Here goes, this might get long. My 99 F-250 a few years ago spit number 4 plug, a shop near-by heli-coiled it. Truck ran good, fixed for $100. Last summer on a 100 mile plus trip to Hodag Country Fest in Rhinelander Wis. truck started missing.Wife, me, truck, camper,300 cans of beer were not turning back. Made it there, had fun,went home. Took truck back to shop, no.4 coil was bad. When removing severely fouled plug, coil came out too. Did more research, found shop with Big-sert kit that would work. $340 later plugs in but engine has a miss. No.4 cylinder has only 10lbs. compression. Piston looked good through their boroscope but they couldn't see valves. I know there is a ton of knowledge on this site so before going any farther I thought I'd check if anyone here has any suggestions. Any input good or bad will be appreciated. Thanks for hanging in this long.

 

This is one way to see if it's a bad valve or scored cylinder. Remove the plug and take a compression test then squirt some oil into the chamber then another compression test. It the compression goes up then you have a cylinder wall or ring problem. If it stays about the same then it's a valve. the oil will seal the rings but not the valves. Old school but it works.

Denny

 

Sounds like a bad valve or seat. Somehow, something went wrong along the way, a piece of metal from the coil job, lean mixture over time, something, and it took out the valve and/or seat.

Turn the engine over until both valves are supposed to be seated on that cylinder, and pressurize the cylinder through the spark plug hole.

Listen to the exhaust and the intake and see which one you hear the air hissing out of.

If you don't hear any air coming out either one, you might have a cam follower that fell off, or lash-adjuster (lifter) not pumping up, which would be an easy fix.

Might be easier to pull the valve cover before going through all that trouble, just for a look-see.

 

What Krewat said. Also known as a "leakdown test".

Really surprised the shop hadn't done that already.... That is a usual go to test when seeing low compression.
Hissing at the tailpipe? Exhaust valve.
Hissing at the throttle body? Intake valve.
Hissing at the dip stick tube? Piston or rings or head gasket
Bubbling in the radiator? Head gasket or cracked block/head