I don't know of any purely mechanical problems other than the headgasket and accessory problems such as the wiring harness, air mass meter, and occasional throttle actuator. I have yet to hear of a bottom end ever wearing out. The M104 unlike the M103 does not have any valve guide or seal problems. Aside from the headgasket problems, I would say its about as good of an engine as Mercedes has ever constructed.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record