I purchased motorcraft spark plugs with the same gap rating as the plugs in my freestar. Part AGSF42FM. Removing the installed part was AGSF42EGM. I have not been able to find a cross reference part for AGSF42EGM online. The electrode on the spark plug looks larger on the original AGSF42EGM vs AGSF42FM. I also purchased plug wires. I changed only the first cylinder. But I have found that the timing seems all messed up when starting with the new plug. Then I assume that I might need to pick another plug to help resolve my issue. Does anyone know what plug I can purchase that should match the existing AGSF42EGM ?

My thinking has been to swap the misfiring plug / wires first, and then if that does not resolve the issue to change the ignition coil as well.

 

New plug left vs old plug right. Notice electrode difference

 

The plug on the left is supposed to be a longer life plug, Platinum.

Plugs can go bad , but not that often.

Would help if we knew the year and engine in the vehicle.