I recently had this problem on a 3 door not so bad though mine would not go below 4.5%! I bought an L8 as I suspected the mixture screw pot on the ecu board, the L8 helped but alos then found the cam timing was out by 1 tooth and this was also affecting the CO! has the car recently had a cam belt? or is the car running well enough not spluttering and has good snappy throttle response?

Nick