QEver since a service, including a pressure check, the expansion tank on my Vetus 417 engine has been leaking. The expansion tank overflowed so I got a new one, but after about four hours’ use, the new tank started overflowing. Any ideas?
On the face of it, a service plus a cooling system pressure check would not cause this, so I think there is something more going on. I suspect either the coolant was changed so new antifreeze could be added, or the pressure check found a leak and blew coolant out of the engine. In either case, I suspect the cooling system was not properly bled free of air so now air is trapped in it.
This is the most likely explanation because you make no mention of overheating. If it overheats as the coolant is ejected, check the hoses in case a clamp was left on one of them.
Air expands far more than water when heated so it blows coolant out of the pressure cap. Try manipulating the large cooling pipes so any air trapped in bends can make its way to the engine or skin tanks and then try bleeding the skin tank, which usually has bleed plugs/screws at the top.
Finally, it could be a symptom of a failed head gasket or internal engine cracks – but eliminate all chances of it being trapped air before thinking the worst.