Technical What's my temperature gauge actually trying to tell me?

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Technical What's my temperature gauge actually trying to tell me?

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On the hard shoulder...
Queuing to drive into Stratford-upon-Avon, this morning, I noticed that my temperature gauge started climbing a little (as you would expect) -- although the fan switched on, as it should, and brought it back down: but not quite to the halfway mark, until I started moving again. So, being the nerd I am, I wondered how this related to actual coolant temperatures.... :rolleyes:

I have one of these permanently plugged into my Punto's OBD socket: so switched the app on my iPhone onto live data -- and this is what I found... :cool:

  • According to the owner handbook, the gauge starts to move once the coolant reaches 50°C. I couldn't test this, as my engine was already warm.
  • Normally -- e.g. when you're travelling at between 30-50 mph (even in today's 25°C temperatures); and when the gauge is rock steady at the halfway point -- the coolant temperature is between 85 and 89°C (and usually at 87°C): which concurs with the thermostat specs in the Haynes manual.
  • If the temperature rises -- e.g. because you're stationary, or moving slowly -- the temperature gauge will still stay at the halfway mark until 100°C.
  • Once it starts crawling upwards, it's out of all proportion to the actual temperature: e.g. at three-quarters (which is the maximum it reached), it was only 103°C. However, if the coolant is going to boil, it's not going to be actually doing any cooling. [According to Wikipedia: "With proper antifreeze (sic), a wide temperature range can be tolerated by the engine coolant, such as -37°C to 129°C for 50% (by volume) propylene glycol diluted with water and a 15 psi pressurized coolant system."]
  • I can't find a maximum figure -- or specification -- for when the temperature warning light comes on: but my belief has always been that this is actually a 'head gasket replacement light'; and that, once it lights up, you're frelled.... [If someone actually has a maximum figure, to complete the picture, I would be extremely grateful.]

Hopefully, this is of interest to someone other than me.... :nutter:
 
I suppose the guages only real job is to stay in the middle so probably not too fussed about being accurate or linear.
http://www.electronics-cooling.com/2009/02/antifreeze-coolants/
Now that I have discovered games on the computer I am considering watercooling it :)
According to this EGW is a fair bit harder to pump than PGW although all get easier with rising temp.
50:50 EGW has 10% less heat capacity than water at operating temp which presumably is a good thing because it will be at a higher temperature for the same heat input and so lose it faster out the rad?
Lower concentrations have better thermal conductivity which presumably is also a good thing in collecting and shedding the heat faster.
 
If someone actually has a maximum figure, to complete the picture, I would be extremely grateful.

Warning light should activate at 120oC +

As you say the coolant wont boil at 100oC like normal water in open air due to the fact its under pressure. This works in a similar way to a pressure cooker, the boiling point increases.
As you say the anti-freeze itself also increases the boiling point and lowers the freezing point.


Alan
 
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