First, Welcome roadbuzz to BenzForum!
One of the benefits of a diesel MBZ is the simplicity of the engine. When the car would not start it appeared to be fuel related to me. The engine starter was doing its job, the engine would turn over. I ruled out the glow plug system since the glow plug light did not indicate a problem and most of the time the engine would promptly start without a problem. I found two obvious fuel system problems. One was the fuel primer pump was weak and the other problem was a fuel leak on the injector pump. My theory was that the fuel leak resulted in air in the fuel lines and the primer pump was having a hard time priming the system. If you cranked the engine long enough it would finally start and run OK.
So, I replaced the primer pump, which was an easy DIY job. And this morning I fixed the leak on the IP. The problem was a leaking O-ring on one of the pressure valves on top of the injection pump. This also turned out to be a reasonable DIY project. The following was the process:
You will need a 14 mm wrench for the lines, a 30 torx wrench for the pressure valve holder lock rings, new o-rings and new copper pressure valve seals. You will also need a splined wrench for the pressure valve holders (617 589 01 09 00) is required, there is no substitute.
Clean all the dirt off the IP and lines.
Remove injection lines at injectors and pump, lay aside (no need to actually remove from car).
Remove lock rings by taking the screw out for each pair.
One by one, remove the pressure valve holders. They simply unscrew using the splined wrench. Watch for the pressure spring that sits on top of the valve, it tends to stick in the holder, then drop on the floor.
Remove o-ring from holder, install new one, then use a pick to lift off the copper seal on top of the pressure valve body. Make sure everything is clean, then put a new seal on top, put the spring back in, and re-install the pressure valve holder.
When you have them all in, torque to 35Nm with a torque wrench.
Re-install injection lines, leaving the injector nut loose.
Crank engine until all the air is out of the injection lines. This take longer than you would think.
Tighten injector nuts and start car.
No grit in the IP, make sure EVERYTHING stays perfectly clean. DO NOT pull the sleeves out of the IP if they get dirty, use some carb or brake cleaner to blast the dirt off!
Don't loose the spring inside the pressure valve holder!
If you don't get the torque correct, the seals will leak, causing running troubles but no other damage: Hard knock and rough idle with no smoke, but engine runs fine at high rpm indicates leaking seals. Just retorque the affected holder, should fix it. At worst, you will need another seal.
So far this repair look to have solved my fuel leak and hopefully the no-strart issue. Time will tell.