I agree that 190F isn't overheating. It's the correct temp for a 4-108.
If a 4-108 is really
overheating, as mine did for several years and despite heroic efforts on my part and that of all the mechanics I could find in Tortola, there could be several causes and a couple of good solutions.
After several years of struggling with the beast, I bit the bullet and flew down to Tortola a friend and his wife. Don was the best engine mechanic
I'd ever seen, and I knew he could find the problem. He did. It took him 45 minutes to find three problems, and about an hour to correct all three. After that, he said I should open her up at full power and run for awhile. We powered from Nanny Cay up to Cooper
Island at full speed. No overheating. Voila. End of problems.
I remember a couple of the problems he found. One was the heat exchanger
which, despite having been acid cleaned and tested still had more back pressure than did the new one I'd brought down with me. We replaced it.
A second problem, much more insidious, was the gasketing on the circulation pump. This pump had been removed, replaced with a new one, and pampered by the local mechanics. It has a thin steel
plate which fits between it and the engine block. Problem is, the gaskets which fit on each side of that steel
plate are not identical.
The local mechanic(s) had fitted identical gaskets, thereby blocking a water port.
A third problem was a small leak in a water line on the "blind side" of the engine, near the oil cooler.
I remember, too, that I hadn't been very good about maintaining a 50/50 mix of engine coolant, in part because I seemed always to be adding water. The proper coolant mix is important both for cooling
and for protecting the water jackets inside the engine from rusting.
Some years later, a hint of the former overheating problem returned and I was then convinced that the heat exchanger on the 4-108 is of only marginal capacity. So, I replaced mine with a larger one from a Perkins
4-236 as I remember. With a little ingenuity and a lot of swearing, I got it to fit more or less in the same place as the original, smaller one.
That was in May of 2000 while the boat was still in the Caribbean
. Now, many thousands of sea, coastal, and inland miles later, that heat exchanger is still doing a wonderful job, and the engine runs exactly at its design temperature no matter how I load it. Touch wood.