Deleted post. Have to think about it some more.

TooMuchControl wrote: ↑

Sat Nov 18, 2023 4:47 pm

Deleted post. Have to think about it some more.

Hmmm. you're keeping us in suspense

After all these years, I never knew the A/C would run during the winter, and that's the way it's supposed to work.

I have an odd ball thing where I am seeing the AC compressor kick on, yet I've disabled the A/C button in the truck!
But the vibration I've had made me look and I saw the clutch pull in and run.

I'm trying to determined if the AC should kick in, even when I've commanded it off at the front controls, when in DEFROST mode. When I hook up my scan tool - it does! The AC enables and cycles in DEFROST mode no matter what I do.

I actually pulled the fuse on the AC, as I think I have a poor performing heater core that needs to be descaled. The circulator pump does operate at high heat 90 DegF and High Fan Speed, but the discharge of the heater line is cold until I rev up the engine with the AC fuse pulled. Cold cold cold.

Because the AC is kicking on, and the heater is poor, it's friggin frigid in the car when I'm trying to warm it up!

I have things to check!

Going back to (I think) around '02, it's Federal law that the A/C is supposed to run in any defrost position, to dehumidify the incoming air. It runs at minimum and it shouldn't actually reduce your heat much unless your heater core has a problem or the mix duct isn't sealing properly. Nissan HVAC's are all set up such that air always passes through the A/C evap, but it's then ducted through the core when you're on a heat setting.

PS - The R51 Pathies with rear A/C had a heater pump to compensate for the cores being up so high relative to the mechanical water pump. Automotive pumps generate a lot of flow but not very much head, so when the core is mounted much higher than the pump then heat at idle can become an issue. So if your problem is poor heat at idle or low RPM, it could be a bad heater pump. Even on R51's without rear A/C (as well as D40 Frontiers and N50 Xterras) the front core is pretty high up, so I've added pumps to a number of them over the years for people who just weren't happy with the OE performance. Gates and several other companies make aftermarket versions of the original OE part 92264-EA50B.

Awesome information!

Thank you for the this. I'm doing a system flush to see how it all starts to come out.

I've seemingly always had issues with heat at idle. The pump - I've checked and it does operate and at the right conditions.

I checked the inlet and outlet pipes, and even when the pump is operating the return is cold, Tap on the gas and it starts to warm up. Maybe it is the pump after all?

There has to be a trick in filling and burping this radiator/engine system...

Lots of threads on burping the system. It's not too tricky, just repetitive: park uphill, rev engine at 2-3K RPM with heat on full blast throughout the truck. Top off coolant when cool, rinse repeat a few times until the level stays steady. Helps to have zero leaks though, including the radiator itself at the tank ends, or you will quickly have bubbles return to the high point in the rear heater core.

FWIW, I did a big flush on my system months ago when the rear heater inlet pipe junction self destructed, and besides flushing broken plastic out I got a *lot* more heat. Since I had everything open already I used garden hose pressure on the rear core both ways by itself, then a flushing gun (air plus garden hose) again on the engine itself without the radiator hoses in place. Then I did a chemical descaler, and then flushed many times with distilled water until it was clear.