Hook a scanner to it and make sure the O2 sensor bank 1 sensor 1 isnt stuck lean. If it is, it will make it smell bad out the back, meaning it's basicly dumping fuel. You wanna see if your O2 sensor is at least switching. If it is, This is where you wanna start looking into your fuel trim. Make sure that at an idle that your fuel trim less that 15% on the negative or positive side. This will at least start telling you things like you may possibly have a malfunction in the system. BTW, is the check engine light on?