Blue smoke and sputtering when cold
New to forum. I recently acquired a 1991 350SDL with 106,000 miles. When cold, the engine starts fine and idles smoothly for about a minute or so, then gets smokey and sputters. It takes some time to get to operating temp (80), but when it does, it runs smoothly and pick up is good. The engine burns little oil and does not need to be topped off between changes. I took the car to a shop that supposedly is experienced, but they recommended having the turbo rebuilt if I wanted to reduce the smoke (before sputtering/misfiring began). $3K later, there was little or no difference except in my wallet. The sputtering only recently began while on a trip to NY. I thought I might have gotten some bad fuel. Added some additive and ran two half tanks of fuel with additional additive. No change whatsoever. Changed pre-filter, fuel filter, oil and oil filter after running diesel purge. No change. Took to another diesel mechanic who suspects it is the injection pump. He recommended taking to Mercedes dealership/shop for the replacement work. Went to Mercedes for diagnostics first before ordering rebuilt injection pump. Mercedes wanted to do a compression test which would have cost about $1k, said there was a leaking injector, and possibly an intake manifold leak. Given the fact that the engine runs fine when warmed up, I don't suspect there is any serious internal engine issues, but a fuel delivery issue that may include air getting into the system and/or an injector issue. I ran diesel purge the other day while the engine was cold to see if there would be any sputtering. The engine runs smooth on diesel purge. I don't know much about these engines other than bent rod issues on some motors. Not sure I believe it is the injector pump. The car is immaculate inside and out and I really want to get this issue figured out. The previous owner (2nd) replaced the Bosch injectors but since one is leaking, maybe it is possible something was done incorrectly. I would be grateful for any insights. Many thanks!