Don't know much about the 603, but on our 617 in the TD, cold starts produced smoke and it ran rough. I expected more than one issue as blue smoke is oil and it running rough I had hoped was not being caused by some oil getting into the cylinders. I began with repairing the GP system and the roughness went away, however the blue smoke was still present. I recently changed the valve stem seals. The blue smoke is gone.

I didn't go the route of a compression and leak down test but that might be a good option.

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1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it