dumb mistake on CAT 3208TA HE
Last year I rebuilt my HEs using brand new end caps (or "bonnets" as CAT calls them). The new caps were a different part number than the originals and have thicker flanges for the gasket surfaces. I only found out about the new caps when I tried to order the old part numbers and was informed they had been superseded. I dutifully put the HEs back together with the 3 required gaskets and one required o-ring per my parts manual and ran the boat all last season with no issues. On Sunday, I went out for a little cruise and my starboard engine started spitting coolant out the exhaust system and the exhaust system bypass raw water outlet. I am 90% sure the only way coolant could get into the exhaust system bypass raw water outlet and the exhaust system is through the HE so I concentrated on that. Well, it turns out the new HE end caps I got require two fewer gaskets and two more o-rings than the old design. I'll pick up the new parts today. Fingers crossed that I found the issue, but it sure looks like a smoking gun...
I learned to study the associated parts diagram much more intently whenever a part number has been superseded.