its a waist of time to remove the thermostat housing i winterize over 250 boats a year it sounds like the drain plugs are getting plugged up with debris and your not getting all the water out. first thing to do is remove the drain plugs from the block and from the manifolds then stick a nail up in the hole and knock all the debris out and make sure you get water out of the holes. then remove the hoses from the thermostat housing and the raw water pump push the engine waterpump hose down as low as you can and let it drain once the block and manifolds have stopped draining pour in your antifreeze through each hose i use 2 gallons total one gallon in the motor through the wp hose and let it run out of the block drains to verify it got any water that was left out then pour into the manifolds hose about a 1/4 of a gallon in each hold your finger over the drain or put the plug back in then release and make sure its antifreeze out of the manifolds next pour down the intake hose untill you get antifreeze out of the hose at the raw water pump and then pour through the hose to the drive to clear it then i put the hoses back on the raw wp and fill the intake hose up then if you want reinstall all hoses and plugs and your ready for summer hope this helps.

Sikinboat, I do stick either a very small screw driver or wire up the holes of the lower drains where all the crud accumulates. Normally, if the water trickles out, and after shoving wire up there and agitating, it starts to flow freely, along with all the mud. But I never thought of pouring AF in the hoses until it flows out of the lower outlets for the block. Good idea, thanks!