Slightly low coolant in the bottle will not cause overheating. Pull codes with a quality scan tool that can read BMW specific codes. I have seen codes for “pump not reaching target speed” before complete failure.

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Dave

'05 325xi
'10 E70 35d
'85 911 Carrera
'66 911 Work in progress
'91 Chevy C1500
'95 Jeep Cherokee
‘18 Jeep JLU