M 104 mechanical problems?
 

What are the most significant mechanical problems that you might encounter on a M104 engine. I am very familiar with the M103 engines since I currently have 2.

For instance valve stem seals, head gasket.

Head gasket
engine wiring harness
Throttle actuator
Mass air flow sensor

Valve seals and guides are generally not a problem on the M104.

Not so significant:
ignition coils
EGR valve tube clean out

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.