I never let it "overheat" - as I either pull over or I turn on the heater to dump heat. By time I hit the driveway its 1-2 bars over normal. If I don't dump heat it almost gets the the final bar. Like I mentioned, the mechanic monitored the OBDII to verify the temp was rising but as another mentioned, the OBDII signal may be the same that goes to the gauge so I will try to replace sensor since its easy and cheap.

A note...it always smells faintly like coolant after driving it. I excused it for a while given the whole engine bay was blasted when the T originally broke and will probably continue to smell everytime the engine bay gets hot. I haven't noticed any coolant loss nor leaks nor bubbles in the overflow lately.

It IS the same signal, not MAY... I don't care what your mechanic says. If your sensor were exhibiting a slight upwards zero shift that would explain your symptoms perfectly, and wouldn't throw any codes either. Again, it's a 15 year old part which was just exposed to an engine overheat (known to kill temp sensors) and soaked in hot coolant which is both conductive and corrosive (known to kill anything).

That sensor should have been the first thing to be replaced given the location of the coolant leak and the fact you're suddenly overheating despite good coolant flow, a brand new thermostat, and no leaks. Your mechanic(s) including the dealership simply don't know the cooling system on this car well enough to troubleshoot it properly, and this is such a rare part to fail it doesn't even show up in the FSM troubleshooting guide for overheating - you have to use your own brain which clearly your mechanics aren't doing.

If you want to be 150% sure it's not the head gasket, do a compression test and have the coolant tested for hydrocarbons and the oil tested for coolant, should run you about $50 for both at Blackstone.... If there was a leak at any point one or the other would have been contaminated. Shops don't recommend doing this because they don't profit off it.

The ECM looks for the resistance change in the sensor and then sends it to the gauge.

You can test the sensor itself by taking it out and putting it in hot/cold water and room temperature. It should than have some different resistance values these are referenced from the AllDataDIY, but probably in the FSM as well.

-10C/14F: 7-11.4 KOhms
20C/68F: 2.1-2.9 KOhms
40C/122F: .68-1 KOhms
90C/194F: .236-.26 KOhms

The water pump is driven by the chain, so unless it's somehow rotted away...I don't think it's that. When my water pump went the bearings were shot and it sounded like a banshee. However, changing the water pump on these is a PITA and the engine can turn over when you do it. It happened to myself and a few other people on here. If that happens then the safest thing to do is rip off the front timing cover and re-align everything back up.

I'm wondering if you have a partial blockage in the radiator or somewhere else in the cooling system.

^^Don't bother troubleshooting the sensor, it isn't worth the time and these things can fail intermittently and test okay but actually be bad. I agree that if the coolant sensor isn't the problem it sounds like a restriction in the flow somewhere, but that's not going to happen unless you haven't changed the coolant for 10 years and/or you've been running straight water.

When I was first removing the hoses on the "T" it was all broken in pieces and at the time I wondered if one of the plastic pieces got into the system and was clogging something. Mechanic says its all possible but unlikely. I think he was talking radiator....I wonder if there is some other thin piping elsewhere in the system?

I also thought about the fact that its only up a STEEP hill....in which the tranny has to work harder...could the tranny be increasing the coolant temps?

No, the trans is unrelated to the coolant temperature, and the plastic pieces would have gone into the heater core due to the direction of flow which would not produce your symptoms.

What happens if you drive it like a racecar in manual mode, redlining every gear and cruising at 4000 RPM at 80 MPH? That's definately more stress on the cooling system than going up a hill at 40.
When my coolant temp sensor went out I could do this just fine but it would randomly start "overheating" on flat ground cruising at the speed limit at 1500 rpms.

Have you replaced the temp sensor yet? I'm dying to know... lol

Correct it generates more heat however it does not generate more pressure. Depending on how many miles are on the engine if you're close to about a 100 k I would plan on just replacing the thermostat anyway as preventive maintenance, also as suggested the temperature sensor is it fairly inexpensive part that can go out so you may as well replace it as well. While you have the system drained down it's simple to pull the thermostat use an infrared temperature gun or a turkey thermometer or something and set your water temperature at about 190 degrees on the stove, toss in the thermostat and it should open.

If it still overheats replace the radiator since you have eliminated the rest of the potential problems.

There's only so much you can ponder before you need to actually start replacing more components. Pressure test the cap if you have not already that is one of those things that can cause issues while under load.

^^
And use the proper coolant as specified in the FSM.
Then pressure test.

Telcoman

Got a socket wrench on it, but no room to turn, can barely get my hand down in there and the heater hose going into the firewall is directly in the way for putting socket in/out. I will see what my mechanic charges to swap it out. More hassle and risk of breaking stuff than its worth for me to do. Who's brilliant idea to locate it there? Ugh.