Who knows exactly which spark plugs would have been installed by the factory in a 1998 3.9L V6?

 

Champions would have been. But I prefer NGK's myself. Had ran a set of Champions and was not impressed with how fast they plugs were wearing out.

 

Autolite 5224. The better one is the single platinum AP5224.

The Champion are known to freeze to the block via rust causing extensive repair costs. This is commonly reported in these threads around the web. The Autolite's like the MotorCraft and AC Delco use a black-oxide coating to prevent that problem.

 

Since platinum plugs were not engineered to run with engine management on this vehicle I wonder if they can have a tendency to contribute to misfire issues? As crappy as the Champions are they were probably used when tuning engine management, so maybe copper plugs like the 5224 are the best choice for these engines.

 

Never had an issue using the single platinum plugs on a 2001 5.2L Magnum engine. Maybe the pre-magnum engines would be better using the old school copper cores. So, 1994-2003 platinum plugs do work.