Completely stumped. 300SDL valve cover will not come off??
 

I removed the 8 holding bolts ( 10mm ) but cannot remove the valve cover. Use a rubber mallet and tap it all round but it would not budge. Put a scraper underneath the gasket and it will not pry open. Something is holding on to it but I cannot figure out what and how. Feel like it is still lock in position. Do I need to turn the engine to unlock something inside the valve cover. I am completely stumped.

Any ideas please?

You're sure that you haven't missed a fastener? I've done that on many occasions. A few times I pried on the parts and broke something.

Could be just glued down due to time and heat. The engine parts don't interfere with the cover removal, so turning the engine won't help.

Be Careful with that Scraper!
 

You're using the Top "Parting Surface" (The sealing surface of the Head) as
a LEVER pry spot.[If you scratch it up ,the valve cover Gasket may not seal.]

No need to rotate crankshaft to remove Valve Cover.

If you've got ALL the bolts out,take the oil fill cap off and use three fingers
inside the opening to pull up on the V.C. , Whilst GENTLY tapping the cover with rubber mallet.

If there are only 8 bolts then I have removed all. The valve cover gasket has a half moon shape in front. Any screw head underneath?

I have removed valve cover before on gas engines and the 300D to adjust valves without issues. They came right off. This is the first time I am stuck. Will try it again and report.

Wipe off the perimeter seam and see if someone used a sealant or silicone. If you are going to use a new gasket and it still remains very stubborn a little heat might help.

Someone mentioned it may have sort of vulcanized on as well. I never had to struggle with a valve pan gasket on a mercedes over the years but there might still be a first time.

Success
 

I pried it with a scraper and tapped it with a rubber mallet at the same time. It came off eventually. The gasket was glues to head real bad by heat over time.

I bought a leak down tester from HF for $29 as it was on sale. Now the fun starts but it will be next week as I had enough today. I am not very heavy duty on engines but I will give it a shot.

Thanks for everyone's help.