My 4D56 TDi was sluggish, powerless and blowing a lot of black smoke. As I have been maintaining it. I knew that the engine was fine but the local mechanics were saying that the engine is getting weak. I searched and contacted few experts from Pajero forums....and acted upon their advices:
1: Gave the whole air intake path a thorough cleaning and changed the air filter.
2: Checked and adjusted Tappets
3: Blocked the EGR valve
4: Checked the turbo waste gate actuator. Waste-gate was not permanently open and working properly.
Believe me it has made a lot of difference. My Pajero is much better, responsive, quick and powerful now and no black smoke at all. Hearing the turbo spooling and giving full boost is much fun now.
Note: Don't just listen to the mechanics; they may mislead you.
But the questions regarding 6g72 DOHC swap are still open please!