Were the grey plastic fuel line retaining clips all broken and pieces
missing? Sometimes these little buggers can fall down into the open intake ports when you lift the intake manifold off.
Plastic and valve seats do not mix.
Usually it is just carbon that is keeping the valve from seating
completely. The OM606 valve springs are very light and don't
have the power to crush carbon down quickly.
You have 2 choices, Take the intake off and inspect all the ports
for debris. Or just rev that puppy up to 3500-4000rpm and try to
blow the carbon away from the valves.
Best of luck.
Dr.D