Excess Vacuum in Fuel Tank

[SHOpar]

It's been awhile since I have checked out the forums, but I have a question I hope you all can help out with:

It seems like there is excess vacuum building in the gas tank after I drive the car. For awhile I have heard a slight whining noise and I can smell fuel vapors after turning off the car and walking over to the passenger side. This weekend I finally looked into it further, so I tried releasing the gas cap. It hissed for a LONG time before the pressure in the tank equalized - I could actually hear the gas tank expand!

I tried this several times over the weekend with the same result. The car is still running perfectly and seems totally unaffected, but I don't think there should be that much vacuum building in the tank. Anything in particular I should check? Is the fuel cap not venting properly - maybe the vent on the top of the tank?

Any help is GREATLY appreciated!

Thanks!

 

[FoSHO]

My gas tank does the same but not as bad. I hear hissing for about few sec. before i completly remove the gas cap. I don't hear the gas tank expand but just hissing. I was woendering it might be the vent hole being cloged up. I don't know for sure. This is the second car that i have that does it.

 

[Shoaz]

Sometimes this is a result of overfilling the tank so that the charcoal canister ingests fuel and gets clogged up. The fuel will eventually evaporate out of the canister, but you can help by resisting the temptation to add fuel after the gas pump hits auto-shutoff.

After I caught on to this I stopped adding fuel after the pump cuts off, but in the summertime I do still get this occassionally. Another way that I get this is after a long descent. If I fill up in Payson or Flagstaff (high altitude), the next time I open the tank in Phoenix it sucks in a lot of air.

 

[SHOpar]

Thanks for the suggestions so far. I doubt my charcoal canister is filling up with fuel, as I don't drive the car too much any more, and fill the tank maybe 2-3 times a month. When I was checking it out this weekend, I only had 3/8's of a tank in it.

I'm keeping my fingers crossed it's just the gas cap, and may pick one up at Napa tonight on the way home.

Doesn't seem to be to common of a problem...

 

[SHOpar]

I just wanted to let everyone know I resolved my fuel tank pressure issues tonight!!!

Thinking I was in for the worst, like a clogged vent on the top of the tank, a bad charcoal canister, a malfunctioning fuel cap, or some other costly item, I decided to take a look at it tonight so I could order up parts tomorrow and fix it this weekend (last weekend I'll be home before I head out for my vacation and the convention!).

After getting the same pressure issue with the fuel cap I stole off my truck and running the codes and coming up with nothing, I crawled under the car to look at the charcoal canister. I took the "T" off that connects the tank vent to the canister and purge solenoid. I was able to blow air through the tank via the vent valve and blow air through the charcoal canister, so I ruled those out.

Then I took a look at the "T" that connected all of this stuff together. Sure enough, it was completely clogged with orange, rusty looking stuff. I cleaned it out, put it back together, and it works PERFECTLY!!! :****: :****:

So for those of you that may have a pressure issue in your tank and have ruled everything out, try checking that "T" fitting. Instead of spending a fortune throwing parts at the car that wouldn't have fixed it, I was able to repair it in less than an hour, for FREE!!!

I wonder if my clutch will be that easy when it comes time...

 

[projectSHO89]

If you have vacuum in the tank after running, replace the fuel cap. It has a vent built into it that allows air into the tank to replace the volume of fuel that has been consumed by the engine . Note: this does not apply to newer OBDII vehicles.

Steve

 

[Shoaz]

SHOpar, thanks for the update, that's useful.