just a faulty valve and some more thoughts
I presume the 'wing nut' was leaking and that's where the excess anti-freeze went. If not, my may need to check around for leak (albeit it could have been the 'bypass' to the hot water heater taking up some of the extra coolant, albeit that sounds like too much). If the wing nut was leaking, then since the engine temp never increased, and the temp sender is just below the 'faulty valve', it pretty-much proves it was just a faulty valve. However, I am wondering about the radiator cap. The normal design of the cap is to allow the engine to build-up presssure, and then during the cooling process, it creates a vacume which allows additional coolant to pass through the lower seal of the cap and into the engine. So, unless the 'overflow' bottle in the lazarette locker emptied after the engine had cooled, I am wondering if your cap may also need to be replaced (my 1988 M25XP took a Stant # R29 -- # 10229 from the local auto parts store). Also, if you are removing the thermostat housing, be sure to get a new gasket and consider a new thermostat (they have a finite life, albeit I just replaced mine this year for the 1st time, and the old one tested ok, but slightly slower and slightly less full-open so I just replaced it). Lastly, the newer version of the engine (M25XPB) has a over-temperature sensor mounted in the thermostat housing, which I think may be a bolt-on replacement for yours. So, depending on price, might be something to look into (in case you find you have to replace the housing instead of just the bleeder valve). This way you could also add a high-temperature alarm (could run this sensor's wire to the oil switch and splice it in, thereby setting off the cockpit panel if either grounds-out). Just a thought... Hope this helps..
