Even a small amount of coolant getting into the combustion chamber will probably erode (or at least discolor) the spark plug over time even if it's not enough coolant to show up in the cylinder on an overnight pressure/leak test. I think a small crack in the cylinder head also will more likely to close back up when cold. If you can put in a tee or adapter fitting so you can pressure test with the cap on then you can test the cap at the same time. I'm planning on putting that in on the E70. Have you had air trapped in the heater core that bubbled through with the heater valve open and the aux pump running?

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)