Just replaced water pump, timing belt, and thermostat in my 1987 325is. yet after reassembling the engine it still heats up..fast. there is no visable coolent leak. the car randomly will start to heat up and i have to shut it off before it overheats. Ive flushed the coolent, re-filled. multiple times. yet i start it up, it starts to warm up, them all of a sudden it heats up fast. any suggestions? Ideas?