Whilst out on a road trip this today, I noticed the water temperature was getting really high. I had to pull over in the end and let the car cool down. Initial thoughts are that the thermostat has jammed shut, as I could see no other problems. Both fans were running fine and we replaced the water pump not long ago. We had to stop several more times on the way home to let Yoko cool down.
After further inspection, I've decided to replace the thermostat, radiator, fan housing and fans in one go, whilst I'm draining the coolant system.
The radiator in Yoko is the original one and has been well battered by stones over the years. Today, I bit the bullet and ordered a Mishimoto performance radiator.
Whilst I am at it, I might as well replace the shroud and both fans.
My new thermostat arrived today.