I have an 1800 k series with a recently fitted Radtec radiator. There is now slightly strange cooling issue.
If I start it let it warm up, it gets hot as normal up to about 80 deg on temp gauge, thermostat opens normally and water circulates to cool. Thermostat then opens and car cools. All pipes get hot. Then thermostat closes.
Then temperature climbs to just under 100 deg but no higher. However if I rev it, thermostat opens and cools to about 80 deg but then temp raises again to just under 100 deg.
The problem seems to be that the thermostat does not open unless I rev it but having said that it opens fine the first time on the heating cycle.