Possible root cause of burnt exhaust valve/low compression 94 V6

[atlasfang]

I've got a 1994 4.0 V6 w/ ~180k miles. A few weeks ago I was driving down the highway when the engine started knocking pretty bad and suddenly started running and idling rough. I brought it to a mechanic the next day who did a compression test and found only 20 psi on the 5th cylinder, most likely due to a burnt exhaust valve.

I'm planning on just swapping out that cylinder head with a new one but I want to make sure I fix the underlying problem that caused the burnt valve in the first place. I suspect a bad fuel injector causing the mixture to run too lean and overheat but I'm curious if anyone has any ideas. It'd be a shame to go through the effort of replacing the head only to have the same thing happen all over.

 

[broncc]

First question is if its the 5th cylinder in Ford or normal car speak. Passenger side rear cyl is #3 according to Ford. They are usually the problem cylinder with heat issues. I would probably run a coolant flush to get rid of any goo that would reduce the cooling efficiency and replace the thermostat. I would also pull all of your plugs to see if the engine is running lean or rich. Depending on how the plugs look I would change the O2 sensors or run injector cleaner. Thats my 2c hope that helps.