The most common reason this happens is a blown hose.

If the hoses are fine, the next most common thing is a pinhole leak during a trip that causes low water and a hot spot in the engine. Then it suddenly turns to steam and blows the water all out the relief valve.

The easiest way to find a non apparent leak is to get UV dye and a light (AutoZone). It works best looking after dark.

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2005 X5 4.4i SterlingGrau, Sport Package, DSP
Build 3/12/2005