I had some time after dinner and played with the X5 some more.

I removed the water pump, thermostat and upper/lower radiator hoses.

Some observations:

The car has been parked and not run for at least 24 hours. When I removed the expansion tank cap, coolant gushed out as if it was under pressure.

The radiator is new. So is the expansion tank (date sticker says it was manufactured in February 2010).

The fan clutch is also new, but stamped "Made in China".

The water pump is newer than the engine, but doesn't look "new-new".

The thermostat and hoses do not look new.

It's apparent that somebody was looking for a cause for it running hot. But did they find it? That is the question!

I have a water pump, thermostat and expansion tank that came from another 3 liter M54 and are known to be good. They were replaced as a preventive measure, but were working properly at the time. I'll stick those in and fill it with BMW coolant, run it some and see how it does. I have new parts coming, but this will get me started until they arrive.

Does anybody have an extra water pump pulley they'll sell cheap? The one I pulled out has a chunk missing out of it.

No. I had already topped it off, but it was blowing past the cap on the expansion tank. If not topped again, I would imagine that it would quickly become low.

Question: Does the E53 use the coolant level sensor in the expansion tank? I ask because mine is not plugged in, but there's no warning light in the cluster. I can't find a connector anywhere in the area of the expansion tank that would plug into the sensor.