Gas Cap Question.
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TECH Enthusiast
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Joined: Jun 2004
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From: Chino Hills, CA
Gas Cap Question. Hey guys, when I take the gas cap off should I hear a noticable hiss from the pressure in the gas tank? This struck me as odd one day when I had to take off the gas cap to relieve pressure, but I didn't notice any release. Every other car I have makes a loud hissing sound when the cap is removed. Could this be the result of a faulty cap? Thanks. 
oh btw it's one of those locking gas caps, it came with the car.
oh btw it's one of those locking gas caps, it came with the car.
TECH Veteran
Joined: Dec 2004
Posts: 4,012
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From: Beaumont, TX
If you aren't hearing any pressure release hiss when you open it, it probably means your cap isn't sealing fully - either it wasn't tightened enough when you closed it, or the cap is defective/damaged. I'm no expert though 
TECH Veteran
Joined: Dec 2004
Posts: 4,012
Likes: 1
From: Beaumont, TX
Heh, think this might be the reason it doesn't do it on these cars but does it on so many others... the evap system! doh. "The emission control system that prevents the escape of fuel vapors from a vehicle's fuel system. Fuel vapors are routed by hoses to a charcoal canister for storage. Later, when the engine is running a purge control valve opens allowing intake vacuum to siphon the fuel vapors into the engine." So that would keep the vapors from building up in the tank in the first place. I should've thought about it more instead of assuming it was the same for all types of cars 
