I owned a '94 E320 wagon for 300,000 miles and it probably has some of the same things your E400 has - like a very expensive evaporator to fix if it goes bad. I think the whole dash needs to come apart to get to it so the labor is ridiculous. The evaporators have a pretty high failure rate too.

Also - I remember something about top oil tubes on either the E400 or the E420 that were plastic and cracked causing quite a problem. A quick search on that might shed some light on it.

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dtf
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (228,000)
1999 E300 Turbodiesel ( died @ 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles) rusted frame - sold to chop shop
2011 Audi A4 Avant (165,000 miles) Seized engine - donated to Salvation Army
BMW 330 xi 6 speed manual (175,034 miles)
2014 E350 4Matic Wagon 128,000 miles
2018 Dodge Ram 21,000 miles