My OM603 pulls like a freight train "at speed" and passing on the highway easy, and I know that I've still got several things to tweak to get it really good. I'm getting about 13 psi of boost.

(1) Install a blocking plate to disable the EGR. When this fails open, it robs power and causes what you describe with lots of smoke at speed.

(2) I also like the plugged exhaust (cat) idea, if the engine cannot breath because of an obstruction in the exhaust, you'll have a similar loss of power.

(3) On the breathing note, check air filter and the MAF flap in the assembly just below the air filter to make sure that flap is not jammed closed.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC