Overheating Ahu Passat radiator cold

[karush]

Hello everybody my 1996 Passat TDI is overheating. I have an aftermarket electronic temperature gauge with its sensor screwed into the cylinder head coolant flange.
After a few minutes of driving it will steadily climb up above 225 Fahrenheit at this point when I check the radiator it is cool to the touch. I suspected a thermostat and went ahead and removed the thermostat and did the hot water boil test on it and it was fine. Started to open at about 90°C was fully open by 95° when cooled it closed again and when heated it would open again so it is not stuck.
I stuck my finger in the thermostat housing while the thermostat was out and checked to see if the impeller was slipping. It was not and the water pump is quite new.
I ran the garden hose into the top radiator hose to check for a plugged radiator and it flowed out the bottom with very little resistance, indicating a free flowing radiator.
. the electric cooling fans turn on as they’re supposed to just fine. they get their signal also from a sensor on the cylinder head coolant flange.
Coolant level is topped off, but interestingly, even though it has overheated according to the gauge, I have not noticed boil over or lost of coolant after the over heating incidents maybe because I stopped each time just before this could occur?
I don’t suspect a head gasket as the head gasket has been replaced quite recently and ran just fine afterwards without any overheating then I let the car sit for two or three years and this issue has now occurred. Also, no mix of oil and coolant or combustion bubbles coming out that I could notice.
could air be in the system somehow that is not purging itself out. I looked on the radiator and there’s no bleed valve that I could see. Is there a trick to getting all the air out of the system? Thanks in advance paolo.

[TDCTDI]

How does the factory gauge compare to the aftermarket one?
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[karush]

The factory gauge is inoperable so I can’t compare, but I’ve tested the electronic one in a pot of boiling water and it seems fairly accurate the fact that the radiators not getting hot iwould indicate some kind of flow issue. All I can think of is that there’s air trapped somewhere .not quite sure how , where. Paolo

[ps2375]

A slipping impeller won't always show itself when cold and by trying to turn it with your finger(s). And if it's got a plastic impeller toss it and install one with a metal impeller. GL

[karush]

Hello, thanks for the replies. I’m trying to get the factory temperature gauge working again. To verify overheating. This is a replacement/swap engine and many of the wires were cut in the process. Including I believe the ones from sensor to temp gauge. Can anyone tell me the color of the wires coming from the temp gauge, through the firewall ,to the sensor. Thanks, Paolo.