Why does my car run bad?

You need to start off with checking your fuel pressure, Sounds like it is too high. When this is done, Ya need to check it at Idle with the vacuum line still hooked up then with the vacuum line disconnected. There should be a sufficient difference. If not check the vacuum line for any signs of fuel in it ( smell it and look at it ) Sometimes the Diaphragm goes bad & it will allow fuel to bypass it, supplying too much fuel into the Intake. ( Just so you know, The Downstream O2 sensor does not have anything to do with the fuel mixture. It reads the emissions after the Catalytic Convertor and determines if the Cat is working. The Upstream O2 will cause the computer to Inject more fuel than needed if it is reading wrong.) Let me know how you make out, if this is good their are a couple more diagnostic steps you can due.

How do I check that?

theJessman....best to have it looked over by a trained mechanic based on what rjr100 says (There's no way in hell that I'd be able to do any of this myself either...LOL!) Good Luck on getting her fixed :)

there should be a shrader valve like on your tire somewhere on the fuel rail you will want to either have a mechanic check the pressure or go to harbor freight and buy yourself a fuel pressure guage to check the fuel pressure yourself and the vacuum lines are pretty easy to check just take one loose and see if it makes a difference in the idle.

Hey Jessman, The other people are correct about a mechanic but if you have some to a little knowledge about cars you can do this step. Yes, you will need a Fuel Pressure gauge, ( sometimes Advance Auto & Auto Zone will lend you these tools at no cost) They want to seel you the parts you may need. Look on the Fuelrail for a little screw-on cap, Take that off on screw on the Guage. Start the car and let it idle while checking the pressure, then (on the same rail ) there will be a Vacuum Diagphram with a single line going to it. While the car is still at idle, unplug the Vacuum line and recheck the pressure ( each car make has a specific pressure it should be ) but if I had to guess I would say at Idle it should be between 25 -35 psi & at Idle with the Vacuum line disconnected it should be about Double that. Hope this helps & don't forget to check with the parts store about borrowing the tools Ya need.

Thanks everyone for the responses. I'll test it soon and let you know of the results

that's some good info rjr100 I hadn't ever had the tools to properly check I'd usually just start the vehicle and my dad would use a key to open that valve to see if there was fuel pressure not saying it was the best way, but in the absense of the guage he did what he could.