Winter Prep


Hello, all,

I will be winterizing my Merc Mag 350 (2004 Sunesta 252) soon, but may not be able to get it to the shop as soon as I would like. I'm in Grapevine,TX (near Dallas/FW). Although it usually doesn't freeze hard in November or most of December, it's not a chance I like to take. So....what is the best way to at least drain the engine block to avoid any surprises till I can can it into the shop, ASAP??

Thanks in advance,


Well-known member
Gold Site Supporter
Everything I find is to add antifreeze. If you just want to drain the water out, call a Merc dealer and they can tell you which hoses and plugs to open up. It may also be in your owners manual. Good Luck. Please let us know how you make out, it may help out someone else.

J Kyle

New member
My 2007 350 Mag (Sunesta 256) has 4 lower plugs and one up top on thermostat housing (this one I loosen to vent). Mine is single point drain out of water distribution housing but I also pull the plug out of cool fuel unit and two other plugs behind the cool fuel unit. Also when you pull the plugs take a wire and probe the holes to make sure you don't have a clog.

Go to and you should be able to look up part diagram for the cooling system you have by engine serial #. I have attached mine as a reference. I have circled the water distribution housing.



Active member
If you have a closed system. It's a 30 second job. Open system is a lot more involved.

Personly no matter which one. I would leave the AF in the engine or heat exchanger. With no AF in the engine means air that will promote rust. I would not buy a boat with out a heat exchanger. But thats me...

Just saying....