My C280 has the same engine as the E320 (M104) and I have had no problems with it at all. It's a straight six with 4 valves per cylinder and variable valve timing on the intake cam. Two common problems were the headgasket which can develop an external oil leak and wiring harness issues. Other than these, the engine is great, and the E320 is a great car.

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Ali Al-Chalabi

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