How hot is too hot for running temp?

After replacing the radiator and head gaskets etc. I installed a dash display that shows engine temp in degrees C. This hooks up to OBDII under dash.
Today here it was above 100 degrees F and my display shows a brief high of 105 deg. C while stopped in traffic, battery charge also dropped to 12 volts.
Would think fans kicked on to high speed. At cruise temp ranged from 92 to 98, seems a little high to me. Of course the temp gauge on dash is pegged at half way (normal) and does not budge. No coolant overflow and no low coolant light,maybe too much info is a bad thing?
1999 XK8

Using an on line calculator (if I am using it correctly) shows that at sea level a radiator cap of 21PSI increases the BP of water to 109.xx deg C
This does not take into account the effect of antifreeze/corrosion inhibitor.
I am sure someone with more knowledge of the subject can clarify.


That doesn’t sound right. I understand a 15lb pressure cap puts the boiling point higher than 120 degrees C. My Honda Fit starts flashing it’s water temperature warning light around 117C. 105C is nothing to worry about in my view…

In Florida with the XK8, it’s not uncommon to see the temp go up to 210F(98c) for me.

At an OAT of 110 here in Vegas, with the AC running, I can see temps approaching 217-220 F , especially if I am going uphill for long stretches. Once I reach level ground, I see the temps drop back near 200.

I’ve just been repairing a fuel injected motorcycle and looked at the cooling specifications in its ROM - the fan is set to come on at 105’c, so I’d judge what you are seeing not to be unusual.

If you look at some similar threads on thermostats which I have posted to recently, you’ll see the engine temperature move about by 6-7’c when the thermsostat is cycling in a stationary car. Given most fuel injected cars now run 88’c 'stats, it means seeing 85-92’c would be absolutely normal.

In some respects, you don’t have control of the temperature - it is simply a reflection of the equilibrium point when heat being generated is balanced by heat being dissipated. Given that the thermostat is moving relatively slowly (compared to change in engine revs) and it takes time for the water to come back around AND it doesn’t know how much water is flowing anyway, it is unsurprising that temperatures move about more than that 6-7’c in that ideal scenario. What is important, is that the temperature drops quickly, once conditions allow it to.

kind regards

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If you are worried about boiling/or overflow due to exceeding the pressure, then you have to be well over 105 deg C to see that happen. (with water) I don’t know what the effect of antifreeze is but allegedly increases the boiling point over that of water.
FYI 15 psi - 121deg C. (Typical steam sterilizing autoclave pressure/temp) So a 15lb (per square inch) cap will substantially increase the boiling point.

Here is a handy calculator:

Note: State 1 is atmospheric pressure in psi 14.696 and 212 F or 100 C. Therefore in status 2 you need to add 15 psi to 14.696 = 29.498 and it will calculate the temp in units of your choice - 15psi ~ 121C or 250F
12 psi = 244F and 6 psi = 235F