Planning the next
AO Silver Member
Join Date: May 2007
Location: United Kingdom
This is only my findings from an experiment on one car, so take it as it is intended which is as an unproven theory.
I recently changed the thermostat on my wife's 156. After completing the work, I fired the car up and allowed it to idle. I then monitored the temperatures with an infra-red thermometer. As the engine temperature rose ( measured on the interior heater hoses and other points ) the top hose only increased in temperature a little bit - as you'd expect.
Eventually, the thermostat opened and the temperature of the top hose increased very rapidly to match the heater hoses.
The point of this ramble is : the temperature gauge in the car was exactly on the intermediate mark between 50°C and 90°C when the thermostat opened.
I'd need to check more cars ( I'll check on my GTV when I get a chance ) but I suspect that the intermediate mark isn't 70°C but the temperature at which the thermostat opens and therefore the normal operating temperature. It obviously it isn't going to be perfectly accurate as the V6, JTS and TS have differently rated thermostats and a common temperature gauge but the mark may represent a temperature of around 80+°C rather than 70°C.
If anyone else has a IR thermometer and a known good thermostat a few more data points would confirm or refute this theory.