The engine temperature gauge on my 230 E usually fluctuates between 80 and 40. depending on acceleration. The faster i accelerate the more it drops downwards towards 40. Also when i am cruising at 140 kmph on the highway , the temp is usually at 40 or 50. Is this OK ?. If not what could be the problem??? I believe the temp should be actually 80 for the engine to operate optimal. Please suggest !!!.