98 F150 4.6 V8. 112000 miles. Tuned up by Ford 8000 miles ago with new plugs and wires. Now misfiring again. Pulled code for misfire on #6. Changed coils, no change. Replaced some of the plugs, #6 look black like it was not firing. Cleaned MFS, no change. Any suggestions? Should I change the wires again? Also changed fuel filter.

This engine uses two coils. 4 cyl per coil. Changed both with new MSD coils.

 

I'm no expert. But if both coils are identical, swap coils and see if the misfire code follows the coil change. Even the best makers of electronic parts can have problems from time to time. If it stays at #6, I would think looking at the plug for oil residue that may cause a good plug to misfire. Could be the plug itself has failed early, which is a cheap fix. I've never been under the hood of a 98 f150, just my 95 and 2001 f250.

 

Found the problem. Bad plug wire. The plug wires on this motor sit down in a hole in the head. The plug wire boot should seal the hole and keep water out. Water got in the hole and the spark plug wire end rusted and statred to short out to the motor. This was hard to find since the wires had only been changed 8000 miles ago and the problem was so bad that I thought I had bigger problems. Changed wire and filled sparkplog boot with die electric grease to keep future water out. Truck now runs great. I have attached a picture of the wire with the boot cut off so you can see the issue.



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