Winterizing Problem with Engine

[jmgallagher]

I own a Maxum 2500 SCR 2002 with a Merc 5.7 and B3 outdrive. All went well with winterizing until I put the muffs on the outdrive and connected it to a 6 gallon bucket that was higher than the engine. I started the engine to warm it up to get the thermostat to open so I could get the purple antifreeze thru everything. At first I got a very low flow and then none, the engine got to normal temp to open the thermostat before I noticed that there was no water flow. Shut the motor down as to not over heat. Unforturunately I live in the northeast and a freeze could happen at any time in addition it is my busy time of year at work. So I opened the two drain plugs on the manifolds and on the block to drain the water. Closed the plugs and poured purple AF into the hoses then opened the drain plugs and repeated until purple poured out of each drain. Then took the Thermostat housing off and did the same. I have two questions. Why did the muffs not work, which means I need to replace the impeller which was installed this past spring. Second did I miss anything on the engine as far as drains that would cause a freezing problem. Thanks in advance for any help