The amount of air or blow by coming out the valve cover hole or the attached tube could be critical if quite a bit of pressure is involved. This excess crankcase pressure if strong enough can shut the injection pump down. It acts on a diaphram inside the injection pump.
We plug the line from the valve cover and count the seconds the engine will continue to run sometimes as a test. This gives some estimation of remaining life of the engine or perhaps how much current wear.
Try to make sure that vent hose is not partially obstructed with crud. Take it off and wash it in a solvent. In fact leave it off for the test as you get a larger hole to breathe through. Also leave your oil cap off and try to start engine. If this is the problem engine should do a little better with the attempt to increase crankcase breathing ability. It may even run.
Very easy to do and let us know if any improvement. Nothing lost by trying. A quite easy little test.