We had a BMW 3 series in today for an MoT test.The owner had been concerned as the car had been loosing coolant and therefore overheating of late. Another mechanic had said that he suspected that the head gasket had started to leak. This is a very expensive and time consuming repair to do properly and would probably not make economic sense on an older car (like this car was). Eventually a coolant leak was traced back to the perished seal on the old radiator cap. There were no visible signs of a blown head gasket at the time, so the radiator cap was replaced for a few pounds. Above is a photo of the old radiator cap. If you look closely, you can make out the perished seal (that would leak coolant when under pressure). The customer is going to have to monitor his car’s temperature gauge very closely over the next few days but hopefully that is the end of the matter.