Since there is only one O2 sensor for all 4 cylinders then it is possible for a cylinder to run leaner or richer than the others, computer just opens other injectors longer to balance exhaust for oxygen level
But there would be other signs of a Lean burn, pinging/knocking would be heard often
And head chamber and piston top would show signs of that, pitting
Burnt exhaust valve is because the valve is not sealing the cylinder when it closes on its valve seat, simple as that.
Explosive gases, when cylinder fires, start to exit at that opening which eats away at the softer metal, the valve
Eventually no seal is possible and cylinder starts to misfire
Yes, lean burn causes higher heat in that cylinder, that's what causes the Pinging/Knocking
That heat can soften the valves BUT.....intake valve usually take the brunt of that because its softer metal than exhaust valve, it "tulips" from the heat, which does happen on the 2.3l
If bevel on valve seat and valve are not the same then poor seal happens(i.e. Valve job)
If valve seat is not being cooled by head coolant circulation, it can get a hot spot, causing poor seal
If valve isn't turning it can get a hot spot, causing poor seal
Valve stem should be flat on top, rocker should also have flat area so valve stem is free to turn, and roller rocker needs to be flat, if it sits at even a bit of an angle it will prevent valve from turning, like its pressing sideways on the of the stem not straight down
Weak valve spring is a long shot but not a no shot, lol