My guess is hose in photo is a heater core supply hose. One reason it could be plugged is because temp/water control valve in cab doesn't operate as designed.
Ok so I’m back on here a year later. Got 60 hours on the rebuild and now have issues with engine missing. I believe it’s a fuel issue. Sometimes it even has the fuel knock. I cracked injector lines twice and it cleared up for a few hours. But now that’s not working. I narrowed it down to one injector and crack that and the engine would actually run better when I crack the line. When I would tightened the line back up it would go back to missing. So today I took injector out and switched it with another one to see if the miss would follow the injector. It did not. So it can’t be the injector. Took the fuel line off and checked for pin holes and line looks good. I took all the lines off the plungers and noticed all of them had fuel to the top of threads except the one that was missing and seen a paint chip in the hole. Cleaned it out put it all back together and still missing. So it has to be inside injection pump? I just had plungers rebuilt and injection pump checked out when engine was out.
Hey Angle sorry I cant help with the problem and I do hope you get it all worked out! I do have a question though if you dont mind me asking. I was born and raised in the south side of Scranton so I was just wondering what you do with an M9 in the big city?