I have a 98 f150(4.6L romeo mtr) that developed a miss about 2 weeks ago. Changed plugs and wires , motorcraft plat. plugs and Auto zone wires. Still had a slight miss, took it to a local garage, they said it had a miss in #5 and #6, said should change plugs and wires. Put on new wires and autolite platium plugs. Ran great for about 75-100 miles than started missing again. I drove it home and pulled the plugs, #8 plug the electrode was gone and the "curved piece" below the eletrode was melted part way away with a small glob of metal on it. Replaced plug still missed, compression test shows only 20-30lbs in #8 cly. Any ideas, sounds like a valve or rings? Thanks Mike

 

Was the engine spark knocking (pinging) before this occured? Unfortunately #8 cylinder was probally burning lean & has a burnt piston. Doing a air leak down test on this cylinder will confirm this.

 

That sounds like a classic case of burned exhaust valve.

If the engine is not burning oil I would suspect the valves before the piston. However there have been plenty of cases where oil burning was not serious enough to indicate a problem.

Still, my guess is exhaust valve.

Chris