[Wuiquen]

I recently picked up a 2018 ZLE with 9k miles on it. I have been reading as much as possible about the coolant in these cars so I could educate myself. While shopping for the car, i saw a bunch with coolant expansion tank full and others totally empty.

This is what I understand so correct me if I am mistaken. The top tank is a expansion/return tank. This should be filled to bottom of the neck. The reservoir is at the bottom and this should be filled to the mark (i believe is like half way). Use the fill tube.

When I got the car, the coolant at the expansion tank was at at the bottom of the neck. Drove home and let the car rest. Next day i saw the expansion tank was like half full and the reservoir was filled past the fill mark. I washed the car so I needed to move it out of the garage, then went to check the coolant level and it was back to how it should be (expansion tank full and reservoir full to the mark).

Drove the car again and it is back to expansion tank half way and reservoir overfilled after the car rest for a day.




Can someone help me understand what is going on? Is this normal?

[Gunkk]

The tank design is counterintuitive. The TOP tank is part of the loop and should always be completely full. If it's not, then you have air in your cooling system.

The bottom tank is the expansion tank, and should be HALF full.

It's a dumb design, but it is what it is.

[Wuiquen]

Ok. Thank you for the explanation. So how do I get the air out? The coolant level had got to be right since the level at both tanks are right some times. It just seems that the fluid is moving around. Maybe someone filled the system improperly?

[Gunkk]

Originally Posted by Wuiquen

Ok. Thank you for the explanation. So how do I get the air out? The coolant level had got to be right since the level at both tanks are right some times. It just seems that the fluid is moving around. Maybe someone filled the system improperly?

More likely it just evaporated and they never filled it (or thought it was normal).

The manual page I posted above explains how to fill it.

TL/DR: With car off and cold, open the cap & add coolant. Close the cap, start the car and run a minute. Rinse, repeat until no more air shows up in the tank. If it gets hot, the top tank will be under pressure so think of it as a traditional radiator cap.

[L78toLT1]

For what it's worth, it appears that the coolant has been replaced in your car. The oem coolant is orange, in the pics yours looks to be red. This alone may not be entirely relevant, but it does indicate that the system has been opened up at some point.

[Wuiquen]

Originally Posted by Gunkk

More likely it just evaporated and they never filled it (or thought it was normal).

The manual page I posted above explains how to fill it.

TL/DR: With car off and cold, open the cap & add coolant. Close the cap, start the car and run a minute. Rinse, repeat until no more air shows up in the tank. If it gets hot, the top tank will be under pressure so think of it as a traditional radiator cap.

But... Given that the fluid has returned to proper levels at both tanks in the past, should I still fill it up? So if I refill the top tank right now, then the bottom tank would be overfill because it is past the fill line.

[Jaxcam02]

I would fill, give it a week of observance for being at correct levels every time and fill if needed.

[Krowser]

Originally Posted by Wuiquen

But... Given that the fluid has returned to proper levels at both tanks in the past, should I still fill it up? So if I refill the top tank right now, then the bottom tank would be overfill because it is past the fill line.

I would refill the top tank to max capacity and observe for a week.

The top tank is not suppose to have that large open space.

[UnknownJinX]

For reference, I have come across 3 6th Gens including mine where I was able to look under the hood. My SS, a V6, and a 2.0T all needed coolant. I even saved the premixed coolant and gave a lot of it to the V6 guy. The lower tank in his car was bone dry and the top tank was barely above the MIN line. Mine wasn't that bad but still needed sone,

Originally Posted by L78toLT1

For what it's worth, it appears that the coolant has been replaced in your car. The oem coolant is orange, in the pics yours looks to be red. This alone may not be entirely relevant, but it does indicate that the system has been opened up at some point.

Other way around. Factory fill is reddish-pink, you can only buy the orange stuff.

[LT4Greg]

I guess I’m lucky in that my 2017 isn’t really losing any coolant which I’m thankful. On past cars I have had problems like that and had to add coolant even if the system checked out okay.

[clg82]

subbed as i want to check my zl1 later today good info in here!

[Wuiquen]

Originally Posted by Jaxcam02

I would fill, give it a week of observance for being at correct levels every time and fill if needed.

What do I do with the lower tank level? Right now it is past half tank meaning there is more fluid than it should have while cold.

[Wuiquen]

What I am afraid of is overfilling the system. Like i said, there has been times or at least one time when I check that the level at the top and lower tanks were proper at full atthe top and half at the bottom

I would be ok filling the too if the lower tank was at the fill line or less but right now it is more than fill line so if I add more coolant to the top tank, wouldnt I be overfilling the system?

[Camaro1973]

Originally Posted by Wuiquen

What do I do with the lower tank level? Right now it is past half tank meaning there is more fluid than it should have while cold.

Only fill the top when cold. It should be at lowest level in top tank when its cold. Mine does weird crap too, bottom tank sometimes has less then half then when its fully warmed and running its back to half. I would only fill the top for now and just watch it. If the bottom tank is already around where it should be i wouldnt touch it.


Monitor it cold/hot for a few days.