You stated that you had 1. changed the started 2. installed a new battery and 3. installed new glowplugs and was still having trouble starting.
They said that you possibly need new injectors or an engine rebuild. I wouldn’t submitt to an engine rebuild just yet.
First of all, you mentioned that it was cranking slow or hard to crank. Starters these days are remanufactured, not new. There are a lot of rebuilt starters on the market that are JUNK with a capital J. I would make sure I had a good starter.
To do that, first make sure the battery you are using is up to the task. Diesel engines require a high amperage battery. Your everyday car battery isn’t stout enough for diesels. Many diesels use 2 batteries in parallel circuit. It’s still 12 volts, but you double the amperage. Make sure your battery is hot (well charged). Try hooking a 2nd battery up to it in parallel. (hook cables + to + and - to -) If it still cranks slow, all may not be lost, but you know for certain it’s not the battery. At this point, the starter is still suspect.
Next, remove the glow plugs and try cranking again with the glow plugs out. The starter should freely spin the engine rapidly. If not, replace the starter with a new one.
While you are cranking with the glow plugs out, look for excessive fuel coming out of one or more cylinders. Some fuel will come out but you should be able to observe the difference between some and excessive. Also look for oil coming out of the glow plug holes (oil would indicate a bad cylinder).
If at this point, it cranks fast, no excessive fuel, no oil shooting out of the glow plug holes. I did notice that you said you stepped on the throttle when you cranked it. DO NOT do that with a diesel. Stepping on the throttle could open the injectors, dump excessive fuel into the cylinders which will hydro lock the engine and make it hard to crank. If it were to start in a hydro locked condition it could bend a rod or the crank shaft.
If you have excessive fuel squirting out of the cylinders it’s a sign that you have a leaking injector. The solution is to change the injector out or have the injector rebuilt at a shop. If you have oil squirting out, it could come from several sources. Worn valve guides (cheap to fix), leaking injector system (depends on the type of injectors and how they are driven) solution is to replace/rebuild injectors.
Another possible is a bad glowplug switch. Even though the glowplugs are new, the electronics (switch, wiring, etc) could be bad causing one or more glow plugs to not heat up. You can check them with a multi meter. Put the ground wire on a good ground and turn the switch on to activate the plugs. Put the positive lead on top of each plug and verify that they are getting 12V+ each. If they are getting power, they should be working.
Hard to start diesels frequently get eithered. That is, someone squirts a shot of either into the intake to make it start. That’s not good for the engine. It will cause the glow plugs to go bad as well as other internals in the cylinders. Diesels can become addicted to either.
Another possible problem you could be having is weak fuel pressure or an air leak. If your injector pump is laying down on you (the most expensive part on a diesel) you could have weak fuel pressure making it hard to start. An air leak anywhere in the fuel system could also cause loss in fuel pressure making the engine difficult to start. Something as simple as a fuel filter gasket not sealed properly could cause it. Make sure everything is tight, and change the fuel filter while you are at it. Diesel engines should have the fuel filter changed very regularly. I change mine with every oil change. Some makes will go every other change, but that’s really a maximum.
My best guess comes from the description of the hard slow cranking engine. That leads me to believe that it’s one of the following:
1. Bad battery or insufficient current to run the starter motor
2. Bad starter
3. Leaking injector(s) causing build up of fuel in 1 or more cylinders creating a hydro locked condition.
Neither of those 3 are as expensive as a bad pump or engine rebuild.
Good luck
Skipper