Magically teleporting coolant
I've done two coolant changes before on this vehicle, an '83 380SL, but I've never used the block drains. I would just cycle four or five rounds of clean distilled water through, draining the rad and expansion tank in between. Costly for the water, but cheap on time and hassle, and no need to raise the car.
This time, I had the entire system full of tap water after flushing, the car was on the ramps, and so I used both block drains. Drained rad and expansion tank too, let the thing drip dry, buttoned up, started to refill with 50/50 coolant/distilled from the expansion tank. Once the rad was full and the expansion tank was level, I said, "Ooops!" This time, because of having used the block drains, that's only half of the system filled.
I had started the car and let it idle briefly just as a test -- but I am leery of running it up to temp to get the thermostat open and allow the rad coolant into the block. Too much risk of damage.
In between pondering and reading the 107 service book to figure out what to do, I said, "Drat!" Remembered that I had not run the heater to drain the tap water from the core along with the rest of the system. I could just ignore it -- it's not that much tap water -- but I really prefer not to have any nondistilled water in the system, having cleaned out mineral crud for years from systems where people had filled with tap.
OK, simple enough. Turn on the heater, start and idle the car for 20 seconds, turn the engine off, pull one of the block drain plugs, expecting a trickle from the heater core leftovers.
Yikes! A cascade of what is without any doubt coolant-colored fluid comes pouring out. I frantically get the plug back in place to stop the flood. Thank goodness it's an Allen.
How in the world did *that* get into the block? The car hasn't run for anywhere nearly long enough to let the thermostat open to admit coolant from the rad.
Could the coolant be getting past the weep hole in the thermostat body? That's a prodigious amount of fluid to get past what I recall to be a tiny little hole, in only a few seconds of running time.