Thread: mercury 312 3 speed manual cooling issue

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Don't see a problem, the "P" is just a bit over halfway, maybe 190? If it never goes any higher, just drive it. As long as it never blows coolant, it's OK.
For peace of mind, there are lots of rad flush compounds available.

I'm of the age that I had done much mechanic work (including racing) with the 312cid engines - and sludge build up in the rad was common. I do recall changing several sets of head gaskets from water seepage - and usually what you end up with is milky/cloudy looking oil and sludge. It isn't usually really bad enough to notice except for those two indicators. When you finally pull the heads, you usually only see a dark band where water and/or exhaust gases have passed - and those that I saw, it usually, and it might only be coincidence, wa between cyls 7 and 8 (left side rear)

Oh yeah - these engines have tender main webs, 5K rpm and higher