Nani Kai, let's review engine cooling
101 and apologies if I am posting
stuff you already know.
TL-DR - gauge only tells you if the coolant is too hot, it doesn't detect cooling defects that can catastrophically impact the exhaust system.
The temperature gauge only measure the coolant
temperature and this is the last thing to change
if a cooling fault develops. The coolant temperature is dependent of the thermostat, the coolant pump and
the raw water
(RW) cooling system. There is a fair amount of hysteresis in the total system, i.e. a delay between an issue and the response of the temperature gauge.
The coolant temperature should never exceed the fully open thermostat setting i.e. even when running at max rpm
continuously, the temperature should remain constant at or below the thermostat rating. If the temperature starts to rise above the thermostat rating, there is an defect somewhere
in the cooling system (coolant or RWC).
If you are using the temperature gauge to detect cooling faults, the following aspects should be noted.
developing issues (e.g. partially blocked heat exchanger
or worn RW pump
etc) will first show up (i.e. temperature rising above thermostat rating)
when operating at max rpm
. At lower power settings, the temperature will remain normal until the defect gets worse (usually it has to get much worse).
developing issues (e.g. blocked RW inlet) will not be detected by the gauge until after other serious problems develop - for instance exhaust system failure (burnt exhaust pipe / melted watercock etc).
So yes, an engine coolant temperature gauge is a handy tool to have and can help to detect slowing trending cooling system faults but won't detect rapidly occurring faults that might destroy the exhaust system.
IMO, the best (and still simple) arrangement is a coolant temperature gauge AND an exhaust temperature alarm.
While there are several types of exhaust temperature alarms, the Vetus one is probably the easiest (but not the cheapest) to install. Of course, an exhaust temperature alarm won't detect all slowly developing cooling faults, you need a gauge for that (