try to google this:
ford focus 2002 TSB carbon build sticking valves
you will see:
......I suspect your rough idle and stalling problem is caused by one or more exhaust valves sticking, which is a well known problem on the 1997-2002 2.0L Ford Escort as well as the 2000-2002 2.0L Ford Focus. The sticking exhaust valves is caused by excessive carbon build-up between the valve stem and valve stem guide. The fix at a Ford dealership is to replace the cylinder head with a newer designed head which has revised valve guides to help prevent carbon build-up. As the problem worsens it will eventually trigger the malfunction indicator light. Here is the TSB Ford issued to address the diagnostic trouble codes that will accompany the MIL warning when the problem worsens:
http://www.justanswer.com/uploads/eb...202142_tsb.pdf
But replacing the original cylinder head with the newer designed head is not necessary. Doing that will "help prevent carbon build-up" but you can easily and quickly clean-up the carbon build-up and restore performance and smooth idle with SeaFoam but you MUST apply it properly to thoroughly de-carbon the valve stems and valve stem guides as well as the combustion chambers, valves, and valve seats. To thoroughly de-carbon the valve stems and valve stem guides you MUST pour 1/3rd of the SeaFoam into the engine crankcase......