Slow coolant loss over 30,000 miles; 2010 Prius II w/ ~85k

Check the caps. They may leak under the slight -vacuum- when the car cools off. This makes the system unable to pull coolant back from the overflow tanks. When this happens people pressure test it and say it's fine, and it is, -under pressure-. But not under the cooldown vacuum condition. It's often caused by a small leak at the rubber seal of the caps. What happens if you have this is the overflow tanks overflow themselves (often through an overflow hose that drains to the ground) during warmup (while you're driving), so you don't notice coolant on the ground. And just to make it even harder to diagnose, often the leak is intermittent.

Do not remove the caps until the engine is cold! You probably already know that, but just in case I thought I'd warn you. Look for imperfections on the surface of the rubber gasket, buildup of "grunge" on the edge of the filler, etc. I've found something as small as a scratch on the rubber causes this, and in another vehicle, couldn't find anything, but the problem went away when I bought new caps.

 

Ours was built in Aug 09, we bought in Nov 10, and both inverter and engine coolant reservoirs are now at or slightly below the low line. I chalk it up to evaporation. It's been like that on any vehicle we've had, maybe more so.

I'm going to knuckle down and buy a 4 liter of the premix.I just wish you could buy a quart/liter.

 

Despite been not been exposed to atmosphere, these coolant somehow do evaporate, I had to top my off 50-100 mL every year, summer time is when it really starts to become evident of coolant level drop. (average outside temps 90*F-105*F in peak summer) just keep any eye on your coolant level every time you fill up your tank.

 

It's not unusual for coolant level to slowly drop in the reservoir. It's by no means air-tight, and heating/cooling would accelerate evaporation I think.

 

^ That guy's full of it, lol. I just topped up our reservoirs, they were a hair below low mark: unscrewing the cap I see:

1. It's got a locking tab, positively holds the cap.
2. there's an O-ring on a protrusion on the underside of the cap, it extends down into the neck of the reservoir. When you pull the cap out there's a definite psst of escaping air.

 

How about this, from my motorcycle....

I tend to lose fluid over time. No obvious leaks. There was a time I could see "puffs" of steam from the radiator. Passed pressure testing every time.

Some leaks are so small, there is no danger of failure or significant loss you can monitor in real time, but over time, it adds up. Finding them can be pure happenstance...being in the right place to see the evidence as it happens.