A misfire can be caused by a ignition or fuel problem. You haven't said the check engine light is on so I will assume it is not a engine management issue. Since you have change the most commom causes here are some things to look into. The coil, faultly ignition module. On the fuel side check for Dirty injectors, low fuel pressure, faulty injectors, and clogged fuel filter. Also check for a vacuum leak and a faultly ERG valve can cause misfires. If all of the above checks out then this leaves a compression problem due to burnt or bent valves or leaking head gasket. FYI a spark plug that has heavy ash deposits may indicate a coolant leak from the head gasket or a cracked head. A misfire can also be caused by weak valve springs or worn cam lobes, This can be detected through the use of a vacuum gauge.
Good luck Backyardmechanic?
Tuesday, December 19th, 2006 AT 8:01 PM