My understanding is that the temperature of an engine is maintained in a steady state by the opening and closing of the thermostat to either restrict or increase coolant flow as required. Am I right so far?

The radiator mounted thermo-switch will operate the fan when the temperature rises as a result of insufficient airflow eg when moving slowly in traffic.

I'm using OE spec thermostat and fan switches but they have quite disparate temperatures. The thermostat opens at 82 degrees and the thermo switch puts the fan onto half speed at 97 degrees and doesn't initiate full speed until 101degrees.

I would have expected given the thermostat temperature, the design temperature of the engine would be in the 85 degree region and that to maintain this temeprature for the fan to cut in at about 90. I cannot understand why the manufacturers want to let the temperature rise considerably higher before the fan cuts in.

I'd appreciate if someone could enlghten me on this. Thank you

An 82c thermostat is a very cold thermostat by any modern standard --- however 82c is only the temperature at which the thermostat starts to open, it will only be fully open about 14 degrees higher eg 96c

Most car thermostats theses days start to open at 88 to 92c.

Coolant will only boil above 120

Fans cut in from 93c upwards (more often 97 these days) with 2nd and speed fans only cutting upwards of 102c and 3 speed fans 108c.

The reasons for the higher design running temperatures of modern engines is connected with fuel efficiency and emmisions.

britishtrident, many thanks for explaining that. I hadn't realised the temperature spread over which the themostat opens.