Sounds like the regulator in the alt is wonky. Im sure they will let you swap. Drain that new coolant back into a clean bucket, thats the fun part.

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740iLDUKE
01' DINAN 7 S62 Swap - Stratus Grau
(1587 produced in SG 440)
02' 4.6is DINAN X5 S2 Supercharged
833/3258 produced | 1/27 Dinan X5's