My '87 300 D-T has 45,500 original miles on it, is in pristine condition, has had all scheduled tune-ups, had the oil changed every 3,000 miles, and uses no oil. Two years ago, when traveling up north during the winter, the engine began to miss and it spewed blue smoke when it was about twenty degrees Fahrenheit but the miss and the smoke disappeared when I increased the RPM's. During warm weather, it always started flawlessly, that is until about 6 months ago, when the engine began misfiring no matter what the temperature was outside but only when during a cold start. My certified Mercedes factory trained master mechanic said it was one of three things: clogged fuel filters, bad glow plug(s), or bad fuel injectors. One by one, he ended up changing all three but the engine still misses! He said the valves were self adjusting so that apparently isn't the problem and the blue smoke comes from unburned diesel fuel but disappears when the cylinder(s) begin firing so the rings are good. I've searched previous posts and most point to bad glow plugs but since I've had new ones installed, that cause has been effectively eliminated. Any suggestions would be greatly appreciated!