Has the engine ever been rebuilt? The reason that I ask is that there are steps that should have been taken to balance the engine and drive train at the time of rebuild, but perhaps were omitted or not done correctly. For example, the moving parts of the engine like rods and pistons are normally balanced with respect to each other by weight, and the rotating parts (like flywheel and clutch) are balanced rotationally. Just thought I'd suggest that you consider whether you have unbalanced vibration versus uneven low-speed combustion.

Kurt

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- '79 240D - engine swap complete! Engine broken in! 28-31 mpg! Lovin' the ride!
- '86 190D (W201-126) - 2.5 NA engine, 5 speed, cloth interior, manual climate controls, 33-34 mpg (sold to forum member).