The North American version of the Haynes manual does not cover the 2.6, only the 4-cylinder. However, you can order the UK version directly from Haynes, in England. Go on their web site. Make sure it is the English site, not Haynes USA. They are very obliging. The manual is pretty good, for engine work. Not so great for electrical.

The other way is to get the official Mercedes manuals on CD. They are a mixed hodge-podge of stuff, and tell you how to do certain operations, rather than a seamless description of a task. And you really have to print it out to use it.

My wild guess would be a burnt valve, possibly caused by carbon build-up from worn valve stem seals. The seals are a known problem, and it is almost routine maintenance to replace them. Good luck!