Lots of rushing air when removing the gas cap

[farmdwg]

Just curious if this is the norm since I am a new owner (4 weeks now). Evertime I go to fill up my '95 80, there is a sound of rushing air when I remove the gas cap. I just had it smogged and they didn't note anything wrong with the gas cap.

I'm kinda at a loss to what is the cause.

Thanks.

 

[cruiserdan]

Rushing out is good, rushing in is bad.

 

[97cruiser]

Perfectly normal.....

Vince

 

[firetruck41]

Normal.

 

[Gumby]

Depends on how you were driving it before you got gas.

 

[Romer]

You tank is pressurized. The first thing you look for when you get an engine light is if the gas cap is tight. When they are loose, the tank does not pressurize and the sensor sees that as a problem. My wife tends to do that from time to time on her GMC.

 

[farmdwg]

Thanks so much. Yeah, air is on the way out.

 

[Gumby]

You tank is pressurized. The first thing you look for when you get an engine light is if the gas cap is tight. When they are loose, the tank does not pressurize and the sensor sees that as a problem. My wife tends to do that from time to time on her GMC.

Kinda.

The light comes on when the tank is pulled into negative pressure by the EVAP and it doesn't hold that negative pressure. Some systems use positive pressure sensing systems, but most I've seen use vacuum.

The EVAP purges the fumes from the tank that build up when the fuel heats. If you don't drive in a condition that the EVAP vents the tank, (steady cruise) you get a pressure build up that whooses out of the non-vented cap when you remove it.

To prove to the OBDII that the EVAP is actually working instead of the massive amount of pollution that you put into the air when all those fumes woosh out, the system pulls the tank into vacuum, then a pressure sensor reads if it stays at vacuum. If it doesn't, the light comes on with an EVAP code. P0441, P0440, P0446 ect.

http://www.findarticles.com/p/articles/mi_m3102/is_1_124/ai_n9480686

Here's a pretty technical article about later, much better Toy systems.

http://www.motor.com/MAGAZINE/Articles/012006_08.html

 

[Brentbba]

A good fart on removal of the gas cap is a healty thing! hehehe