Radiator cap? coolant leak

Originally Posted by Darwins Child

In April we had a pre-purchase inspection done to Z. The one and only thing noted in the inspection was a leaking coolant cap. After we purchased the vehicle, the dealership handed me a new cap under warranty and I replaced it.

Since that inspection we've put on the vehicle only about 600 km. (The Mobil 1 that I put in shortly after purchase isn't even dirty yet.) Because I assumed (yeah, I know) that the coolant leak had been stopped by the new cap, I did not check if the coolant was still leaking. Yesterday, when I checked the oil level, there was the coolant leak staring me in the face. It is shown in the photos below of the area, the sealing side of the cap and the aluminum coolant pipe's cap-opening.

The rubber gasket on the cap, as well as the sealing ring on the aluminum pipe seem fine to me, but, obviously, something is indeed leaking.

This couldn't be an over-pressurization, issue, could it? Coolant temp is normal. Level is still OK in tank.

Is it possible that the machining of the opening on the aluminum pipe is faulty and not allowing the cap's gasket to press down with enough pressure on the aluminum sealing surface? Should I polish (NOT grind) that sealing surface to a mirror finish?

Today I'm taking the vehicle back to Nissan for their assessment. But before I do that, because I doubt if our Z is the one and only Z to have this problem, I thought I'd ask here if other members of the forum have had this same problem and, if so, what the solution was.

Thanks.