BMW Repair: 535i 1989 Losing coolant, losing coolant, apparent leak

Question
Hi, I've a 1989 535i BMW and it's losing coolant from not apparent leak or place, everything started went overheating and it blow the radiator which I replaced, it keep on doing it and I replaced the thermostat and the Fan Clush, Everything was OK for a week, but It overheated again, but because it loss the coolant I refill it and it did it again i replaced the cap in the reservoir because I noticed that It was dumping it from the over flood valve, I checked the engine oil and transmission oil looks OK, Any suggestions?

Answer
The problem is probably a head gasket and now having been over heated a couple of times I would have the head checked for flatness and resurfaced if needed.The reason for not being able to see the leak is you are more than likely burning it.You can check the plugs for any abnormal burning.A compression test and cylinder leak down test will help determine how deep you'll have to invest.