I've noticed over the years, that some jet aircraft engines emit a large volume of smoke(blue in color) from the engines after start up in the AM,,,rarely in the PM.
This occurs on 737-300/500 series,,,,never seen it on a -400 series. Some Airbus, 757 and also the L-1011 when they were in the later stages before most airlines quit using them.
I've never seen this on 727- or a 747 or a DC-10
after a start up.
Why do they smoke and how come it's usually on low /under wing engine pod aircraft? Am I wrong?