Seems when it rains, it pours. Took the e39 on a drive (do it 3 times weekly) where I can get the rpms up to 4000 on successive shifts. With my daughter in the back seat, almost back at home, our last little acceleration from 2nd to 3rd gear (with the automatic shifting in sport mode), hitting the 4k rpm where I always ease up a bit to let it go into the next gear, I felt what was like a mud flap pop or like a slip in the engine. I immediately left off the gas, we drove the last 2km home, and the car ran fine (and the thermostat stayed dead center, not fluctuating either way).

Upon opening the hood, I was greeted with bright, brilliant green rad fluid on the thermostat housing, around the belt, on top of the P/S reservoir, well, you get the idea. I immediately found a crack in the plastic thermostat housing where rad fluid is leaking out. I know I've been living on borrowed time with this thing, as it is the original thermostat housing when the car came off the factory floor.

My questions:
is there an easy way if I can tell if my water pump is still ok? I mean, how would I know it didn't freeze up and cause pressure to build to where the thermostat housing cracked??

I've done my radiator (and associated parts) a few times, as most of you know. But my question is there a trick to where I can change this thermostat plastic housing without losing much rad fluid?? I mean, the rad fluid is basically brand new in there, as is the radiator itself, the hoses, clamps and all. Could I just wait until the car is only lukewarm, and then maybe get hose clamps and clamp those two rad hoses off (and get at the housing, assuming the thermostat will close as the car cools)?? Or will I just destroy those new hoses clamping them off??

I have lost access to the garage/carport for the winter, so this all (in addition to my very recent windshield washer pump failing) is going to be a tough fix out in the cold, freezing, snow/rain weather.

Would greatly appreciate any tips and/or hints on the two above questions (how do I know my water pump is ok, the pump propellers and all, and next, is there a trick to changing the thermostat housing out on the I6 where I don't have to do a full rad fluid drain and change).

If it helps, here's a schematic of my 1996 I6 thermostat housing, thermostat and water pump. The crack on the thermostat housing is exactly where the dotted #10 line passes over the thick part of the right radiator tube portion of the housing itself:
1996 I6 thermostat housing, thermostat, and water pump schematic

Thank you for any prompt replies (I already have the fan shroud and fan clutch out of the car, a royal pain getting that thing loose in this weather!!).