Hello everyone,
For as long as I’ve owned my camaro (1987 v6) it has almost always made a hissing sound when I open the gas cap, and I can smell and see fumes escaping.
What is causing that to happen? Could it be a bad fuel pump? I have a suspicion I have a bad fuel pump because the gas gauge marks way past full all the time & whenever I try to accelerate a lot it loses power/feels like it’s choking.
I already replaced fuel filter & cleaned up MAF sensor.
Thanks for the help!

 

Pinched or clogged vent tube

 

Do a search on this site for "UFO" valve.

 

If you take that "UFO" valve off and disassemble and clean it really well that will likely cure the issue. Just be careful seeing as how they can get a little brittle.

 

Something to consider here ;

The "UFO valve" is supposed to be a "Last Resort" , and is not supposed to be the normal way excess pressure is relieved from the fuel tank . If the UFO valve opens , both the excess pressure and the fuel vapors will be released to the atmosphere , which is something the EPA hasn't allowed cars to do since the 1970s . Under normal operating conditions , the "EVAP" system is what's supposed to vent any excess pressure in the fuel tank by allowing that pressure and the fuel vapors along with it to be vented through the charcoal canister , with the pressure being relieved and the vapors being stored to be burned later during the "EVAP Purge Cycle" . To have such a pressure buildup as the original poster describes would have to involve a failure of both the UFO valve AND the EVAP system , so his troubleshooting should include both the UFO and the EVAP system .

 

Lol... last resort should be a simple vented fuel-tank filler-cap (gas cap).

 

When I did mine I replaced all those rotted hoses that led to the evap canister too, they were in rough shape.

 

Agreed. I was also under the impression that the UFO valve is also a double or 2-way valve. Not only is it supposed to open if the pressure in the fuel tank gets too high, but it also is supposed to work in conjunction with the rest of the EVAP system to OPEN to let fresh air INTO the tank as the vapors are drawn out by engine vacuum.

 

Your 100% correct , it is a 2 way valve and somewhere here at TGO (that I can't seem to find at the moment) I saw specs for the pressure and vacuum it's designed to operate at .

 

Some hissing is normal. If you are not having any other issues I would not worry about it. Hissing is caused by pressure or vacuum buildup.

The UFO valve is really designed as a one-way valve to allow air into the tank but not out. However, in the event of too much pressure it should open and vent around 8 psi. I tested mine and it begins to creep open at about 5 psi. There isn't much in them to fail. If you aren't having any other problems I would just leave it alone.

 

My '86 has done this since new. It still does. I've changed the filter pad under the EVAP canister more than once, and the system seems to operate just fine with the minor pressure differentials created by temperature and purge cycles.