Altitude vs diesel knock
I live at 9,000 ft in Colorado and I recently drove my 220D to Ohio and back. I noticed a funny change in the engine's behaviour along the way. I don't mean that it had more power at lower elevations, because of course it did. What's interesting is that at high altitiudes the car sounds much different at idle. For example, there seem to be many more random knocking sounds. Also, there is a trace of blue smoke in the exhaust. When I accelerate, there's a nasty knocking sound at about mid rpm range. It makes this noise at sea level when accelerating, but only when cold. The engine has only 5,000 miles since a complete rebuild and in 2,500 miles of cross-country driving it has not used a drop of oil. It has plenty of power and the timing seems to be about right. It basically runs perfectly at sea level.
I also have a Jeep Cherokee with a turbo diesel motor (factory! I've never seen another), and I have noticed the same sorts of symptoms with it. It has blue smoke and is harder to start at high altitudes. As sonn as the boost pressure comes up, it runs fine, because then it doesn't know it's not at sea level.
This seems to me like a case of too much fuel being delivered for the thin air of Colorado. But that doesn't seem to explain everything.
My question is: is this this an issue of fuel, or of timing? What can be done to address the problem? Why the blue smoke?
I appreciate your comments.