Is it normal for the gas tank to be under a big vacuum? My 97 300 is the only car that I've ever owned that makes a big sucking sound every time you open the gas cap. It's been doing this ever since I bought the car many years (8) ago and it runs fine. My worry is that the cap or system isn't venting properly and that it will eventually toast the fuel pump. My other cars, including an LS, might have a slight vacuum relief hiss, but nothing like my SC produces. Anybody else notice this?

 

Happens to my '95 as well, she just Queefing some trapped air...

 

I don't think it's something to worry about. Although I noticed that it only do it when the weather is cold and I maximize the gas. When it's hot weather or there's some gas left it doesn't do the sound that loud.

 

Maybe you have a strong fuel pump that wont give up.

 

mine does it too, so far I've only filled mine up when I'm real close to empty, I'm sure it's not as ridiculous sounding when the tank is a bit more full. I don't think it's a problem, I assume this car just has a really strong fuel pump.

 

Back in the days of mechanical fuel pumps my Dad's car would shut down for no obvious reason after it had been driven for 20 minutes or so. After sitting for an appropriate length of time it would start back up and everything would be good. He had the dealership look it over and they replaced the carb.....no good.....then another shop replaced the fuel pump.....no good. Someone diagnosed the problem as being vapor lock and they wrapped the fuel lines in aluminum foil. This went on for 6 months or so until he pulled into a station for gas (this was before the days of self service). The attendant went to put gas in an heard the whoosh. He walked up to the drivers window and said, "sir your gas cap is not venting, probably rusted shut. If you don't replace it it'll build up vacuum in the tank to the point that your fuel pump can't overcome it and your car will shut down due to fuel starvation." 2 bucks later, problem solved.

I know modern fuel systems are designed completely different to eliminate gas emissions escaping from the system, but you'd think that there would have to be a one-way vent to allow air to replace the volume of fuel pumped from the tank.

 

Its not vacuum, its a pressure build up. Its normal.

 

am i the only one that has a gas cap the blows OUT and not sucks IN? lol

 

yep, pressurized, normal;. Just means the gas cap is doing it's job correctly.

 

Heh - I've been suspicious that something is wrong with my evap canister because of all the pressure. I'm replacing all vac lines anyway since they're 20 years old.

 

Mine blows out a lot of fumes.. at least a few psi in that tank haha it's obnoxious especially on a hot day when its low on gas

 

mine not only blows out tons of air but sometimes even little droplets of gas lol

 

all 3 of my sc do this, so far no problem, was told that this is normal

 

Mine friends bought a sc that needed a new gas tank. So he got one from lkq. Well it was a little warm dkwn here in texas yesterday and his trunk gets so much pressure the gas cap will shoot off if ur not carefull. He said it expanded and dented out today... Maybe this IS somethi g to wkrry about