I've noticed that the coolant reservoir tank is almost empty, well below the MIN level. OK to leave alone, or do I add distilled water to above the minimum level? Car otherwise is running great.

 

Add some water, preferably with antifreeze in it, but that little bit of plain water will be just fine. Fill it to the max level.

Being water, it does evaporate over time, but monitor the level just in case.

 

Add water/antifreeze mix with the engine cold.

Had the same problem. The clamp on the lower hose attached to the radiator snapped due to corrosion. When you're looking at the reservoir look at the bottom of the radiator and check for clamp corrosion.

 

Filled up the reservoir tank to "MAX" yesterday, and when I re-checked it this afternoon after driving a bit, it was empty again!

The radiator itself is full when I open the radiator cap. Car is NOT overheating. I pulled the tank out but it looks intact with no cracks or holes.

What is going on?

 

small leak somewhere. I have the same problem. Not sure how fast it dissappears, I haven't control tested it yet. I would bet upper or lower radiator hose has pin point leak or a crack in the reservoir. Coolant is pressurized when run through the engine. A pinpoint leak can still loose noticeable amounts I think.

 

you dont add pure distilled water to the reservoir, it still needs to be 50/50 because that coolant will over time make its way into the system

if your rad is always full and the reservoir is always empty, the reservoir itself probably has a crack on the bottom of it. all the reservoir does is hold the coolant your rad pukes out when its warm instead of dumping it to the road like old school cars (environmental reasons aka animals drinking it). so if your rad is always full, obviously its the reservoir itself thats leaking

and coolant does not just "evaporate". if you have a ghetto coolant overflow made of a open Gatorade or Coke bottle with no lid, i suppose it can evaporate. but OEM coolant reservoirs always have a snap on or twist on cap so nothing should evaporate

you can leave it below the minimum if you want, doesnt hurt anything. technically you dont even need a puke tank. just make sure you check the level again in a week or so, if the little bit in the reservoir right now dissappears i would check for a leak somewhere.