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E46General discussion and technical help for 1999-2004 3 series cars.
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Your Ride: 1994 BMW 525i, 1987 325is, 1986 325
Low coolant issue, leak somewhere in the system, air bubble in the system, bad radiator cap, failing water pump or clogged radiator causing an overheat, failing cooling fans causing an overheat (that's the basic stuff). Or more seriously a warped head/failed head gasket (which is less likely but there's not enough info to go off of in your question).
Big Evil, The BMW driving monster of the American Southwest (Currently on BMW #5)
I would remove the bleeder screw next to the fill cap, fill it up until liquid comes out the bleed screw, (key on, heater full hot setting) and then watch for leaks over the next few days. On an 11 year old car, the first two suspects would be the expansion reservoir (where you add the water) may be brittle and cracked, or the O-ring on the water temperature sensor on the lower radiator hose. That will leak only when the car is hot and there is pressure.
If you have over 150k miles, the water pump may be leaking. That is tough to see until you get all the stuff removed from the front of the engine. By the time you get that much work done, you will want to replace the water pump as preventative maintenance even if it turns out to be still OK.