I had an 88 with a locking gas cap and when I would take it off you would hear the pressure release.

I bought another 88 that does not have the locking gas cap and I never hear the pressure release with it. I have read other threads that mention the pressure release. Could I have a bad gas cap?

Thanks for your responses.

Keith

Mine hisses too, does it with both genuine and locking caps. The locking cap im using is the same locking cap that i used on my 87, i returned the original cap with the car and kept my locking cap.

it's not so much the locking or not, it is if the cap is vented or not... and your's should not be vented.

Quote:

Thanks for the reply Bogus. I guess I can buy that at a local auto parts store. Just curious, how does having the vented affect the car?

Thanks,

Keith

Good question.

On pre-1996 cars, no effect. The vented cap, tho, will vent gas fumes into the air. Those fumes are bad for ground level smog. The unchained hydrocarbons interact with the air and the sun and make ozone. It's the #1 reason why during ozone alerts they ask folks to not pump gas during the daylight hours. When it's really hot and sunny, it makes matters worse.

The C4 has a vapour recovery system that consists of a coffee can full of activated charcoal. The gasses from the tank are passed through that and are vented accordingly. That's a simplified description, but it's the basic idea.

On 1996 and newer (OBDII) cars, the system is monitored for pressure loss. If the cap isn't on tight enough or otherwise fails, it will throw a code. Won't effect operation, but it will throw a code.

Good info, Andy. Also, since the charcoal cannister retains these vapors for introduction into the intake system, it actually will aid overall efficiency in a minuscule way. After all, it's the gasoline vapors that burn, not the liquid. This is why anything that aids in the atomization of the air/fuel mixture contributes to getting the most power and efficiency from a given quantity of fuel. For the sake of our air and your pocket book, please don't disable these systems, on a street car especially!

Excellent point.

Those systems do NOTHING to hurt performance. They add complexity, sure, but they are rather simple in function and very reliable. I mean, my 92 has 165k miles on it and all those components still work perfectly on mine. No codes = no problems, AFAIC!!!

And yes, I agree with Notorious - the extra vapours get burned, that cannot hurt consumption. It has to provide SOME positive benefit by not wasting those vapours.

Great replies. I will be purchasing one tomorrow.

Thanks again!!

When I remove the cap on my '94 it hisses as well.

Why should it hiss? Isn't the system "vented" through the charcoal canister? If it hisses, doesn't it mean that vent path is blocked, causing the fuel pump to pull a vacuum on the fuel tank?

From my 1996 owners manual.
I know it does not explain exactly why. But the fuel system is slightly pressurized, and even more pressure in warmer weather.

Hmmm. Thanks -- there goes my theory.