Good morning, everyone. I have a 2007 Mercury Mountaineer with the 4.0L V6 engine and is AWD, right around 175,000 miles and going strong.

I had a hose crack while on the expressway a couple months ago, the fix was easy and life was fine. While under the hood I noticed that my thermostat housing was leaking, just enough to annoy me. So I swapped that out, replaced the housing and put a failsafe 192F thermostat in.

It's all sealed up tight, no visible signs of leaking, but my coolant level keeps dropping. Coolant temp usually tops out around 192F, and that will go up or down depending on driving and weather. It's been hot here in Michigan, this morning it maxed out at 199F until it leveled off to the thermostat temp (192F).

Pressure looks right, I can't see any white smoke from either the engine compartment, or exaust system.

I'm at a loss. I have no idea why the level keeps dropping. I need to check the rear heater core for leakes, it's given me issues a few years ago with bad hose clamps, but other than that I'm open to any suggestions.

Maybe someone can point me in the right direction?