Should there be a vacuum in the gas tank when the gas cap is opened?

I ask because when I open my gas tank to fill up, I hear air being sucked in.

 

Vacuum or pressure. It's dissapated when the engine is running by the EVAP system or when you open the gas cap.

 

Yes. It used to just be a gas cap...now it's part of the "fuel system"

 

i just thought that since your car need fuel pressure, and when you open the gas cap it releaves it. sort of like removing the fuse for the fuel pump to change out a fuel filter.? am i correct to asume?

 

I don't understand what you said.

 

So is this vacuum normal?

 

Yes vacuum is normal. The car has a pump that maintains negative pressure to reduce unburned hydrocarbon emissions. It keeps gas fumes from leaking out of the car.

 

hmmm, this make me wonder now, because lately when going to fill up my tank or put gas in I do not hear the air sucking in. I noticed during the summer, but now it has stopped. Could this be a sign of a problem in the fuel system to come?? Thanks in advance.

 

I would say that it's normal to hear more air during the summer. It's probably due a different expansion rate for different temperatures. I notice the same thing that you do.

 

Your car is going to blow up.

 

The vacuum is there and it's normal on my 91 but a different system is used and it is not there on my 03.

It's-a-no-problamo

 

I have the exact opposite reaction when i take off the gas cap. I hear and smell pressurized gas vapors hiss out for a brief second.

anyone think my car is broke?

 

ok my gas cap not hissing anymore so i buy new one (non nissan)...now it seems work fine.. the hissing come again but with strange fuel gauge...after driving sometime, the fuel gauge drop half strip. So i try open the fuel cap, theres a lot of hissing and the fuel gauge going up again half strip...any advise whats wrong here?

 

Dude its perfectly fine...

Some sucking sort of noise happens sometimes but not all the time...

lol, In my 510 when i had my LZ23 in it, due to there being no evap or return system for the fuel whenever i would get past a half a tank i would have to release the vacuum created by the lowered gas level...if not the carbs would get choked up due to lack of fuel and kill the car

New cars get the same thing but due to evap and a return line system from our EFI this is reduced to a minimum. It can be caused by the gas level changing, as well as vapors expanding/contracting...its all controlled so it doesnt have any ill effects like what i said before in my 510 lol

 

Also if you dont have your Evap hooked up correctly like i did for a month, i lost about 1/2 my MPG and you smell gas constantly. it also creates an intake vacuum leak.