Gas cap pressure?

[BeeDuhb]

Hello I'm new to the forum and loving it! Just picked up this little guy a week or two ago and working thru some of the issues it's got.

One thing that worries me is there either a pressure build up or vacuum happening in the gas tank. When I go to fill it up, while removing the gas cap a lot of wooshing air noise happens. On old jeeps I've had the gas cap had a breather to avoid pressure build ups.

Is the pressure or whatever normal? Thanks for any help!

90 Festiva 5 speed


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[Dragonhealer]

Yes, the pressure is normal, helps reduce evaporative emissions.

[TorqueEffect]

What's would be wrong if your tank doesn't build any pressure at all? After I changed my fuel tank, I no longer have the "whooshing" when removing the gas cap.

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[Dragonhealer]

Assuming every thing is hooked up properly;
gas cap or seal defective
fuel pump module seal defective
Purge valve defective, missing, not getting correct electrical signal
Split or hole in hose or pipe
split or hole in vapor tank

Don't have the manual in front of me, possibly other

[TorqueEffect]

Assuming every thing is hooked up properly;
gas cap or seal defective
fuel pump module seal defective
Purge valve defective, missing, not getting correct electrical signal
Split or hole in hose or pipe
split or hole in vapor tank

Don't have the manual in front of me, possibly other

When my B6 was installed, zoom zoom deleted the purge valve and canister, but plugged up the line with a bolt and sealant. I wish he hadn't deleted it, because is does pull a code for the purge valve. But the light doesn't come on. (He deleted it because he didn't think the B6 ECU used it.) though the stock gas cap seal looks pretty rotten.

Would that valve normally hold a vacuum on the gas tank?

I am only really asking these questions, because I think that this might be hurting my fuel economy by causing gas to evaporate, and that maybe the running condition may be effected by having no vacuum on the tank.

[Dragonhealer]

Would that valve normally hold a vacuum on that vapor line?

Good question !
I think the tank system holds vaccum also, but am not certain. It cannot be much, as the tank would collapse beyond a very small amount of vaccum. Pressure wise, it's like 1/4 psi
all the EFI B series use a purge system, in fact, should be all USD cars with EFI from '90 on

[bravekozak]

I removed my evaporative system for purposes of testing.
I do not have an ECU. I vented a gas cap I found at the junkyard. No woosh.
No trouble codes. Instant throttle response. No problems yet. I will continue the test for at least another twenty years and report my findings.

[BeeDuhb]

Good to know its normal. It's definitely pressure


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[TorqueEffect]

I removed my evaporative system for purposes of testing.
I do not have an ECU. I vented a gas cap I found at the junkyard. No woosh.
No trouble codes. Instant throttle response. No problems yet. I will continue the test for at least another twenty years and report my findings.

I just picked up a normal off the shelf gas cap today for the Festiva that is vented, they also have push button vented ones you can get too. But I can technically remove that EVAP crap now that I have a vented gas cap.

Still wondering why Mazda decided to put such a complex EVAP system there, when all they needed was a vented gas cap? Obviously there is something behind it, because that system has to be far more expensive to make than a simple vented cap. lol

Obviously it's for emissions, but how is that weird device better than a vented cap when brand new cars don't need this weird device, and just use the vented cap? Did cars just not have vented caps up until the 90s?

[Dragonhealer]

Ha!!!
You missd it! I like to burn the fuel I buy, not lose it to atmosphere, out here in the desert the losses would be large

[ricko1966]

Dragon healer gets it! The system holds pressure.That pressure is gasoline trying to escape but cannot,there fore your fuel does not evaporate into the atmosphere.Saves the planet and your gas money.A simplified example put a jar of gas outside with a sealed lid.it can sit indefinitely,remove the lid,how long before the gas is gone.Working Evap is a win win.

[BeeDuhb]

Now I get it! Good to know!


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[bravekozak]

The only reason I have a vented cap is because I want air to enter as the gas gets sucked into the engine. The amount evaporating out of the tank is negligible with the cap in place. I am willing to bet that the gas wastage carrying around a charcoal casnister + fuel/vapor separator tank + spaghetti hose is greater than the vented cap. Your simplified example should include a jar of gas with the lid loosened about a quarter turn.

[Dragonhealer]

Not sure why you want O2 to get into the fuel tank, as oxygen is, well, an oxidizer. Though these days, in most parts of the country,the fuel is oxygenated in the winter, which may help emissions but doesn't help preserve the fuel system (ruins fuel economy too).
fuel blends are done in each locality monthly according to the weather forecast to have a compatible positive vapor pressure for best starting and running. Sloshing gasoline in the tank, at any temperature a gasoline IC engine will run, will create a slight positive pressure. The fuel tank won't collapse due to vacuum.

[Dragonhealer]

In 2010 Toyota announced its newest evaporative system would loose to atmosphere only one cup of gasoline a year, making it 100 times better than the average cars system. So, 100 cups of gasoline lost a year..........
a fuel tank can only hold a little pressure. At elevated temperatures the overpressure goes out through the charcoal canister, and thus, when saturated, the rest is lost.