I have a 1991 w250 with 142,000 miles this morning for the first time the engine started loosing power while driving up a long hill--first I looked at my guages and all looked normal--then looking in the rear view mirror I saw blue smoke pouring from the tail pipe. At the top of the hill the engine seemed to clear out and run normal--even under heavy acceleration. On the return trip the engine smoked going up the hill but didnt seem to loose power.... im only going 25mph up these hills with an empty truck. I looked in the discharge of turbo tubing and it was clean and dry. Any ideas?

 

Sounds like a fuel supply problem to me. Change the fuel filter and see if that gets you anywhere.

 

Did you check for a torn or cracked intercooler boot or a plugged puke tube?
ClayBob

 

Lift pump maybe. Def sounds like a fuel issue.