Mercruiser Engine Cooling System.

[Thepipdoc]

Someone once said "Ask a question and feel like a fool for five minute, or don't ask the question and be a fool for the rest of your life"!
So here goes..........!
I have a twin engined Mercruiser 4.3l petrol boat and try as I might, I can't find the engine water cooling header tank. Is this because the engines are raw water cooled?
Can I also ask, if it is only cooled by raw water I pressume there is a stop cock somewhere. Where is this likely to be.
I've checked the manuals I have and whislt they provide basic information they don't give specific info regarding the engine set-up.
Any help appreciated.

 

[Cliveshep]

Generally water comes into the boat via a fitting in the hull and through a sea-cock (tap?) and into the engine. So - look for hoses that end at the hull as that will be where the sea cock is. At the other end it will go generally through a raw water pump, and thence to the engine. If you have a large water jacket around the manifold (heat exchanger that might have a rad cap on the top) it will be indirect cooled, but if the water goes into the engine itself and exits via another pipe into the exhaust it is likely to be direct or raw water cooled.

Having said that some units have water pick up and exhaust in the drive leg, I do not know for sure about Mercruiser units what system they employ.

 

[enterprise]

Hi there, yes your engines as you suspect are raw water cooled, the water is pumped up from the leg through the transom shield and into the cooling jacket which surrounds the engine, it will also cool the exhaust, before being ejected back through the transom/exhaust via the exhaust injection bend.

There is no seacock, as long as the engines are not running the water pump will not pump water, if you look in your manual the water pump is located in the lower housing on your sterndrive.

Only problem with freshwater cooling as opposed to closed cooling (header tank / heat exchanger) is the problem of corrosion, with closed cooling you can add an inhibiter so your engine doesn't eat itself,with freshwater cooling you cant, you will need to check your manifolds and rissers aprox every 5 years, as they are a known weakpoint,expect to pay around £1200 (aftermarket) for new manifolds and risers for a twin engine setup.

 

[Thepipdoc]

Thanks for both of these answers - they both answer the question perfectly.