Hamish,
I have done this on water cooled VW's as descrided - remove the sparks, and put a pressure fitting in the spark plug hole you are working on. However, this is not your basic DIY job. You have to have some skill, knowledge and confidence, since you are getting into the heart of the valve train. I have not worked gas MB engines, but suspect this technique would work as well. The key is that you have to be well beyond just doing the basic oil change and will have to invest in some equipment and tooling if you already don't have it. This is the type of job you don't want to tow to your local show half way through it! If you have any question, it is well worth paying a professional to do the work.

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Deezel
87 300TDT
150,000 miles