Engine overheat

[JG4_Helofly]

Recently, I flew on a dogfight server and was chased by a spitfire (I flew a 109E1). After several minutes flying at full power at about 2500 rmp, the engine began to shake and finaly died. Rads were fully open. Now the problem is, that water temp. is nearly always the same in any situation from take off to landing. Even after shutting down the engine, water temp. remains unchanged. Oil temp. is never a problem. Is this a bug or something realistic I didn't know about? And how do you know when it's time to throttle down and cool the engine?