After driving for 3 hours on the interstate, I stopped for gas. I had about 1/2 tank showing, but I wanted to drain the kidneys and top it off. When I cracked the gas cap, it started venting. I figured no big deal I've seen this before. So I waited a few seconds and then loosened the cap some more. It was still blowing and now it started spraying gas out along with the vapor. So, I tightened the cap back up some to where it stopped spraying gas and just let it vent. I'll bet it was something like 30 seconds before it quit venting. That just doesn't seem right. Makes me wonder what pressure I had in the gas tanks. Is there a pressure vent/relief for the gas tank system. Is it supposed to maintain pressuer by venting throught the charcoal canister system? What if this doesn't work? Is there a backup vent? I had visions of the tanks rupturing and unloading all their fuel on a hot exhaust system while driving down the highway.
I have checked the gas cap many times since then and have not seen it do this again.
Any help on this would be mucho appreciated

 

That sounds wild "fuel unloading on a hot exhaust system" Hopefully it doesn't happen. I have had a 3 second WHOOSH come out of the tank when I was on dead E and driving on FUMES. Nothing like what you described though. Keep us posted if you find out whats up.

 

The whoosh you hear when you open the gas cap is air going into the tank(s). The EVAP system pulls air and gas vapors from the gas tank(s) to the intake manifold. Unless your EVAP system has completely failed, the gas tanks should be under pressured rather than over pressured.

Keith

 

Wonder why on the gas cap it says "Caution, remove slowly, fuel spray may cause injury"? Do you think that was because if the EVAP system malfunctions then that's what happens? This is on a 98 C5 by the way. So, following that line of logic, if the EVAP systems failed is there a secondary relief system to keep the fuel tanks from overpressurizing?

 

Just wanted to post what I found out from service manual information. There is supposed to be a "relief valve" for the gas tanks behind the right rear wheel. The valve is supposed to relieve at 1 psig. I'll be checking this out when I get the chance.