Hi,

Maybe I'm missing something here!
That voltage up to 4.2 is totally a wrong reading at the ECU pin 13.
It seems that you are checking the ECT connector out at the engine?
That would be applied voltage coming to the ECT from the ECU.
The chart is the readings with the resistance of the ECU applied "in circuit" across the pin #13 to ground.
Doing it this way you are checking everything. The ECT and all the wiring too and from the engine.

Are you sitting at the back of the ECU with the rear cover off?
You need to back probe the terminal block @ pin #13 while it's all connected up.
According to the chart or graph, that Art provided, it's saying the temperature is zero degrees or less.

In normal "sweat shirt" temperatures of 50 to 75 degrees the voltage is meant to be in the upper 2.0 volts range.
A lower reading will happen if the engine is warmer.
With the engine up to operating temperature (92C to 87C) it should be .5 V to .3 volts.
With the key on or even the engine running, the red lead of the Voltage meter, is put in the socket of the proper color of the #13 wire and the other lead to a good ground point on the body of the car or ECU chassis.

You either have the sensor disconnected from the sensor while reading at the ECU or that sensor is bad or wiring no matter how much you clean the connection pins.

When you are talking about grounding of the ECT in the head, you do not want to use any kind of thread sealant, as that will insulate the ECT from having a good ground!
The threads should be of taper lock design and should not leak.
There is a "slight" misnomer about thread sealants tapes (Teflon) and compounds when it comes to these type of threads.
It does not seal them but lubricants the threads so they will tighten in easier or make a smoother engagement.
Especially, if the two materials of the mating are the same or alike to prevent galding!
The threads need to be made with good sharp taps and dies so no tearing of the thread lands or faces occur.
If the depths of the two threads are made correctly on each part, they should work dry as their design name implies, "dry seal threads" and tighten appropriately.

The British actually dealt with this in the early years of modern aircraft design. Probably around WWI or II. Or as I suspect the advent of jet engine racing.
They called it a "gas" thread! BSPT or BS or BSPP for straight pipe threads.
Our NPT system is a copy, even though, I have not research this!
I taught it or made statements, in my classes, as we probably "stole the ideas" by changing it slightly somewhere along during the war lines.
It helped keep the students more awake "thinking" that "we" could do such a thing! (:-)
Edit: I found this out of curiosity and running my mouth, again!

https://www.ralstoninst.com/news/story/the-difference-between-npt-bspp-and-bspt-seals


Phil