Rod knock on 2 seperate engines in the same car. What am I doing wrong?

[ROLLGUY]

Am I doing something wrong, or do I just have bad luck? Last year my 300 CD developed a rod knock after trying to start it cold. I replaced that engine with a good used one last month, and the same thing happened again. I have put less than a hundred miles on this engine in this car. I tried to start the car this morning, and it would crank,but would not run for more than a second. I tested the glow plugs and relay, and found a bad relay. I replaced it and got it started. To get it started, I gave it a little throttle thinking it would help to keep it running. It started to knock right away. Is there a common problem with oil not staying between the bearing and journals? or did I just get an engine that was on it's last leg anyway?
I figured that the glow plug relay might have been the cause of the first engine failure (trying to start it with a little throttle), but I am thinking there must be another reason. Is there something I can do to prevent this from happening in the future? I now have two OM 617 engines with bad rod bearings, and they are getting harder to find. Anyone have any ideas???