Brian, Despite your pronouncement, pressure in the cooling system accompanied by NO coolant/oil loss or cross contamination happens on some (not most) OM61x and OM60x engines and is of NO consequence. I have owned both 61x and 60x engines that intermittantly did this and neither ever exibited any additional symptoms or caused any problem. Some of the Mercedes gasoline engines do it too (I've owned several that did as well) and I suspect it's a radiator cap failure.

If there is coolant loss, then there is a problem. The most usual cause IS a failed head gasket or cracked head. A blown head gasket is a rare problem on OM61x engines (much more common on 60x engines), but it does happen. Cracked heads are even less common on 61x engines.

Marshall