welcome to the hassles of winterizing raw water cooled I/Os! While fresh water boaters might not see the need for a simple half closed cooling system, I bet you'll agree after doing this a few times that you'd be better off if that were standard equipment because it makes the engine much easier to winterize, in addition to extending engine life. What you should do is post up some pix of your engine on both sides so we can see where the drain plugs are. The block drains can be difficult to get at, because they are down low on the block. I have a small adjustable wrench that I use to loosen the metal drain plugs in mine. On the starboard side it is right in front of the starter, on the port side it is behind the dipstick tube that goes down to the oil pan. I've done this at least 15 times over the years. I warm up the engine first and change the motor oil, filter and drive oil. Then run it again and check for leaks. If a carbed engine, fog the engine till it stalls, and let it cool off. That water in the block will be like at 140* at least so let it cool....
Next I disconnect the big hose that connects the front water pump on the engine with the thermostat housing and let it drain. Next I pull out each engine block drain plug, and be sure to poke the holes with a pick or thin screwdriver. Raw water cooling on a cast iron engine = rust flakes that can clog drains. After it drains, I coat the drain plug threads with gasket sealer (OMC gasket sealer, Merc perfect seal or Permatex Aviation sealer). Then replace the plugs and re-connect the bottom end of that hose. Now remove the drain plugs for the exhaust manifolds, they will be on the rear side of the manifolds, opposite from were the water feed hose is on the front. Poke holes, make sure it drains...then coat drain plugs with gasket sealer and replace. Go up to the front of the engine and mark the hoses on the impeller housing, so you don't mix them up. Disconnect the one that goes up to the thermostat housing and let drain, then reconnect. Now disconnect the one that comes from the transom mount (raw water intake hose). Point it down in the bilge, to drain. I like to put the outdrive all the way down, then fill that hose with -100 marine AF till it comes out the outdrive water intakes, that pushes water out of the P/S cooler on the rear of the engine, and flushes water out of the hoses that connect with the transom mount as well. Then reconnect that hose.
Most will say this is adequate but if you want max protection from internal corrosion, you can: disconnect the feed hoses for the exhaust manifolds at the thermo housing, and fill each one with Marine AF (-100 is best) till it runs out the exhaust. Re-connect those hoses.
Disconnect the big hose for the front water pump on the engine at the thermo housing, fill this hose till AF spurts out of the thermo neck. This will fill the engine 100%. Keeps out air, and reduces corrosion with Marine AF with corrosion inhibitors. DO not use auto antifreeze, it is toxic and will get all over your driveway when you start up in spring. You must use no-tox marine antifreeze for storing engines, not plumbing antifreeze for pipes etc. I have tested -50 and -60 in the freezer, they both get hard at approx. zero* F. -75 and -100 will stay liquid. I feel for a cast iron engine, you want AF that stays liquid! Don't care what other back yard mechanics or hacks who work at marinas say. Those numbers are burst temp for pipes, not freeze temps. While they are not supposed to expand, I feel that AF that stays liquid, is safer if you want to use AF.
That's what I do and it has worked well for me. BUT, for my next engine, I will for sure install a half closed cooling system. That way I can use a winterizing tank and don't have to pull drain plugs, at all. Well worth it for that and other reasons (longer engine life, no internal corrosion, better control of temperature, pressurized cooling etc).
Last point, you wondered about the exhaust manifolds and elbows...well...some freshwater can be corrosive...so look at that joint between the manifold and the elbow....if you see rust trails coming from that spot, that's water leaking and it can leak inside...right into a cylinder via an open exhaust valve. So keep that in mind. In salt water we check them every 5 seasons...that joint on the wet style exhaust is a known problem area. In fact Volvo has revised the gaskets several times and Mercruiser re-designed their exhaust systems to use a dry joint design approx. 17 years ago. Better design than Volvo but the cast iron can still rust through.
_________________ 88 Four Winns 200 Horizon 4.3 OMC Cobra-4bbl 2002 Walker Bay 10/2012 Suzuki 2.5 2008 Walker Bay 8
1998 Jeep Grand Cherokee 4.0/Selectrac 2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II
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