Yes, FSOV is Fuel Shut Off Solenoid. It may be working, but as batteries age, their voltage when cranking drops. Starter will still work perfectly, but FSOV will close while cranking. You are unable to see this from the cab as a single person. Pull it back and zip tie it to test, or just remove it, only a few 10mm fasteners. Also make sure the large plastic fuel shut off lever is securely attached to the pivot shaft that goes into the pump. We have seen one be loose and cause an engine to not shut off. It should be spring loaded into the run position though, even if it slips.
Once confirmed OK, then move to fuel suction lines. If never replaced, they are due. Known weak area on these trucks, they are all old now.
If you have a frame mounted fuel filter (behind the driver side tire), check that it is tight, and the square washer at the top under the thru-bolt is fully seated in the square recess area of the mount head, rubber side down.
After that, the overflow valve is a spring loaded pressure relief check ball built into the fuel outlet port of the injection pump. An un-official way to field clean it is to pinch this line with Vice-Grips for 30 seconds while running and then release. Do this a few times. Or, also easy to just remove, clean, inspect, and re-install it.
If all above OK, move to fuel selector valve (if twin fuel tank truck). The O rings in it are a common air intrusion path. Easy to disassemble, clean, lube, and inspect.
Then go to the fuel tank dip tube. This is also a known air leak point as these trucks age. Check the tube itself for pinholes, and especially the solder joint at the top where the tube joins the fitting. It may be cracked.
Check the engine mounted spin on fuel filter drain valve at bottom is tight, the filter is tight, has no rust holes in it, the hard fuel line fittings on the engine and pump are tight and have their copper washers on each side of each one, and that the bleed bolts are all tight (10mm head). One on pump, one are rear of intake manifold where hard line goes from pump to engine mounted filter.
See if you have a fuel pressure sensor on top of the engine mounted mechanical pump. If so, this is for diagnostics only, and leak prone. Remove this sensor and plug this opening.
Once running, you can install a clear section of hose on the fuel return side to see if there are any air bubbles indicating an air leak. Easiest way to use the numb-thumb primer button on these engines via a tool-less method is to spin loose the engine mounted fuel filter, prime as needed, then retighten it.
