Had the same prob on my 02 12r .I checked all the obvious,hoses, clamps,blocked rad+rad cap no joy.Do u have blowback into your reserve tank or a loss of coolant .i did the problem was a bastard to trace .It turned out to be a pin prick hole in the small connection hose from the water pump to the engine .The problem in tracing the fault was when the engine is running & up to temp the coolant was escaping as steam & when the engine was switched of the hose was no longer under pressure ergo no sign of a leak .Fixed the prob with a rad hose cut to fit from a ford .While your at it fit a thormo override switch cheap & useful.PS if u are losing coolant pressure from anywhere in the system u will overheat Ride Safe
Last edited by GILBY; 08-14-2018 at 07:57 AM.