Hey all, please post any water sources you have encountered and your fix. Here are the ones I've experienced.
HOT WATER
Freeze plug plates on block:
Under the exhaust manifolds, there are 2" approx dia plates secured by two machine screw to side of block. If you have ever had an incomplete weatherization, you might notice that these plates are bulged out. They can do this and not leak, but eventually they will leak hot water into bilge. Ti fix this, take the plates off and beat them back into shape gently with a small hammer. wire wheel the gasket surfaces. Reinstall them with marine sealant and let them cure overnight.
Transmission cooler (V drive) This is a cylindrical brass tube approx 2" in dia and 12" long. It is mounted on the starboard (driver) side bilge wall adjacent to the block drain. My boat was not winterized properly and this cooler froze and one end popped off. I was able to remove it in the water and take it to a shop. We used JB Weld quick weld to put it back together, it has been fine ever since. The boat mechanic at the time told me it was a common Mercruiser part.
Exhaust hose, muffler leak. This will be a big leak and can be catastrophic, one reason you should have your bilge pump in auto when boat is on water. Exhaust hoses can degrade if you overheat, and will collapse inside. They can blow off the engine and cause a huge leak and no run situation. If you have straight ( non-ribbed) exhaust hose it might be original. The ribbed kind is much easier to replace with engine in boat. If your muffler rubs through, it is a big leak but boat will still run. You can pull it out and fiberglass it.
Thermostat housing: You should always pull thermostat when dry winterizing. There is water behind it. When reinstalling, be careful that thermostat, rubber gasket is seated in housing. You can put it in crooked and cause a leak.
Bad heater core, this can split and cause a hot leak near driver. Replacement heater core requires slight modification of the case but no big deal.
I know that the external water pump can fail and cause leaks but have not experience that. Any other hot water sources???
