This particular engine runs a 17 Amp fan, and a 7.5 Amp water pump. PWM speed control of these components looks ideal.
Do you have 2 free FET outputs (almost certainly) that you can parallel to drive that safely? (the econoseal pin is also for 10A only at higher temps of its range)
Maybe it should increase pump flow rate based on engine coolant temp measured at the heads, and increase fan speed based on coolant temp drop across the radiator. There may have to be some fuzzy logic or something to adapt to the different thermal transfer rates of various radiators and engines, and the flow rates of various pumps.
The coolant systems have such a high thermal mass, and low precision requirements (who cares if temp changes +-1 C?), that any refined control has only marginal benefits. The simple radiator bypass valve and fan on/off is quite efficient and works well but if you enjoy playing with it...
I'd definitely put energy into BoostController instead, where there is a lot of places for improvement (handling, TPS dependence, rampup and peak, changing settings easily etc...) .
Some notes about cooling from IRC discussion:
''Jorgen: On the Audi I have a cap that doesn't seem to let anything in or out. The hoses get hard as steel tubes when the car is at working temp and if you open the cap up then you better run as the car will instantly start to boil locally
mcel: mine rarely reaches 100C
Jorgen: Locally the heads have hot spots that are much hotter then the temp the gauge shows. They rely on the pressure to avoid a steam pocket to build. If the pressure is removed the steampocket will prevent cooling of that spot and that will usually lead to detonation.
Jorgen: As an example a friends Honda's cap must be fitted when the engine is cool, otherwise it won't build enough pressure and it will detonate. It's beyond me why the local boiling spots don't increase system pressure to prevent it from boiling. But if the cap is fitted when the engine is hot the engine will detonate.
Jorgen: The pump usually increase the local pressure in the engine even more, there is a few schools to how to build a cooling system. Some insist on running it unrestricted and try to flush away the heat while only relying on the static pressure to prevent hot spots. Some use restrictions to let the pump build pressure in the engine, that can allow areas of the head to reach around 150c without creating a steam pocket. The lower flow can make the car run a few degrees hotter but the maximum allowable temperatue is also much higher.