Exhaust valve for #4 cylinder sticks open about a 1/4 inch in 1987 MB 560 SEL engine.

Some info leading up to problem, my 560 overheated after a 3-4 mile drive home from work and began KNOCKING loudly. The noise was so loud I figured it had to be coming from bottom end. I pulled valve cover and found a bad hydraulic valve clearance compensating element (ball stud, lifter) for exhaust valve for #4 cylinder. So I replaced it and checked it with the Go-No-Go tool (it’s the right height). I removed and measured both valve springs and both are correct length. I can spin and move the valve up and down freely and I can see only a small amount of carbon on bottom of valve (1-2 mm thick) while looking through the spark plug hole. So does anyone have any suggestions on what the problem is or what I need to do?

Thanks,

Justin