Yes way, the heat transfer function of the radiator only works when air flows over the fins. I did mentioned when engine is idling meant car is stop which there's no air flow over the fins and if engine is already at operating temp you will hardly can tell the different between the inlet and outlet, at least this is how both of my E39 cars act.
If you have air trapped in the thermostat housing, air doesn't conduct well to temperature it faked your thermostat not to open widely restricted the flow, the lack of flow caused temperature to rise.
Yes, correct the heat transfer works when air passes through the fins over the radiator, that's why we have the vc fan constantly spining even at idle. And that's why the VC fan will spin faster when things gets hot and the Aux fan comes into play above 91C to move the air for heat transfer. And as the coolant travel in radiator, the fin keeps on robbing out the heat for it so more or less, the exit point of the radiator will be cooler. If you mount 2 temp sensor on both end of the radiator, for sure you will see the different in temp.
I bet it is hard to tell the difference if the car being warm at idle, the T-stat opening and closing also play it's part at affecting the temp shows on the radiator as well. If you let the car warm up to operating temp, it will reach right up to the temp of the t-stat opens. Most likely it is partially open and have a slow coolant flow to the radiator and closed the coolant went below the t-stat opening temp.
If you have the car drive around and reach up to the temp where the aux fan start to kicks in, you will feel the different in temp between both side of the radiator, as that's what the radiator suppose to do, takes in the hot coolant and cool it off as it leave the radiator.
If you have air trapped in the thermostat housing, air doesn't conduct well to temperature it faked your thermostat not to open widely restricted the flow, the lack of flow caused temperature to rise
Nothing wrong with using 100% water if you live in a climate that ambient temp never dipped below freezing point, as a matter of fact water has better heat transfer then any antifreeze cooling.
What concerned me the most is are you loosing cooling now? If you do your head gasket is busted. I hope this is not the case.
Try to bleed again but this time using cold bleeding method first.
I agreed, water is the best liquid for heat transfer, but the pressure it created, and being evaporated is annoying. I used both the bleed screw on the t-stat housing and the radiator, and had the coolant level stay the same for a week, so I rule out the air part.
The Aux fan is not working, so I guess that should explain the cooling system is not working properly. And the VC fan is a bit questionable now..... After switching from water to coolant, at the same temp, the hose is not pressurized so I am not having exhaust gas in the cooling system. For the save side, I will do a compression test on the cylinder over this weekend to make sure the gasket is ok.
Geeeeeee, I thought my Discovey was painful for the cooling system...........