Michael1

You might try a cooling system filter in such cases. Many people are not aware of such devices, but GM actually sells them. They are used by large diesel truck engines, and have heater hose size fittings on them. The filter looks like a PF-30 oil filter (about the same size, but different gasket and filter media). I put one on a old Buick I bought, in which the previous owner never changed his coolant. Talk about rust and scale! The filter worked like a charm, and the cooling system stayed very clean. I believe the filters are available from Napa Auto, and you may be able to get the filter housing there, too (otherwise check with a GM truck dealer).

Michael

Poorsche

There are closed cooling kits without Merc logos for a lot less--and we all know that Merc doesn't manufacture them--they "outsource" to some 3rd party
I'vee seen them for around $400

bob

I have added closed cooling to three different boats which were fresh water boats to begin with and had zero problems. I did flush them a couple times initially with system cleaners but that was about it.

Phazar454Mag

I keep my boat in the water too during the summer season, and when installing new engines I decided to install SuperFlush from http://www.superflushsales.com/
and I am very happy with it. Flushing couldn't be easier, you don't have to run the engines during flushing, very little added weight (compared to closed cooling) and the entire engine is flushed including exhaust risers/elbows.
I believe it is pretty much the same system as the MST Guardian, but I think MST Guardian went out of business based on the info on their web page:
http://www.mstguardian.com/index.html
under August 05 Update.

Phazar454Mag

Sorry. I now realise this is an old thread.

mikebrls

flush....flush.....flush...it's a must in salt water