Another possibility for causing lower power: EGR is failed in the open position, feeding exhaust gasses back into the intake, so there is not enough oxygen to burn the fuel. Engine will produce smoke and have low power.

__________________
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