Montreal Blue 1996 BMW 328is

[Jake]

I'd bet your fan clutch is gone or starting to go. Do that cooling system work ASAP. It stayed cold on 81 because you were going 75 mph and forcing a ton of air through the radiator. As soon as you got off on Port, the constant air supply went away. The fan should help mitigate that (motor was just spinning at 3k for a few hours, it's hot) but if the clutch is not engaging, the fan won't spin. There is a test you can do so try that, but just plan on replacing everything since you already have the parts.