Engine Mechanical Check

WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.

Check for carbon buildup. Remove the carbon with a top engine cleaner product. Follow the manufacturer instructions on the container.
Check for excessive oil entering the combustion chamber. See appropriate article in ENGINES.
Check for incorrect basic engine parts such as the cam, the heads, the pistons, etc.