will bad O2 sensor cause rough running like the car has fouled spark plugs? My plugs are new and I know I need to replace my upstream o2 sensor on the driver side but the car runs really rough at idle...if I rev the engine up to about 2000 rpm it steadys out but let it go back to idle and its rough again. I do have some leaky valve cover gaskets..so i guess its possible I have fouled plugs if oil has gotten to them???