Coolant Blow Out...

Jeffrey - next step. You may have to run an earth wire between the cap and earth as these corrosion spots are usually generated by galvanic or electrolytic corrosion. Essentially 2 differing metals in an electrolyte (the coolant). Incidentally that earth strap should also connect your very expensive tank to earth as well. And that earth strap will also prevent the cap from coming off (unlikely) in flight and finding its way into your prop. ( I fly pusher gyros)

You can actually test this by placing a very sensitive meter (millivolts) between your cap and the crank case earth. The motor doesn't have to be running and you can disconnect the battery but you most probably will be able to read a voltage depending on the age of the coolant and your aircraft plumbing.

Regards.....Chook.

Wayne - Interesting.  I'll get my multi-meter out and do some testing.  And I guess I have more to learn about galvanic corrosion.  Since the expansion tank sits on a rubber pad and all other connections are to hoses it would make sense that voltage generated between the tank and the cap would have no place else to go.  Have our friends at Rotax mentioned anything about this?  During my troubleshooting process, I reviewed the cooling system description from the Installation Manual to make sure my understanding was complete. In a past life, I was an ASE certified auto mechanic but wanted to make sure Rotax didn't have any different thinking on the operation of the cooling system.    And I care for several Searey seaplanes so I follow your thinking on keeping things out of the prop.