My cousin had the same prob with her Cavilier... It'll go only like 5-10 mph and putt. We took out the spark plugs they were so fouled up with oil and if i remember i think one plugs was sorta melted (symptoms of detonation). The boot on a plug wire was melted off. The engine bay was caked with oil, couldn't find the cause of that. After changing the plugs and wires it wouldnt even start anymore. We gave up on that point and sold the car as-is. Just had to give you a horror story... A lot of luck to you!