im new to this but i have a similar problem. mine is 2004 ls v8 3.9. I GASSED IT ONE DAY AND MY CHECK ENGINE LIGHT CAME ON. THE COMMOM LS PROBLEM!!! COP IS PROBABLY OUT! I COULD FEEL IT WHEN IT WAS QUIET AND IN IDLE, BUT NO BIG POWER LOSS/ DIFFERENCE, 1 WEEK PASSED APPROX AND I CHECKED THE OBD...