TL;DR: It's time for us to make a gauge to monitor our coolant recovery tank level and let us know as soon as the levels indicate a leak or air incursion into the cooling system. Here's an article on different ways to measure coolant level. Let's noodle on how to do this, and build the missing coolant gauge. Below is a discussion of what the coolant tank does, how the level changes, and why this guage is important.
One of the insidious issues that seems to be emerging with this car are coolant leaks. We had a rash of reports lately. The leaks are small, weeping from the coolant block, old radiators, etc. The problem is that it seems that you can lose quite a bit of coolant, to the point where there is air in the head and it needs to be burped. The coolant temperature gauge is noted for being insensitive, so this can go unnoticed. Finally, there is a theory that a cylinder head with air in it can lead to overheated cam bearings, galling and eventual seizure and snapping of the camshaft - at least, that is one theory about camshaft failure. It seems that no one will be immune to this - they seem to be an eventuality, can go unnoticed, and potentially result in the end of an Insight's life.
The thing is, you can get advanced warning of this simply by checking the coolant recovery tank. This is not some "boilover" or arbitrary tank - it actually is a simple, clever but necessary piece of engineering. It's purpose is to permit a cooling system to be almost completely free of air and completely sealed.
The problem with a sealed system is that the coolant and any residual air expands as the temperature rises and contracts as the temperature falls. For an example of this, don some oven mitts and pour an ounce or two of boiling water into a coke bottle, swish it around for a few seconds, dump it out and quickly cap the bottle. Within a few seconds the hot air inside will cool and by the time it has reached room temperature, the bottle has collapsed.
The same problem happens in a sealed cooling system. What the overflow tank does, then, is when the engine gets hot, the little bit of trapped air expands, as does the coolant to a much lesser extent. The trapped air increases in pressure. Eventually the pressure opens a valve in the radiator cap and coolant in the radiator is pushed into the overflow tank. That's why it's good to put a sticker on the overflow tank and mark the level when it's hot - it's going to be highest when the engine is hottest and that air pocket has expanded the most.
As the engine cools the pressure drops as the air takes up less space and sucks in coolant from the expansion tank, like sucking soda from a straw. That's why it is SO IMPORTANT to make sure that straw is attached and well-secured and there is no leak where it attaches to the cap. I have seen and read about several of these fallen off and when they do, only air gets sucked in and since air expands much more than coolant, next time around a lot more coolant is pushed into the tank! Anyhow, when the engine is coolest the pocket is smallest and the most coolant has been pulled back into the car and a second mark is placed on the tape.
This is also why if you ever find that tank empty, GULP! the cooling system is sucking in air! Refilling it is not enough - Time to burp the car!
With the two marks we can now see the range of coolant being drawn into and out of the car on each cycle. But what a PITA to lift the hood and check the tank every time you start and finish a drive to check those two levels, especially when the high and low points change very little over months of driving! What we need is a something to do this automatically, that makes sure that the coolant never goes too low when cold, too high when hot, and that the distance between high and low does not grow too large, indicating an incursion of air in the system.
I've put an order into Amazon for some stuff that I think will work, but before I discuss it here, let's see what ideas people can come up with for measuring and reporting the coolant recovery tank level.