Coolant loss- 95 525, M50

[peterllo]

I first posted on this problem this past March. Loss of coolant, frequent top ups, most severe when really cold. Replaced the auxiliary tank orings, problem persisted. The issue was resolved by replacing the radiator which had been replaced twice before once at 79k, then at 122k. The March fix was done at 150k followed by the replacement of the auxiliary pump and hoses. I detected the leak at the the inner lower corner of the radiator where the aluminum clamps to the plastic expansion tank. There was white residue (dried coolant) on the surrounding area and I could see a tiny (pinprick) of wet in the corner. The mechanic said the leak was due to thermal contraction, when it was cold outside the the plastic and aluminum contracted at different rates allowing the leak, once the car warmed up no problem, the expansion of materials closed off the source.

[I]To the present,[/I] low coolant light comes on, the auxiliary tank down to the top of the sensor. Car now has 163k on it. Topped up and everything is fine. No drips on the ground, hose fittings, water pump, firewall or inside. Seems like my old friend the corner leak has returned, residue on the expansion tank, a damp corner. This seems very odd to reoccur twice, likely three times but I did not own the car the first two radiator replacements.

So the question is why? Is it pressure related? What should I check? I have seen posts about checking for an internal head gasket leat by checking the plugs, I thought I would do that. There is a very slight, but steady, seepage of oil on the pass. side of the block just above the motor mount.

Any ideas would be appreciated.