hello when i open my gas cap off it gets massive amount of air rushing in or rushing out not sure if it is vacume or built up pressure but anyway if it gets 2 high will it make your car run like crap i was driveing down the road and i could not raise the rpm over 3000 i was down to 1/4 tank of gas so i stopped at a gas station and thats when my gas cap had the air rush so i put in 5 worth of gas and it ran fine after that so what do ya think

sorry its a 90 firebird 3.1 litter

do you go up or down in elevation frequently?

every morning just take off your gas cap for a second to equalize the pressure

check your evap system. there should be a diagram under the hood (that little sticker) make sure everything is routed and hooked up right. you should have no problem figuring it out. other wise the hose or valve may be clogged or stuck

It's supposed to do that.

It's pressure. The reason it does it more when you let the car run dry of gas like you said you did, is because in a FI car, the fuel is the coolant for the fuel pump, which gets hot enough to burn itself up if not immersed in fuel; so your fuel is getting hot in the tank. Keep your tank at a higher level if you'd like for your fuel pump to live as long a life as possible.

The reason your car ran so bad and then ran better when you put gas in it was because you were about to run out of gas, and your fuel pressure was so low that the engine wasn't getting enough fuel to run properly. Again, keep your tank fuller, it won't happen.

In fact, alot of new cars will set a "Check Engine" light if the pressure fails to build up in the tank; and if you pull the code, it will tell you "gas cap not installed properly". In California, one of the things they check in emissions inspection, is the gas cap's ability to hold pressure; if it doesn't hold pressure, then the car will be spewing raw fuel into the atmosphere 100% of the time, by evaporation.

Don't try to "fix" it, it's not broken. It will not affect how your car runs.

Good ole PO440-Evap system leak.