Thread: Small air space in coolant reservoir

I had a curious observation about coolant reservoir in my 05 grand prix base. I think the hose from the radiator filler neck to the reservoir should be airtight. So when the car heat up or cools down, coolant can be pushed out of sucked in to the radiator.

It is strange to me why the coolant reservoir has a U-shaped structure (as the red line shows) where the hose enters the reservoir. And the hose does not connect to the bottom of the reservoir. If the whole cooling system is airtight the short column where the hose enter the reservoir should be full of coolant. The cold mark on the larger side of reservoir is lower than the small column. So when it is cold, there is a tendency that coolant will flow from the small column to the larger side. The only reason that it does not flow to the larger side is because the cooling system is completely airtight so it can maintain this uneven level. This is a strange design to me. If the hose just connect to the bottom of the reservoir, will it be better? This way, we are not artificially create a tendency to draw something in.

As I circled in yellow line in the attached picture, this space is above the entrance of hose to reservoir and I always find a small air bubble here. I lean the reservoir and let the air bubble out, but it will appear again. I am confused where this air comes from. Maybe it is because I did not completely purge the air out when I flush the system so the small air bubbles walks its way out and end up being in this high place. It has been almost two month since I flush the cooling system and all the air bubbles should already walked its way out.

Any idea why I always see the small bubbles there? The coolant rise and fall as the car heat up and cool down normally.$(KGrHqZ,!hgFCF2BSE0vBQtSO-PfNg~~60_1.JPG

Ever used a bottle of Sta-bil? same basic reason, you'd want the amount of coolant in the tank not to effect the pressure against the cap.

Originally Posted by addzradd

Ever used a bottle of Sta-bil? same basic reason, you'd want the amount of coolant in the tank not to effect the pressure against the cap.

Thanks! Would you please explain it a little bit more? It seems to me the difference in liquid level between the two sides will influence the pressure against the cap.

For the sta-bil bottle, I guess the bottle of is designed for the convenience of measuring and pouring a certain amount of liquid.

The level will be lowest when cold, and it should be above the opening from that u-bend. It's like that to create a vacuum and to help keep your cooling system at the correct pressure. That plus the radiator cap.

Overfilling the reservoir won't damage your engine, at worst it overflows out the overflow opening on the back of the reservoir. But under filling it can if it causes air to be introduced into the cooling system.

Originally Posted by nik12937

The level will be lowest when cold, and it should be above the opening from that u-bend. It's like that to create a vacuum and to help keep your cooling system at the correct pressure. That plus the radiator cap.

Overfilling the reservoir won't damage your engine, at worst it overflows out the overflow opening on the back of the reservoir. But under filling it can if it causes air to be introduced into the cooling system.

Thank you! I understand that. The cold mark is above the U-bend so when the engine is cold, the radiator suck in coolant, not air. What I don't understand is why they design the shape like U-bend. They could easily put the hose on the bottom of the reservoir or sidewall that below the cold mark. That way, they can still prevent air coming into the radiator.

Also, I always find a small air bubble in the yellow circled area. The bubble will come back after I get it out of there by leaning the reservoir a bit and drive for a while. Do you have any idea how this air bubble get there?

"If the hose just connect to the bottom of the reservoir, will it be better?"
If the overflow hose from the radiator was positioned at the very bottom of the overflow tank, any "crud" (dirt, corrosion, gloop) that settles out from the coolant will find it's way back into the radiator, and could possibly clog the overflow bottle hole/hose.

i would change the location of that hole. it would make a difference but why change it when its not causing any problem?

Originally Posted by marines542

It could get there by the slightest amount of air being in the system and getting worked out

Sent from my DROID RAZR using Tapatalk 2

I get the cooling system flushed two month ago. Maybe some small air bubbles gradually walk their way out.

So do you think if the system is completely out of any air bubbles, the yellow circled space should be full of coolant and no air should appear?

Originally Posted by QUICKSILVER462

"If the hose just connect to the bottom of the reservoir, will it be better?"
If the overflow hose from the radiator was positioned at the very bottom of the overflow tank, any "crud" (dirt, corrosion, gloop) that settles out from the coolant will find it's way back into the radiator, and could possibly clog the overflow bottle hole/hose.

Good point. This totally make sense. I do notice there are some dirt precipitate on the bottom of the bottle. So Putting it sideway instead of bottom can avoid this problem.

But what is the purpose of making a U bend instead of just having the hose connect to the side wall? Hope you have another genius explanation

Originally Posted by marines542

i would change the location of that hole. it would make a difference but why change it when its not causing any problem?

Hmmm...I was just curious why they design a shape like this. Isn't it a little easier to just make it a bottle and put the hose on the side wall? It seems to me that it makes it more complicated to make a U bend if the U bend does not give any benefits.

sorry i meant would not change the position of the hole unless your having some sort of problem

its amazing what a small typo will do

Originally Posted by marines542

sorry i meant would not change the position of the hole unless your having some sort of problem

its amazing what a small typo will do

I see. i am not going to change or modify the reservoir. I am just curious about the logic behind this design.

Originally Posted by Coldwaves

I see. i am not going to change or modify the reservoir. I am just curious about the logic behind this design.

well there didnt have to be any logic...just that some jack as* that makes way to much da*n money was like this is the most efficient designa you should use this. Then another over paid dumb as* gets paid way to much to agree with the jack as* and so thats how the design is put into use in the car

Originally Posted by marines542

It could get there by the slightest amount of air being in the system and getting worked out

Sent from my DROID RAZR using Tapatalk 2

I still keep getting air in that yellow circled area. Every time I saw small air column there, I get it out. But the the air keeps coming back. It seems the air only comes back when the car cools down and then being driven for a while. The car does not overheat or do anything wrong. Oil and oil filler cap look good. No sign of moisture. No performance issue either. I am so confused how the air comes there. Maybe there are still little bit of air trapped in the cooling system and walking out itself?

Originally Posted by marines542

Yea there could be. If I were u I wouldn't worry about it unless u start loosing coolant

Sent from my DROID RAZR using Tapatalk 2

I fill the coolant to the cold line when the car sit overnight last week. The coolant level hasn't changed much yet. It is a little over the coolant line now since the weather is much warmer this week. I will keep watching it.

I have tried get the air out of the yellow circled space maybe 10 times (Maybe total 100 or so air space) and I am amazed how much air can be trapped in the cooling system after a flush. I will keep purging the air out of the reservoir and see whether I can get all the air out and the air stops coming back to the yellow circled area.