We had recently purchased a 1994 gmc with the 6.5 in it . Ran ok seamed like it had like a shake around 1500 rpm and recently it started running rough at idle and white smoke at idle also driving ! Also putting oil in it regularly. Has around 250,000 miles

 

I had the same truck. It had a bad *** dump on it 1 ton. I hate to say it but the conversion Diesel was the biggest mistake GM did, converting a 454 into a diesel. The motor is a train wreck. I had one in the early 2000's and started blowing white smoke, and running rough. Head gaskets...left bank went and cost me 3000 to fix and the other side was going. I could not sell it fast enough. Thanked the good lord when I did.. sorry to be the bearer of bad news....RUN

 

White smoke= not enough fuel but if your regularaly adding oil i wouldnt doubt u have a cracked piston by the sounds of it. ...itd b my guess if your coolant and oil arent mixing

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^ the 6.5 is definetly not a converted 454. By any means

 

sounds like you need to do a compression test.