I think Betown's right. WEP just won't work if your temp hits the red. Of course, if you turn your engine off for a bit, you'll cool down really fast. (not recommended while fighting obviously)
Also, if your radiator gets hit and you're running back to base, watch the temp gauge. When it gets in the red, kill your engine (as in totally off, not just throttled back) and glide a bit. When the temp gets out of the yellow, turn the engine back on again. It's kinda gamey, but you can milk your way back home doing this if you don't have the alt to glide her all the way in. Managed a Spit V back home from 25 miles out with only 7000 ft of alt doing this.