So I'm curious what could be the cause, I got my fresh 509 broken in and last weekend I let it rip took it to 5K for maybe a 40sec blast. When I came down to a idle it was steaming from the starboard side exhaust. The engine never went over 140 degrees, the impeller is brand new as the engine including it is a brand new block. The engine water pump is brand new too. It is dry tail risers they mix water with the exhaust at the transom. Port side doesn't do it and after a minute of idling it stops steaming. Water flow out of the tips looks to be fairly equal as well.

I was worried of a bad head gasket on a new engine, it is fully studded and all fasteners are ARP, oil is nice and clean. The sea pump is not new only thing I didn't replace, entire transom assembly is a BRAND NEW HP unit too.