Fuel cap has a lot of air coming out, and terrible gas mileage

[Bjankins1]

So my gas mileage suddenly got way worse and every time I take the fuel cap off, there is a bunch of built up pressure, do these cars have purge canisters

The car is a 92 cutlass coupe 

[jiggity76]

They do.  It's mounted in the rear quarter panel area behind the left rear tire.  

[Schurkey]

Verify fuel tank venting.

I've seen charcoal canisters fill up with liquid fuel because the purge valve doesn't work right.

Wouldn't hurt to connect a scan tool to see what the O2 sensors are showing.  I'd expect "rich" code(s) if the vent/canister system is failing so that the engine is pulling in excess raw fuel.  While you're in there, verify ALL sensors, and might as well check the fuel pressure, too.