I have done all the work on my 400E which has the same engine as the E420 that your looking at. I've had really no trouble working in the space that's given in the engine compartment and have been able to finish every job I've started.

As far as I'm concerned the M119 engine is a more reliable, less costly unit than the M104 based on what I have read on the forums.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500