6.5 diesel again lol

The oil pressure sending unit serves two purposes. Besides sending a signal to the oil pressure gauge, it also provides voltage to the lift pump once oil pressure builds up. While cranking the engine, the lift pump gets it voltage (temporarily) from the starting circuit. Once the engine starts and oil pressure builds up, the contacts close inside the oil sending unit and then voltage is provided to the lift pump. These switches are "under-designed" and don't hold up very long. On our '95 Suburban, I've probably changed the switch at least 10 times. Used to be a dealer only item but now you can buy them at most auto parts stores. Cost about $25 or more.

The only test you need to do is while engine is running, place hand on lift pump and if its running, the oil switch is working. I'm about to install a green LED to provide a "pump running" indicator so I don't have to check it so often. The engine will run without the lift pump but not under heavy load. Also it helps if the tank is full or near full.

If you're getting pressure while cranking, then make sure you've bled all air out of the filter and only fuel is coming out the vent. You may have to crank awhile to get it running. You can also open up the air cleaner and spray some WD40 while cranking. Don't use ether on the 6.5!!!

If it doesn't crank, then I would expect a fuel stop solenoid failure or injection pump failure.

Update to your new info:

When you say that fuel is coming out the return line, I believe that could be coming from the lift pump, but I'm not 100% sure of that statement. At this point I would pull the intake manifold off to get to the line fittings right at the pump head. Crack those and if no fuel spraying out there, then likely a bad pump. (If you do end up doing that, be careful of the diesel spray--protect eyes and skin.) Like the other folks said though, check wiring harness and grounds too. And there could be a relay in the system that's bad. Have you tried the WD 40 into the intake while cranking? If you can get it spinning over under fire it might just clear any air still in the pump.

Oh, another idea to help with the bleeding of air--you can send a (temporary) 12 volt source right to the lift pump connection so you don't have to grind on the starter while bleeding the system. I know you said it's already been done, but I've thought the same thing before and bled again and then she lit off.

That DS4 pump uses a fuel cutoff solenoid and a "fuel solenoid".

The fuel cutoff solenoid is actvated as soon as the "ignition" switch is turned on. The mechanicals of the pump are very similar to the "DB" mechanical pump it replaced, with the exception of the second "fuel solenoid".

This solenoid is commanded to CLOSE by the 'puter (through the PMD) just before EACH injection event.

The precise fraction of a second that solenoid (FS)closes determines the START of injection (timing), and the exact millisecond the solenoid is commanded "OPEN" determines the LENGTH of each injection cycle, and, therefore, the fuel rate.

When the FS opens, the remaining fuel from that injection cycle is "wasted" into the pump housing, rather than being forced through the injector.

You need a signal from the crank position sensor TO the 'puter, as well as the "fine" position signal from the optointerrupter in the IP for the "puter to get a "fix" on when to begine injection. Then the 'puter has to be functioning to interpret all that, and feed a signal to the FSD, which powers the fuel solenoid, as required, as well the the fuel solenoid on the pump's "head" to do what the 'puter, though the PMD commands it to do. So, there's a LOT that can go wrong.

Start by checking ALL the fuses and fusible links, as well as ALL engine grounds.

There MAY be trouble codes as well.

Here's a photo of the pumps internal "fuel solenoid", from the "Real Diesel" website:

• Real Diesel