Okay, the friend's father went to take a look at the car today. Cosmetically, it looks to be perfect. However, the engine issue appears to be different than how the friend was told (or, at least, how he described it to me).

According to the friend's father (who has worked on big trucks in the past but isn't too horribly familiar with the OM616/617), one cylinder is not "doing its job". Furthermore, there's a knock that was loud enough that he wasn't willing to try and drive the car. He had to keep some throttle on to keep the engine running; I don't know if it was capable of revving or not (I think the knocking kept him from trying).

My first thought was that the valves are out of adjustment, except the owner said that this happened suddenly, and I presume that a valve adjustment issue would take some time to develop. In the owner's words, "all of a sudden, the engine bogged down".

I'm thinking that either there's a problem with one injector, or there's a mechanical failure. Problem is, at least as far as I know, if one cylinder wasn't firing at all, it seems to me that the engine would still be capable of running; it would just be uneven and would have a noticeable loss of power. The inability to idle suggests fuel starvation, but doesn't explain the knocking (unless there's two problems).

Does anyone have any ideas? This is very frusturating for me since everything I'm getting is 3rd or 4th hand, and I haven't been able to play with the car myself (it's about 500 miles away).

Any comments would be appreciated. Thanks in advance!