i have a check engine light on. when put on scanner it first said mis fire cylynder 6&7 so we changed all plugs. wires light went out for short time. came back on said cylinder8 mis fire rechanged that plug. also got new coil pac . came back on saying 8 again and added catyletic converter code. the light is still on with an intermittent miss mainly when accelerating. can you help. also from time to time when heater is on you can smell antifreeze. thought maybe heater core however i still have heat.