From my experience on bad cores, you usually only see that with a large corroded area, or if the corrosion is on the bottom, or the leak is at the o-ring.
It just seeps into the carpet padding underneath the carpet, the absorbent side is face down so you don't feel it until its very very full.
It can leak from around the o-ring on the outlet, or on the top row of the core(closest to the pass side) and it will suck a bit of air in as the coolant passes through.
I've seen it do that once (sucking air in), so its just a theory and not something I've seen multiple times.

 

There are 2 sensors next to the thermostat housing, both sense temps. one for the dash gauge, one for the computer.

 

Get a cheap IR Temp gun & take readings of the thermostat housing, top hose, and bottom hose. That will give you a good idea of your temp readings and how the coolant is entering/leaving the radiator.

 

Can the torque app read the ECM temp sensor data so I can cross reference it with the gauge sensor?

 

Yes. You will find there are only three readings on the gauge.... cold, normal, overheated. The gauge is not reliable for any practical purpose.

One last item to check, the belt routing.

 

You can check the sensor with a multimeter. I don’t think you even need to remove it. Just take the cover off and test it from there.

I used this link to test my sensor (skip to 3:50 or so):


I found this pic on the forum, you want to test the brown plug. It’s a 19mm if you remove, and some coolant will spill out.

 

I got distracted with an old Volvo that ended up in my driveway, but managed to put a little time in on the Rover today.

I swapped the lines going to the heater core, drove 34 miles to work at 70mph+ and the temps stayed steady at 195*, no waterfall noises behind the dash, so I think this problem might be licked. Thank you everyone for your help!

On to the next one. Now I've got a noticeable lack of power with no codes, and the exhaust has been making some rattling noises, so I suspect one of the cats is plugged up. I'm gonna get some pre and post temps this evening and see if there's anything obviously amiss.