CAT 3306 engine 250 HP black smoke????

My Cat 3306 engine with 250 HP. Engine arrangement 6N1309
I started it and black smoke came out of the exhaust, let it run for a while 5/10 minutes and still black smoke.
The last time I used it was fine no black smoke.
Maybe some one messed around with it in the field!
Air cleaner is clean.
What could be the problem?
What could some one have messed with to cause the black smoke.
Last time I changed oil I had an oil analysis done it was report iron was high. hours on engine 3998

CAT 3306 engine 250 HP black smoke????