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You might expect the head to be at a bit higher temperature than the thermostat housing, but not 30 degrees hotter!   What coolant are you using water/antifreeze or waterless  - and are there any indications of overheating that made you do the measurement?

(infrared thermometers can give odd readings depending on the surface being measured, but read low rather than high)

 

Edited by RobH
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Hi Rob, thanks for the reply. The temp gauge is reading hot but I suspect it maybe the volt stabiliser and I have ordered a new solid state one, the car doesn't boil over and the rad cap is 7lbs but I have kept my eye on the cooling because of the temp gauge hence the check and it has only just been rebuilt.

What should the temp difference be between the head and the thermostat housing ?

Regards

Ron

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17 minutes ago, ronhatch said:

What should the temp difference be between the head and the thermostat housing ?

 

Good question Ron - I doubt there is a 'correct' answer as there will be some variation depending on a number of factors such as how it has been run and whether it has stood idling for a while. Your car might be expected to run a slightly hotter if the engine has just been rebuilt, because it is probably still a bit tight. 

 Water under 21.7 lb pressure (14.7 atmospheric + 7 for the rad cap) will boil at 111 degrees C so if your cylinder head gets to 112  it's getting remarkably close (given there will be some temperature drop from head to coolant so the water will be a few degrees below that.)   The thermostat housing is so close to the head it is difficult to believe it will be 30 degrees cooler, but I've never measured it. 

Next time I have my car out  -probably tomorrow -  I will do that and let you know.  

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My advice is

to forget measuring temperature with a infra red gun when you don't know the emission rate of the surface,

and forget measuring if you can't adjust the emission rate on the gun (standard adjustment is 90%),

also if you cannot hold the gun 90 degree to the surface,

and forget IR measuring temperature at all on metal surfaces because of their EXTREMELY different emission rate.

Ciao, Marco

 

Edited by Z320
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OK - just back from a good country-road thrash so the car is fully warm. Measurement taken with the engine still idling about a minute after coming to a halt.

Thermostat housing was at 84 degrees C and the cylinder head was 94 degrees C, measured using a thermocouple meter. After the engine was stopped the heat-soak raised the temperatures to 94 and 101 respectively. 

I tried using an infra-red thermometer also for a comparison but the readings were so erratic that I could place no reliance on them. 

Edited by RobH
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The temperature gauge was reading 185 degrees F  (85C) when idling  Andy,  and rose towards 200F (94C) when the engine was stopped.  That is remarkably good I think as the temp gauge is hardly a precision unit. 

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Thanks Rob for your stalwart efforts, I was probably over thinking the job, if the thermostat is reading ok and the engine is not boiling over it should be fine. I was probably just a bit anxious after the rebuild, I've adjusted the fan to cut in at a lower temp and will adjust as needed. Thanks to all for your input.

Regards

Ron

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There is an element of the water temperature rising just after switching off - the remaining heat from combustion will still pass from the hotter spots in the head and block to water jacket.  However if the water pump isn't flowing the water local to the hot metal will heat up and with it the water temp. There will be some thermal syphoning and limited cooling by the rad, but in short the engine water temps will be hotter for a little while after switching off.

Modern cars tend to have their electric fan running for a while (after stopping) to cool the water what may circulate into the rad and if they have an electric water pump it will often be set to run for a while too so that localised hot spots don't develop.

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