Stock 6.0 bucks n puffs black smoke 50-60 when towing
 

Hello all. New to light truck diesels don't know a thing about them really. I bout a 2003 f250 from a friend for $6000 it's got 197,000 on her oil change is due in bout 1900 miles. Far as I k ow the engine is bone stock. Egr is there no tuners and friend hasn't had an issue with it for 20,000 miles used it lightly and owner before him was a owner of a crane company no fifth wheel or goose neck hitch and rear recover hitch barly looks used. So truck I think had had a pretty easy life. I got a 25 ft camper and a 26 ft boat and I have noticed that if I don't use the tow haul mode on jt at 50-60 after a while will puff black smoke and buck. Does fine on interstate towing. Other tha. That it runs perfect no odd sounds no blue or white smoke in fact. Even when i romp on it I struggle to get a lil black smoke outta it. No check engine light comes on at all and no fluids leak for all I know. Besides when I'm towing it runs perfect. And after I put tow haul mode on and romp on it a few times it clears up and runs perfect. What causes that?? Do powerstroke not like to lag at all like that ? I'm a truck driver and my dietroit I just put pedal to floor and it goes lol. Any help with why it does this thanks. 😃💨

when the last time the fuel filter been changed there two of them.
On the motor and one under the truck on the frame