The only way for your coolant quantity to increase is for oil (ATF or engine) to be leaking into your coolant. This can occur via a bad radiator (ATF), bad head gasket, cylinder head, etc. for engine oil.

Check your engine oil and transmission fluid levels to see if one or both are decreasing as your coolant level is increasing. Also check coolant reservoir cap. There should be some yellowish, foamy stuff on the inside of the cap if you have oil contaminated coolant.

Also note that the coolant will expand significantly as your engine warms up. Sometimes this is mistaken for increasing coolant quantity.

Hope this helps.

Hatch

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1980 240d 4sp
ASE Master Auto Technician
ASE Advanced Engine Performance