Diesel is not going through to the injectors. There is a new diesel filter in. Up to the injector pump there is diesel but not in the injectors itself. What could be the problem?

diesel knocks are from 2 things
bad injectors and incorrect pump timimg
bad injectors dribble fuel into the cylinder causing combustion to far advanced so the piston is slammed against the cylinder wall
incorrect pump timing does the same thing
get it inspected by an accredited diesel specialist shop or engine failure will occur

you will need the engine turned on to the run position to deactivate the stop system in the pump
injector lines are bled at the injectors
loosen the nut off and crank the engine over until the diesel drops are seen

black smoke = excessive fuel = bad injector tips , low injector pressure , turbo not working properly
take the car to an accredited diesel specialist professional for a pump pressure and injector pressure test, injector calibration and tip replacement

have you bled the air out of the injector lines or out of the injector pump
look in e-bay --type in injector pumps for onan diesel engine-- and see what is available
go google and type in --injector pump for onan diesel engine model 8000--and see are there are any diesel repair businesses with sirs for new and reconditioned pumps

Electromagnetic or electronic fuel injectors are self-priming and all that is needed is to crank the engine, though I believe on later systems the injectors are coded and will not work unless the ECU has been informed the codes have changed. I think this applies more to diesels than petrol engines but on such matters I am not expert...

Staying with diesels for a moment - new injectors on either engine would usually mean fitting a new fuel filter. Sometimes this would require the primary diesel circuit to be bled in order for the high pressure pump to work.
The amount of starter cranking required can be cut by leaving each injector pipe loose in turn until diesel is dribbling from it.

Not enough information. What vehicle? What year? What kind of injection system? There are basically two kinds of diesel injection - injector pump and common rail.
I did notice you talking about an injector pump, but both these systems need a high compression pump to operate the injectors. On the older systems, the injector pump is connected via a gear system to either the crank or the camshaft, and its task is to supply fuel to the injectors in very much the same way as a distributor does on a petrol car.
Now, do you mean the header pump that supplies diesel to the injector pump, or do you mean there is diesel coming out of the injector pump between the pump and the injectors?
On the other system, which I assume you are talking about, diesel is supplied to a high pressure pump which keeps a rail at high pressure. All the injectors are connected to this rail - from there the name "common rail" - and the timing is supplied by the Engine Control Module (ECM) via electrical impulses. Now ,if you have diesel supplied to the common rail but the injectors are not popping, the problem is most likely with the ECM.
However, there is no way of helping you unless you supply more information.

These are self bleeding so if diesel is getting to the pump it won't need bleeding. If you still feel the need to bleed the system then loosen one injector pipe turn the engine until you see diesel coming tighten again it should fire up. If you have a good flow of diesel coming from the filter to the pump and diesel at the injector pipe then your pump pressure may be low. To check remove any injector reconnect the pipe to it with the injector end where you can see it. Turn the engine with the starter a few times the diesel should be making a mist. Wear eye protection for this test

check diesel filter rubber ring or diesel main pipe puncher

anytime you change the filter in a diesel you have to bleed the air out of the fuel system. The pump won't auto bleed the fuel system for you. every place in the fuel system that has a flare type connection needs to be bleed, start at the connection closest to the tank, loosen the connection and then crank the engine until you see diesel fuel drips from that connection. Tighten the connection while the engine is still cranking. Move to the next connection that is going toward the engine. Continue on in that manner toward the engine until you reach the injector pump/injector. Once you reach the injector pump be VERY aware that the fuel is at VERY high pressure. Do NOT put your body parts in the stream of high pressure diesel fuel. It could kill or injure you! Once you bleed all the way to the injector pump/ injector, the truck SHOULD start. Do not bleed after the injector pump because of the pressure involved, it is WAY too dangerous.

Good luck!

Does it have diesel?
Could be fuel filter or fuel pump.
Glow plugs?
Injectors? these trucks have been known to have bad injectors.
Possibly the oil pump, the injectors run off the oil pump.