Run down to your local GM dealer and grab a can of "Top End Cleaner" and use it, along with any other advice Steve gives you. Cleaning out the carbon around the valves and in the cylinders will improve your emissions and your inline 6 will love you for it.

p.s. and please don't listen to Running Too Hot's doomsday prophecies of using this product.

It does work.

Good luck,

-Paul-