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I would flush all of the coolant with only distilled water. Drain and fill 2-3 times to get everything out, then pour 50/50 mix. Just don't leave it parked outside if the temp drops to freezing.

Hot coolant naturally rises over cold coolant. So even though there's no flow directly past the sensor when the tstat is closed, the warmer coolant will still make its way to the top of the system, which is why they put the temp sensor there. That sensor location is also why the heater core and intake warming circuits run back to the cold side. Both of those circuits are dumping heat, so the coolant coming out of them will be cooler than the actual engine temperature. If this cooler coolant was introduced to the system at the top, it would mix with the hot coolant around the sensor, giving the computer an inaccurate idea of how hot the engine was. Figuring out the rest of the coolant flow was more difficult than it had a right to be, mostly because flowcharts were a terrible way to try and explain this system, but also because Nissan doesn't seem to understand them either. Now that I'm really looking at them, the flowcharts in both the '95 and '87 manuals are just straight-up wrong. They show the water pump only being fed when the tstat is closed, and the tstat and rad forming a loop on their own, sans pump and engine, when the tstat is open. So, yeah, that ain't right. The Fronty flowchart makes a lot more sense. To confirm that it's the same as the VG30 cooling system, I pulled up yet another manual, this time for the '85 300ZX (which also had a SOHC VG30). That diagram looks very similar to the Fronty diagram, apart from the turbocharger of course. So, yeah, near as I can tell, that Fronty diagram should be correct for this application. It looks like the end of the thermostat is what blocks off the bypass hose. So as the thermostat opens, it's allowing more flow from the radiator, while throttling flow from the bypass hose. (Kinda like the hot/cold mixer knob in a shower.) This one-wax-motor-moves-two-valves thing would've been explained much better in a cutaway than it is in those stupid flowcharts! I did find a cutaway in the '93 300ZX manual, so check that out if you're struggling to visualize this--though that car has the DOHC VG, and I don't know what else changed on that variant, so don't trust it too far. Hopefully that explains it! I did not expect the rabbit hole to go that deep, or the rabbit at the bottom to be that vicious. I wonder if the borked flowcharts were drawn by the same guy who copy/pasted that fuel pressure test. (For those who don't know--the fuel pressure test in the WD21 service manual mentions a fuel pressure regulator control system, which was not fitted to these, but was present in some early 300ZXs.)

My bad, I was thinking in VG30! The pulleys on mine are bolted to the balancer. Looks like the VG33 combined the two so you can't mess it up. No idea why the marks are in the wrong place then, unless you've got the timing light on the wrong plug wire, or the rubber in the balancer has let go and allowed the pulley ring to slip. Good to hear the mileage is back up, at least! The noise could be a lot of things, and I don't know the R50 engine bay too well. Run it without the belts (briefly) to see if the noise goes away. If it does, check the bearings in the accessories and the idler pulley. Could be one of those singing the song of its people.

I'm confused where you said the pulley can mount six different ways, on this truck the pulley and the balancer are one and the same no? The truck came with this new pulley because the old one had a stripped removal bolt. I put the new one in doing the timing belt and saw that there is only one way it can fit on the crankshaft as the crankshaft has a little "tooth" on it that guides the pulley on. Perhaps I missed something? Or maybe the pulley can be taken apart? Very easily could be a vacuum leak as well, I do notice a loud "whirring/ vacuum whine" near the air box but can't place where its coming from. At any rate, I'm getting 15mpg around the city now! Major improvement from before.