fuel cap is removed when there's virtually no vacuum (for example, when the vent valve just opened, or long after shut down) ==> No or little woosh.
- No woosh ever ==> Vent valve stuck open
- Woosh is occasionally much stronger than normal (for example, right after a long drive that consumed a lot of fuel) ==> Vent valve plugged

But this is just a guess.

[Alan S.]

Re: Gas tank on '70 makes "woosh" sound when removing cap. Normal?

Hi Mark,
The information I have access to is in the 1971 EMISSIONS CONTROLS/THEORY Manual ST 343-71. I believe this was sent to the dealer service departments.
In the ECS System Section there's mention of pressure in the tank, it's causes and remedies. In the summary for the Diagnosis section appears: "In operating conditions of high temperatures or with high volatility fuel, some pressure may be normally encountered in the fuel tank".
Perhaps this applies to 70 tanks too?
Regards,
Alan

[Mark E.]

Re: Gas tank on '70 makes "woosh" sound when removing cap. Normal?

Wasn't '71 the first year with a closed evaporative fuel system on every car? In '70, gas fumes vented to the atmosphere (either through a vented cap or the fuel tank's vent valve), except with option NA9, CA emissions, which added a closed system.

So I'm not sure if the '71 Emission Controls manual applies for '70, at least without NA9.