Nailing

A friend of mine got a 1985 Mercedes 300D about a week ago. He drove it about 500 miles and it drove beautifully. Going up a steep hill it started nailing. At least, that's what I would call it. He said pushing the accelerator all the way to the floor didn't accelerate the car. However he did drive the 10 miles more to where he was going and 20 back with it driving just as it always does just with the loud nailing noise.

The inline fuel filter was very dirty. I had gone to his home to change his filters and was doing a diesel purge until he stopped me. The diesel purge only ran for about 30 seconds and went from its clear color to a brownish yellow in that time. The nailing didn't diminish though. He has since stopped driving it.

It doesn't smoke at all and the glowplug light goes off after just a few seconds. It starts very easily.

I'm wondering if some gunk blew through his filter and clogged an injector making it nail. I would appreciate any thoughts on what you think it might be and any thoughts on how to go about fixing it or testing to find the problem.

Thanks for reading this and for your comments.