Duramax smoking badly

[Bill and Ann]

The truck has really started putting out black smoke when idling. Took it to the dealer and as there aren't any warning lights showing they say they are going to have to do a bunch of different tests to see if they can find out what is happening to cause the smoke.
At $100. per hour this should be interesting. The injectors have been changed at around 30,000 miles. Now have around 115,000.