W140 S600,

The search function will find some posts on the subject. For the most part the removal of the intake manifold is the most time consuming step, however, there are a number of posts on the subject of these glow plugs being broken off in the head. When this happens the fix is to remove the head and drill the remnant out without damaging the head. The problem lies in the new glow plug configuration and how they fail. When the glow plug fails it gets stuck in the head, and twisting too hard on the hex on the outside leads to having the outside of the plug snap off in the wrench.

In the end, it is probably not a good idea to change plugs if they are working ok. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)