Currently owning both engines (97 E320 and 01 SLK 320), I'd have to say that the M104 idles a lot smoother than the M112. During idle, the M104 even sounds better than the M112. The M112 has always "ticked" during idle that I have heard in pretty much all the M112 cars, so I know it's nothing wrong with mine. Once you get going, both engines are smooth as silk. The M112 does have a much flatter torque curve.

This is not saying any one engine is better than the other, just some facts I've noticed owning both.

I can't say for longevity for either, since both my cars are considered low mileage for a Mercedes (SLK has 24k miles on it now and the E class has 99,000). The M104 in my E class has not missed a beat and I've had no head gasket or transmission issues whatsoever.

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1997 E320