Gas tank vacuum & MPG - Problem?

[TheV6Bird]

Hey guys,

Everytime I open up my gas cap to fill up my tank, I always have to hear a big "whoosh" sound as air fills into the gas tank (this happens cold and hot weather). I used to think whatever, no biggie.

But after filling up other people's cars and hearing their vacuum sound compared to mine, I must have a VERY tight seal from my gas cap! Everyone else's cars do not seem to have the big whoosh that my car has.

Could this contribute to poor gas mileage? I've heard that excessive vacuum in the gas tank is bad for mileage. Thanks everyone [img]smile.gif[/img]

EDIT: I have a 95 3.4 A4 Firebird, and an aftermarket locking gas cap.

[ April 09, 2005, 05:37 PM: Message edited by: TheV6Bird ]

[Need4Camaro]

I doubt it seriously. and trust me I wouldn't mess with it either because if you mess up your gas cap your PCM can set off codes and give you bad gas milage.

[TheV6Bird]

Thanks man. Its an aftermarket locking gas cap BTW

[l1tech]

A 95 shouldn't set codes for the evap system (which your gas cap is part of). Are your sure that air isn't coming out instead of being pulled in. If air is being pulled in then you definetely need to get a new gas cap because the valve in the cap is bad and not allowing air to be drawn in. If air is coming...pressure being released...then this is perfectly normal and you should leave things the way they are

[TheV6Bird]

Originally posted by l1tech:
A 95 shouldn't set codes for the evap system (which your gas cap is part of). Are your sure that air isn't coming out instead of being pulled in. If air is being pulled in then you definetely need to get a new gas cap because the valve in the cap is bad and not allowing air to be drawn in. If air is coming...pressure being released...then this is perfectly normal and you should leave things the way they are

Thanks man. I've never felt any rush of air pushing out (but then again I've never looked. I'll see if I can feel something next time I fill up with my hands, or I'll try to get a piece of paper or a napkin to place near the cap and see if it gets sucked in or pushed out.

[l1tech]

For what it's worth, the aftermarket locking gas caps are probably the worst thing that you could use...there is a check valve down in the filler neck that makes it almost impossible to push a siphon hose past...

[TheV6Bird]

Originally posted by l1tech:
For what it's worth, the aftermarket locking gas caps are probably the worst thing that you could use...there is a check valve down in the filler neck that makes it almost impossible to push a siphon hose past...

Wow that's pretty interesting! Didn't know about a check valve. I bought my gas cap more on the lines of vandalism (soda or sugar in the gas tank) instead of siphoning.

[Dizturbed One]

I'd look into some sort of lock for the fuel filler door.

I never mess with gascaps on any fuel injected car unless its an OEM replacement.

I've had numerous problems all because of a stupid gascap. (stumbling, poor mileage, even hard starting)

[Renzo]

My car makes that whoosing sound. Last tank lasted me for about 330 miles. I have a 95 Firebird A4

[Die Ford V8]

what kind of mileage are you getting. and do you have your old gas cap to use for a while and compare wooshing? or to compare mileage.

[TheV6Bird]

Originally posted by Die Ford V8:
what kind of mileage are you getting. and do you have your old gas cap to use for a while and compare wooshing? or to compare mileage.

I am getting around 16-17 MPG in the city. I don't have the old one because when I bought the car, the gas cap didn't seal well at all, and when I drove the car around with a full tank, gas actually rose up and spilled out of the tank and onto the side of my car! Hence why I bought a replacement.

The only way I'd be able to get anything original would be to go to the dealer.