Usually valve guide seals will only cause the smoke when the engine is first started, or it can happen under full accelleration. Since yours only happens when the engine is warm, I have a feeling the ports in the cylender head are partially plugged and cauising oil to drain back the oil pan slowly. As a result, there is a build up of oil in the head causing the smoke.
Have you ever removed the valve cover?
Thursday, August 19th, 2010 AT 9:02 PM