Looking for some advice on how to winterize a 496 MAG HO. I recently watched a very informative video and read the owner’s manual that explained exactly how to do it, but I have a few questions for those who have done it before. In the video the gentleman uses the pump to pump out all of the water, and then flushes using RV antifreeze. My questions are as follows.

1. Is it really even necessary to pump out all the water if you are going to flush with antifreeze?

2. Since he pumped all the water out and turned over the engine to get all the water out, wont it hurt the sea water pump when I start the motor to flush the antifreeze since its not pressurized and there is no water in the sea water pump?

3. After you use Mercury’s pump to pump all of the water out and wait for it all to drain, do you then relieve the pressure using the manual release valve (green indicators go away) before flushing the motor with the antifreeze? Or do you leave the pressure in there until summer time when you are ready to launch the boat.


Thanks for the help.

1. Draining engine of water is not required but is a common practice to ensure highest concentration of anti freeze. The fear is any left over water will dilute the antifreeze increasing the freezing point.

2. Using the engine water pump to remove any remaining liquids won't harm the pump as long as it is done for 8-10 seconds. Starve the impeller of water and run it too long and you'll burn out the impeller.

3. Not sure what Mercury pump you are referring too. We always use a submersible pump and turn on engine to help circulate antifreeze. In any event the engine and exhaust is not a enclosed pressure vessel and vent to ambient air.

Hope this helps.

another good thing to check is your cloosed cooling antifreeze,some times a slight leak in the heat exchanger will cause the coolant-antifreeze to be diluted and can be a freeze hazard if not checked

There is alot more to it than just the water side for a winterization and thinking the air system will do it for you.

Not even even close for a proper winterization. .

You have the cool fuel side and you have the complete fuel system, fuel pumps and you have the fuel tank that all will need to be properly protected for lay up storage and protected against condensation along with freezing temps., All of these sides need a form of corrosion protection and lubricity including the fuel injectors plus be water free 100 %.

It is wise to drain the heat exchanger 100 % completely of water as well. Thats is very easy but most think the air system does a complete job of draining water. It does not especially for the GEN III cool fuel side. Also there is blue drain plugs that can be pulled to make sure the water is drained if in fact the air system did not completely drain the water sides of the engine. I always pull them and stab those holes deep with a tie wrap or wire. Especially boats without sea strainers.

The water pump housing needs to be 100 % water free as well.

the outdrive needs to be 100 % free off holding any water including the water intake inlet and hoses transom side and engine side. Also no water in the gear oil plus change the gear oil completely.

All oils & filters along with all fuel filters should be changed.

Clean flame arrestor.

Next is testing the anti freeze - one for corrosion protection and then for freeze protection.

Also for the fuel side it is very important to make sure no water is trapped in the system including the seperator in the gen III cool fuel or the GEN II spin fuel filter. Fuel samples from different parts of the fuel systems are a must for winterization for condition of the fuel and water. Water even just a couple of oz's in the fuel system causes havac especially when stored.

These are steps for a 100 % professional winterization that offer problem free boating year after year..

Thanks everyone for the input.

Why are you asking? Seems like the wrong season to be worried about winterizing.


I know winterizing the engines is the easiest part of my fall ritual. It also includes all the other systems on my RV of a boat, plus plenty of anxiety and depression associated with putting the cover on for the last time this year.

Luckily, I have an outboard on the pontoon for that nice day in January.

Remember, the air pump drain valve system is not forceable replacing the water in the sea water side of the engine.
ALL it is doing is opening two air valve actuators on the bottom of the motor, allowing water to DRAIN out.

Before you run antifreeze through the motor for the full winterization, you MUST pull the pin to release the air pressure out of the drain system.

This system was designed to extend your boating season, into months where temperatures dip into freezing; but you must still do a full winterization the old fashioned way, every time.

ALL of BUP's information is correct, and there are so many threads on 496 Winterization here it's not even close to funny.

Brain when I get chance I will post to you why the heat exchanger sometimes can not even drain completely of water. It has to do with the end cap gaskets. The pics I have to send - you will not believe it and only 230 hours all freshwater. These gaskets cause alot of issues especially an over heat and or very hot exhaust temps. These gaskets actually sucked into the ports of the heat exchanger. Another reason why the heat exchanger can not drain fully of water during winterization.

And so far I have repalced in the past 2 months 3 cool fuel GEN III systems leaking water because they were not drained of cooling water and they used air pump but it just does not drain the cool fuel GEN III. Just an FYI and we were not even that cold for winter, It only takes about a full day to solid freeze and that cool fuel will crack.

I know of exactly what you are describing. My gaskets were all sucked in on both ends of both of my heat exchangers. Now that I have seen this with my own two eyes, I am a advocate of taking the ends off and cleaning regularly. It's cheap (almost free) insurance.

Same here. I pull every other year, clean HE and replace gaskets.