I've owned my 400E (same M119 engine as yours) since it had 25 k miles. It has always had a less-than-smooth idle. I've had it back to the dealer under warranty and was told "All the V-8's do that." Not sure I believe it, mind you, but whatever it is, it is persistant and not progressive. I now have 119 k miles. It got a bit better after replacing motor mounts but there is still a subtle vibration at idle, in gear, esp. in warm weather. I notice it more after I've been stopped (say at a red light) for 10 seconds or so. Personally, in my case it's minor enough I've learned to live with it since the engine performs so well otherwise.

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Steve
'93 400E