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All of which means that the engine is running at the thermostat temp, Ian - confirming thermostat function, as it should…
You also need to measure the temp at the sensor - which is what the temp gauge is getting. The sensor just varies its resistance and indeed checking its resistance against another sensor as you propose - and swapping the two sensor may show something. The sensor and gauge fitted must be a matched pair - and two versions of both are available…
All ‘our’ gauges use ‘straight up’ to show 90C - about 11 o’clock at 80, so your needle in the ‘white’ section means that the sensor is mismatched to the gauge, or faulty. It’s all about instruments; the engine is running at the proper temp - all xk engines are created equal…
these IR thermometers are just great, aren’t they? Your engine couldn’t to any better than what you measured. Now leave that t-stat alone and take care of the sensor and/or the gauge.
I was fooled twice by a slowly failing sensor and I have made use of two otherwise identical (fuel) guages with a considerable difference in reading. Change the sensor and I bet you’ll be back in the game. The variance in guage reading is immaterial given you use the guage primarily to detect changes.
One way to check is to Ebay a $25 Digital water temp Gauge & Sender, you will likely need a thread adaptor for a few $.
I have done this to verify factory gauge.
You CANNOT tee a water temp sender, it MUST be fitted with its body immersed same place as original, or it will not be accurate
I may do this for my 420G, which has NORMAL, runs between R and L, even in ambient temps above 30C, but I would still like to know what those represent in actual degrees C
The one I was using suddenly became innacurate by about 15C, so I have fitted it against the engine block (of another vehicle) that has a red area of 112- 117C, as it has a red light that comes on at 100C (so actual 115C on engine block would activate warning light)
I also found this a bitch of a job, but only because I didn’t realise that the housing itself had a tiny crack through which water was leaking. After the fifth attempt to stop the leak with new gaskets and different sorts of sealant I finally thought to use a mirror to look underneath and see where the leak was coming from. I was lucky enough to be able to source a second-hand replacement housing at a very reasonable price from Ken Jenkins in Worksop (UK) which is a superb place. Fitted that and everything worked again. Parts like this are now getting hard to find, so I feel I was very lucky to find one. Moral of the story - metal can crack and leak, even though you might not expect it!