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Discussion Starter #1 (Edited)
Many members are experiencing coolant loss over time and venting through the reservoir (surge tank) cap. Symptoms include coolant loss and coolant odors both outside and inside the car.

The reservoir vent channel it this plastic piece on the left side of the cap:



If your cap is venting you will see moisture build-up under this channel and/or white and orange deposits. This post only covers issues surrounding coolant loss through this vent channel.

If you are losing coolant vapor through the vent channel the vapor can get into the car through the HVAC intake. This is supposed to be sealed from the engine compartment, but many cars exhibit a gap in the cowl seal right near the coolant reservoir. When coolant vapor exits the tank it is free to pass into the HVAC intake through this gap. Here's a crude cross section diagram of how this happens:



The foam gasket between the cowl cover and cowl tub doesn't seal properly on many cars, allowing coolant vapor into the car. With the hood open you can check your car by shining a flashlight under the cowl near the tank and looking for light through the open grate in the cowl cover. Credit for this information must go to member BlueSkyMontana, who worked extremely patiently with his dealer's service department to figure this out.

I personally had to replace my reservoir since there was a defect in the throat opening. The rough spot in the throat made it impossible for the o-ring in the cap to seal properly. Here's the throat:



And here's the defect as good as I could capture it:





STEP ONE of curing your coolant venting issues will be to inspect the throat of your reservoir and make sure it is smooth and defect free. No o-ring will seal properly to a rough/damaged surface.

Even after replacing my reservoir and cap I was still getting random coolant odors and some slow coolant loss.

At the 2014 Lordstown meet I showed a few people something I was up to. I installed a thicker o-ring in the surge tank cap that dramatically increased the seal between the cap and tank. With this new o-ring I actually ran my surge tank slightly over-filled (coolant level ~1/8" above the cold fill line) and experienced no signs of venting... my tank vent was bone dry.

The problem seems to be with the lower o-ring in the cap taking a compression set (deforming) over time and allowing coolant vapor to escape. In this picture you can see how the round profile of the o-ring can change over time and offer less sealing capacity:



The o-ring on the right is after being removed from the car, the one on the left is the same o-ring fresh out of the package. To be clear, this is the lower o-ring in the reservoir cap, the one at the top of the photo:



Replacing this o-ring with a thicker version works, but replacing it with one made from a superior material also works. The original o-ring is most likely made from Buna-N, a popular material for o-rings, and measures 23mm Inner Diameter and 4.0mm thick.

This o-ring is also 23mm ID x 4.0mm thick but is made from Viton, a slightly stiffer material that resists taking a compression set. This is the o-ring I'm currently using with great success and I recommend as a first try for anyone with venting issues:


Some people have a reservoir that has an extremely loose cap fit, meaning there is very little compression of the cap o-ring as the cap is screwed into place. Here's a video I shot of a brand new '14 LT in my local dealer's showroom:


If you have a very loose cap fit and the 4.0mm Viton o-ring doesn't fix your cap venting (i.e. you still get traces of moisture under the vent channel), you may wish to try a thicker 4.5mm Buna-N o-ring:


This 4.5mm o-ring may be tight to install, so I recommend wiping a little bit of coolant on it before installing the cap for the first time.

I recommend ordering one of each since the Buna-N part is so cheap. In fact, ordering more than one of each is ideal since the shipping cost is likely to exceed the value of the o-rings, and you'll have a spare in case you somehow damage one getting it installed.

Lots of members, many who don't even know who they are, have allowed their coolant levels to drop to a level where they stabilize. As long as this level is safely above the coolant outlet in the bottom of the surge tank this seems to be working OK. The issue here is the reduced pressure in the cooling system will effectively lower the coolant boiling temperature and risk excessive boiling in the hotter parts of the cylinder head and maybe even the turbocharger. Excessive boiling can lead to poor metal temperature control, allowing hot spots to form and increasing the potential for damaging levels of heat cycling of the metal.

This lowered boiling point will be of even greater concern to anyone living at higher altitudes.

The Cruze cooling system should operate properly fully topped up. A reservoir is properly filled when the car is parked on a level surface, the engine is cold, and the coolant level is at the highest rib on the bottom half of the tank. This rib has an arrow pointing to it, indicating it as the cold fill line:



Thanks to @obermd for this photo.

I would also like to point out that I still had coolant odors under the hood after getting my reservoir and cap figured out. I had to replace the small steam hose that connects the reservoir to the water outlet on the cylinder head. This is covered in more detail in CruzeEcoBlueTopaz's thread here:


I hope this thread helps some people rid their cars of coolant odors. Remember also that the Cruze water pump is a popular source of leaks and is now covered under an extended 10yr warranty.

Good luck, and leave your feedback if you try a new o-ring for your reservoir cap. :)
 

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I installed the Viton 4.0mm ring a couple weeks ago along with topping up my coolant tank. I still have a lowering coolant level along with the cabin smell, so I ordered a bottle of dye and a UV flashlight to try and see if it will help locate the source of the leak.

The Viton ring was much better fitting on cap installation than the one in my new tank cap, but I think it's entirely possible since I still have my issues that it's still venting. I don't want to cram on the 4.5mm ring unless it's entirely necessary, though.
 

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To add to this thread, I was having the coolant venting issues as described in the OP. After discussing the issue with Blue Angel, I've been using the 4.5mm Buna-N o-ring since September 2014. I have not experienced any noticeable coolant loss since installing the new thicker o-ring. I haven't checked to see how well it is resisting compression set, but as long as it stays properly sealed I am happy. Great job on the write up!
 

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Discussion Starter #5
...I think it's entirely possible since I still have my issues that it's still venting. I don't want to cram on the 4.5mm ring unless it's entirely necessary, though.
Are you still seeing moisture forming in the vent channel? Is the tank throat smooth? If yes to both, it's entirely possible your cap is not functioning properly. If you have a new cap and both new and old caps are leaking, well, I'm not sure what to tel you other than to try the 4.5mm o-ring in case your reservoir throat is oversized.

Let us know if you make progress.
 

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Are you still seeing moisture forming in the vent channel? Is the tank throat smooth? If yes to both, it's entirely possible your cap is not functioning properly. If you have a new cap and both new and old caps are leaking, well, I'm not sure what to tel you other than to try the 4.5mm o-ring in case your reservoir throat is oversized.

Let us know if you make progress.
I haven't noticed moisture in the vent channel since replacing the tank/cap and installing the O-ring. That's why instead of doing more changes to the surge tank, I'm going for the dye test. I have a suspicion I may have a seperate problem.
 

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Can't check my vent channel since it was REMOVED to add the longer vent tube per GM's wonder smell "fix". They did tape a longer vent tube on, which I have had to reattached a dozen times since the tape can't handle underhood heat or vent moisture. Would have to replace my surge tank because of said "fix", not really happy about them destroying part of my car for something that doesn't do crap to solve the problem.

Wonder if I contacted CS if they would foot the bill? The vent tube that would have lasted the life of my car was replaced with a rubber tube and gorilla tape! Anyone have the part number on the surge tank?
 

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Discussion Starter #8
Anyone have the part number on the surge tank?
13393368

13393368 | RESERVOIR | 2012 Chevrolet Cruze

Doorman has a tank and cap for $16.57 through RockAuto:

https://www.rockauto.com/catalog/moreinfo.php?pk=5927844&cc=1501801

I'm not sure if that's a factory part or not? If it's actually a different (Doorman made) part it might be worth giving it a shot! If so, I would hold off on any o-ring mods until you prove the tank out.

I would give GM Customer Service a shot. Getting your dealer to agree to replace it it probably a waste of your time.

Just a thought, if you wanted to check your tank you could put a piece of clear packing tape over the vent channel opening. Any moisture would surely condense on the underside of the tape.
 

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I ordered the 4.5 mm ring before BlueAngel was working with the Viton ring. Here's a few pointers for installing this larger ring.


1. Put the o-ring on and lubricate it with coolant.

2. Carefully install the cap, if it doesn't thread all the way stop. Leave the cap as tight as it is, and leave the car parked for 24hrs. This gives the o-ring time to conform to the bottle.

3. The next day remove, relubricate and install the cap a little further, until it threads all the way.

I initially could not turn the cap all the way. I was about 3/4 of a turn away from complete tightness. Allowing the cap to sit under pressure in the tank for a day allowed the new o-ring to mold to the tank.

The cap now goes all the way on in one attempt, just like a factory cap would. It's that first tightening that can be somewhat over tight. Give it time, don't force it, or you will be replacing the tank!

Works great. I would agree to start with the smaller Viton O-ring, but given the price of shipping of these rings, I didn't want to reorder them again.
 

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I have one of the larger openings so I need the Buna o-ring. Watch out for this one - it doesn't handle the cold well and will vent after a while when it's cold but the cooling system is starting to build pressure. I need to order some more to deal with this.
 

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I'm glad I ordered a half a dozen. I did notice that even the BUNA 4.5mm ring takes a visual compression really fast. In three weeks the new o-ring visually looks as flat and smooth as the old cap ring combo.

Nice idea with the tape over the vent to see if it's leaking. I'll put a piece of scotch tape on it for observation. I've had really good luck with Dorman parts. Hopefully 6 rings will provide several years of excellent coolant service. If not, I'll be replacing the tank and cap with the DORMAN before I'll ever go back to ACDelco for this part.

Thanks to both obermd and BlueAngel for this fix. My heat at the heater core is much more of a uniform temperature over time. I always knew the coolant was boiling, but I never realized how bad it was.

I had a Cruze rental in Arizona that was always running the radiator fans even on 40F mornings. I think that car may have vented itself to it's death. I feel sorry for whoever buys it as a used car. What an 0-ring will fix.
 

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I ordered a new ring probably about 4 months ago or so, after meeting with Mike (Obermd) at a meet we had here in Denver we were checking out the coolant resevoir because of the amount of coolant I was using and I siwtched to the one he recommended (Can't remember the number off hand) but I haven't had any issues since and I also switched over to the coolant boost from amsoil after I stopped having venting issues.
 

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Slammed - you have the Buna ring.
 

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What he said ^ :D
 

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Discussion Starter #15
I have one of the larger openings so I need the Buna o-ring. Watch out for this one - it doesn't handle the cold well and will vent after a while when it's cold but the cooling system is starting to build pressure.
Mike, I noticed when it was really cold out, like -18C (0F) cold, that the cap would occasionally let off a little steam. I was at first concerned, but then the vent was dry for days afterward until another cold spike. I got thinking...

Thermal expansion varies with temperature; the greater the temperature difference, the greater the expansion difference. On really cold nights the coolant will contract more than normal, sucking more air through the cap (as designed). Then it heats up to the same temperature as normal, but from a lower starting temperature. When this happens it reaches the cap's operating pressure and a small amount of steam escapes.

To clarify, I believe this is the pressure relief in the cap operating as designed, not a leak past the o-ring.

I think this is normal operation and I didn't bother reporting it, though I probably should have. If the car goes back to a dry vent in warmer weather I don't think you've got anything to worry about.
 

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I'm going to order several Buna o-rings later this week and replace the one I have. My coolant is down to the bottom of the arrow and there are no other signs of a leak.
 

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Discussion Starter #17
I'm going to order several Buna o-rings later this week and replace the one I have. My coolant is down to the bottom of the arrow and there are no other signs of a leak.
I confirmed that my water pump is leaking this past week, so that was likely the source of my glacially slow coolant loss, since solving my cap o-rings and replacing my coolant vent tube. For me it was checking my coolant level for the first time in about a month and finding it at the bottom of the tank, and tell-tale signs of coolant below the water pump pulley... time for my car's first dealership visit since new. :(
 

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time for my car's first dealership visit since new. :(
I never thought the day would come, where Blue Angel's baby would see the dealership for service! I noticed that the water pumps are now back in the Rockauto catalog for about $50. For a while they were totally missing, and I was thinking due to the extended warranty on the pump, manufacturers wouldn't be selling/stocking a part that most average people may not buy.

I haven't been in either, and I still have a splash shield that hasn't been cut. I really like how it keeps the bottom of the engine compartment so clean.

The trick with the water pump would be supporting the engine. I'm not a fan of a jack on the aluminum oil pan, but Amazon does have a 3 point engine support bar for $60.

Good luck with your first dealer visit.
 

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Great write up! I bought some rings. I was wondering if you could explain how to interact with this cooling system. If I take the cap off and replace the old ring in there do I have to go through the routine of letting the car run without the cap off before I put it back on or is that only for when I add fluid? I also overfilled my tank a little bit and the level is actually to the joint above your mark in the picture do you think that will be a problem?

thanks for all you info I appreciate it
 
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