A few months back, I noticed a hissing sound coming from the rear of the car. It’s a clear-sounding hiss, that almost sounds as if it’s coming from inside. I’ve also noticed that it’s 100% louder and more distinct when I’m running at a full tank.

I’ve already replaced the gas cap once this year, when I saw gasoline running out of it and down the side of my car. I know this doesn’t mean that the new gas cap isn’t at fault, but I haven’t seen any fuel running down the outside of the car anymore. This kind of worries me, because I have noticed a considerable drop in my mpg. I used to average around 14-15 mpg, and have recently been getting about 10.

Any input or ideas would be appreciated.

I think 15 mpg average is low. Maybe if you drove in town almost all the time.

Have you tried to track the sound down? Listen to it and see where it is the loudest? Could the new cap be causing it?

Did you replace the cap with a vented or non-vented style? Your car uses a non-vented cap. If it is a vented cap then the engine is operating with a huge vacuum leak. The tank is always supposed to be under a slight vacuum while the engine is running.

There is also a white plastic, two way valve located fwd of the tank, on the left side, that vents the tank to atmosphere if there is too much pressure buildup inside after sitting for a bit on a hot day with a full tank. The valve body is white plastic and is almost impossible to remove. If you suspect this valve is bad (it is venting to atmosphere all the time, thus making a vacuum leak), it is easier to take it apart while on the car (you are inside the wheelwell) and replace the guts only. It's an easy job and doesn't cost much.

I'll replace the gas cap one more time...I bought the current one at Wal-Mart, instead of the auto parts store. It's very possible that the new cap, although better than the old one, still isn't working right. That thought aside, how would you know (appearance-wise) if you put on a vented vs non-vented cap?

If it is vented, there will be a hole in the center of the cap. If the cap is a non-vented style, then measure the o-ring outside diameter and make sure it is seating in the fuel filler neck. There is a problem with the OEM caps on 94 Firebirds, but I don't know if this problem carried through to later years. In the 94s, the plastic threads of the cap would swell over time so that the cap could be screwed in and appear to tighten up, but the o-ring would not contact the mating surface of the filler tube. When that happened, the cap would be tight, but there would be a vacuum leak. The solution is to replace the cap. To ensure that the cap fits and seals properly, maybe it's a good idea to only use an OEM cap.

Agreed.

The new cap seems to have corrected the noise. I looked at both caps side by side, and the only difference I could make out was with the rings. The new gas cap has a thicker ring (it's softer too). Anyway, buying the OEM one was seemingly the answer.

Just gotta keep an eye on the gas mileage now...hopefully, I'm at least back to the 15 mpg range.

Thanks for the info and suggestions. I won't buy anymore gas caps from Wal-Mart.

when it comes to stuff like that...dont buy anything at walmart!...take it from an employee

Update:

The hissing sound coming from the inside of the car, is almost non-existent now. However, I still hear something around the rear of the car from the outside. Maybe that two-way valve needs checked out... Anyone know what the official name of that part would be?

Yup, it's called the pressure relief valve for the gas tank. However, in my experience, it rarely fails...but, it could. They are relatively cheapy affairs to buy. Forget removing it from the car, not possible! However, it does come apart, just replace the guts and clean it out inside if needed. I would think that the vacuum lines from the front of the car back to the vapor canister would be the biggest problem. Those little rubber lines like to dry rot and come unstuck. Not hard to get your hand back there once the panel is removed. You'll need a flashlight to trace the lines. I can almost guaranty that you will find that the rubber hose that is glued to the nylon line has come loose and is leaking, but don't think you could hear that from inside the car.