Yanmar Injection Pump Not Pumping Fuel

My Injection Pump Won't Pump Fuel

*This assumes you have a steady stream of fuel going

 into the pump but nothing coming out of the pump when cranking over the engine with throttle in the "fast" position. 

The injection pump on most of the YM series Yanmar engines is really just a series of individual pumps inside one housing. Each cylinder gets its own pump and it does its own thing- independent of the other cylinders.

These cylinders are basically just little pistons that ride on a camshaft and, when the camshaft rotates upward, the pistons go up and pressurize a measured amount of fuel. Then the spring on the top pushes them back down as the cam rotates back down. These pistons do not have orings or seals on them. They are very precisely made to fit in very precise cylinders and seal just from being a good tight fit.  

If you have had water in your fuel then these highly precise surfaces can flash rust and the plungers can get stuck in the up position (springs are not strong enough to push them back down). This most often happens when you run out of fuel and the diesel is not there to keep them lubricated. 

You can often GENTLY tap on the side of the injection pump (while cranking over the engine) and sometimes free up the pistons and get them pump pumping fuel again.

If you do get the pump working again then you have dodged an expensive bullet and need to consider this your sign that you need to flush your fuel with fresh, water-free, diesel.