Gun5

I was reading saw the 93-97's came with vented gas caps? I got a locking one months ago (so hating *******s wouldn't **** her up) does it need to vent or what? I know whenever I pull the gas cap off it releases pressure.. and my car always smells like gas (believe its the no Cats though?)

razor02097

yes you need a vented gas cap. Not having a vent the depleting fuel will create a vacuum and the fuel pump needs to work harder to pump the fuel. Eventually the car will throw a code and or the fuel pump will fail.

shoebox

I'd say that was all wrong.

4th gens use non-vented caps. It's required for the EVAP system to work properly. There is a vent valve plumbed into the lines that vents the tank if a certain amount of pressure is exceeded.

razor02097

How come my 94 Z has a vented cap then? There is a red valve inside the cap I know this cause I filled the tank over an hour ago

shoebox

Can't tell you. They are supposed to be non-vented. Perhaps someone put the wrong cap on it. Lots of times you hear a hissing sound when you remove the cap when the tank is low. If the cap were vented, you would never hear this.

razor02097

errr... well crap... I never hear hissing... how bad is it if the cap is vented? Every car I've owned had a vented cap so I thought nothing of it.

shoebox

GM has had sealed systems for a long time. It probably is making the EVAP system ineffective, but apparently is not causing a SES because of it. Remember when computer controlled vehicles first came out and people would say tightening the gas cap might get rid of an SES light? That's because the system was being vented (by the loose cap) when it was supposed to be sealed.

Injuneer

Because someone switched your cap for a vented one. I bought my 94 new, and the cap was, and still is 12+ years later the correct unvented one. There is a pressure control valve in the EVAP system that corrects for vacuum and pressure in the tank. OBD-I cars can get away without the unvented cap. Put a vented cap on an OBD-II car and it sets a code and turns on the SES light. Emissions inspections will test the cap to insure it will not leak under pressure.

Gord's Green Z28

How do they do that test?

razor02097

they have a machine that they screw the cap into... wierd thing is my gas cap passed...

Gun5

hmm so my hissing noise is normal just letting the pressure go

shoebox

Yes, hissing is normal.