Something I've noticed on my car: it runs much better if I just filled it up. Trying to pin down the problem, I decided to unscrew the gas cap, and magically the car started performing better. Here's what I mean by performing better: noticebely faster acceleration, less noise, and no pinging (rarely happens). I mean the difference is definately noticeable. And the gas mileage is awful when the pressure is too high.
I've replaced the feul filter less than 500 miles ago.

Anyone know what the problem is or how to fix it?

Thanks

 

You might have the lines to your charcoal canister inspected, as well as the canister itself.
The charcoal canister is there to filter all the fuel vapors out but allow internal pressure in the gas tank to vent. I've never heard of it being such an issue to cause accelleration loss, etc, but if the difference between the gas cap being on and off is that amazingly noticeable, thats possibly the cause.

Edit: doing a quick google search I found refrences to it as both the charcoal canister and the vapor canister, they are one and the same part

 

Great, Quality Post

 

Ok.. I'll get that checked out ASAP. A quick google search said that the canister vents the fuel system to ambient pressure. Its probably clogged.

Mods: Thanks for putting this in the right section

 

"A purge valve is used to control the vapor flow into the engine. The purge valve is operated by engine vacuum. One common problem with this system is that the purge valve goes bad and engine vacuum draws fuel directly into the intake system. This enriches the fuel mixture and will foul the spark plugs. Most charcoal canisters have a filter that should be replaced periodically. This system should be checked when fuel mileage drops."

http://www.familycar.com/Classroom/e...IVE%20CONTROLS

Looks like another thing for me to check.