Cooling System Overflow Tank

[RoyDaGeneral]

I've noticed that coolant level of my QX4 level is very low. I don't have an overheating problem. I've done some research and spoke with a few people and answers I'm getting are all over the place. Some say that overflow tank if for when system gets too hot, coolant will flow from radiator to the overflow container, reason it needs to be empty. Others are telling me that I must be having a small leak somewhere the reason coolant level in the overflow tank is that low. So, here I'm posting here to get a more reliable answer. So, shall I top it off or just leave it where it is now? Also, tank is not cracked or leaking as I checked it very closely. Thanks.

[VStar650CL]

Leaks, especially slow ones, can be vapor and not just liquid. The simplest way to find a slow leak is UV dye in the coolant and a week of driving around. Even if it's a vapor leak, the magic flashlight and glasses will show you where it is.

The overflow tank doesn't have a pressure pump, nor does it violate physics. If it could put coolant into a hot motor then it would need one or the other. It actually works by pressure and vacuum. Air or steam are always on top if they're in the system, so when you have a tiny leak, it gets forced out the overflow first instead of liquid as the engine heats up and the coolant pressurizes. When the engine cools a vacuum is created instead of pressure, and that sucks liquid from the bottom of the bottle back into the system to replenish what leaked out. This leads to a gradual drop in the bottle's liquid level.

[RoyDaGeneral]

That's a very thorough answer and very easy for me to follow. My first sentence should have been, "I've noticed that overflow tank/reservoir coolant level of my QX4 level is very low." That's what you responded to, thanks for seeing thru my fog! Do you need a special type of flashlight to see UV dye? How about those glasses? I'm in the process of getting a cooling system pressure tester which is currently in the mail.

[VStar650CL]

Do you need a special type of flashlight to see UV dye? How about those glasses? I'm in the process of getting a cooling system pressure tester which is currently in the mail.

Yes, but you can get a whole kit off eBay for about $15:

https://www.ebay.com/itm/192099365242

I got mine at Harbor Freight. I don't recall them having a kit, but I think the glasses and flashlight came to about $20. A small bottle of dye is about $8 at any parts store.

[AlanAZ]

I've noticed that coolant level of my QX4 level is very low. I don't have an overheating problem.

How quickly did it lose this amount of coolant? I top up my coolant, once a year and 5K miles, both this vehicle and a similar age Camry with similar overflow tank design. It's usually 1/4 of the way down from the max mark to the min mark (look closely at your tank, there are upper and lower horizontal lines molded into the tank.) These tanks are not sealed, so a certain amount of evaporation is normal. The age of the coolant can make a difference, older goes quicker. And the age of the radiator cap, as the spring fatigues and loses pressure, more hot coolant goes into the overflow tank, increasing the evaporation. I replace mine at 150K miles or before, with a factory OEM part. Beware that the radiator all eventually fail, I replaced mine a couple of years ago at just 140K, look for dried coolant sprayed over part of the engine bay and drips on the floor. I had a drip, on the right side behind the axle, from the hose connecting large diameter metal pipes below that head, I only needed to tighten the wire clamps.