"Raw water pump" do you think of the pump on the sterndrive and "circulation pump" the one on the engine or are there only one? (sorry if its is a stupid question)I will start the engine tonight and take some pictures of the gauges and feel the riser temp.
Not a stupid question at all........
You have 2 water pumps. One is on the back of the drive and is called the raw water pump. It pumps water from the "lake" (It's also known as a "Sea Water Pump")
The other pump is the Circulating Pump. It's job is to (only) circulate water through the engine block and heads.
That water is regulated as needed by a thermostat which allows a small amount of water coming from the raw water circuit to be "mixed" in a proportion controlled by the thermostat..
When you start the engine and it's cold, the thermostat is usually closed and little or no water from the raw water circuit is added and mixed with the water circulating in the block.
The Raw Water pump on the other hand is pumping water from the drive up to the engine and whatever doesn't get mixed ends up going directly to the exhaust manifold(s) and then goes overboard (and is mixed) with the exhaust out the riser.
The Raw water pump is actually a multi-vane (positive displacement type) rubber impeller pump.
http://www.sterndrive.info/id207.html
Here's a pretty good video outlining how to replace one.
https://www.youtube.com/watch?v=pH6dNUXKJ6o
Because you have a cast iron engine, manifold and riser, and are operating in salt water, there will be a point where the rust in the raw water passages will become so thick as to clog those passages (eventually completely) When that happens the engine will start to overheat at lower and lower power settings until the engine will overheat at even idle. You would replace the manifold and riser maybe a few times before that would happen.
The same thing will happen with the engine block. When the block cooling passages become clogged, the engine must be replaced.
Since the circulating pump is also cast iron, it also can become clogged with rust.
Flushing the engine with clean fresh water after every use (assuming you're not leaving it in the water all the time) and using Neutra-Salt or other salt mitigation compounds will prolong the life of the engine, manifold and riser but they do not stop the corrosion.
If your manifold and/or riser is plugged with rust, it could be the cause of an over heat.
However, if your raw water pump hasn't been inspected or replaced in the last year, I would look there first.
Regards,
Rick
