low coolant / add water?

jeromep wrote:
I had a slow, "fillable", coolant leak this summer. For a couple short trips I filled the reserve tank to compensate for the leak, but any time I got the rig up to temperature, I'd see the leak under the rig after I parked, presumably directly below where the compromise existed, and the leak would only really occur when the system was at pressure. Once the engine had cooled off and pressure had returned to atmospheric levels, the leak would disappear. But the reserve tank always told the true story because it was always low on the old marker when the engine was cold.

I refilled my overfill tank with straight coolant (older 460 V-8, so old tech), just to make sure I didn't run low or overheat. I added much less than a half gallon over the few weeks I had to run with the leak, which shouldn't have messed significantly with the coolant/water ratio, much.

Took the rig into a mechanic, and after a rather expensive total radiator replacement, no leak, as it should be.

Unless you where checking the pressurized part of the system when it was cold, you where gambling. Engine gets up to temp, coolant expands, forcing some into the tank. Then as it cools/contracts it is drawn from the tank to keep full. But a leak that can let air in means the tank can stay full but the system gets low.
In the OP's case, where the engine is setting and maybe slowly bleeding, the level can drop enough to let engine rapidly overheat when started, and the gauge never read hot because the sending unit is dry.