I have a 2000 v6 mustang. all was good until i replaced the plugs and wires and now It is backfiring. I tested it by pulling each wire to see which one does not effect the idle and narrowed it down to one. I have tested to ensure I am getting spark at the plug and I do.I even replaced the plug thinking I may have got a dud. Any thing else i could try to get this fixed without a shop?