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FIX: Coolant Odors/Coolant Loss From Reservoir

116378 Views 117 Replies 39 Participants Last post by  Blasirl
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 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.
Anyone have the part number on the surge tank?

13393368 | RESERVOIR | 2012 Chevrolet Cruze

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

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 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.
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. :(
You don't need to worry about bleeding the cooling system unless you think you have air in it. Just taking the cap off and replacing it is fine, no worries there.

As far as the high coolant level, I wouldn't worry about it. If you see signs of venting and the fluid is too high, that's probably why. The system should loose a little water over time from that steam and the level should drop. If the level is correct and it's still venting, that's when you know something's still not right.
carbon, it hasn't been into the dealer yet! If I could get my hands on a $50 water pump I would probably do the job myself. Chances of that happening are pretty slim for me up here in Canader, where the dealerships LOVE to charge for parts.

My car has to go in for a recall notice (air bag coil), so I might have to bite the bullet and just get the pump done at the same time. Drat.
Blue Angel looks like you increased sales of their o-ring :) looks like it is backordered.
Time to call them up and collect my commission! :D
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The connection that heads to the back must maybe go to the throttle body? I suspect that's heated, you can't see that connection all that well. or maybe somehow it's tied into the heater core hose at the firewall?
The rear output of the water outlet goes straight to the heater core, the throttle bodies on these cars are not heated.
I would just clean the vent (it pops off the side of the reservoir) and then monitor for any signs of venting. The vent isn't a functional sealing component so you won't be disturbing the delicate balance of your cooling system. :)
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I wonder if there's a high demand on the water outlets due to leaks?

Regarding the revised part number, it's likely if the part number changed then something functional changed with the part. If early parts were failing that could be why. Do you happen to have the two part numbers handy? The current part number listed through Cultrag is 55565334:

55565334 | WATER OUTLET | 2012 Chevrolet Cruze

For $21 it includes the coolant temp sensor, seems to be a decent value.
carbon, the connection is just an open hose connection. There's a small circular feature on the reservoir where the hose connects. If you watch it closely, every now and then a few small air bubbles will get trapped in it and spin around, indicating the flow into the reservoir.

It is my understanding that the flow through that line is actually quite high. Someone (obermd I think) posted when they had dye added to the cooling system, they saw it circulate back through the tank shortly after being added.
Actually, the caps on newer cars do click into place. Here's a video I took of a '14 (I think) in a showroom:

I believe all caps have the locking feature, but only the newer tanks have a stop high enough to engage the first of the two raised tabs on the cap, giving that click just before seating.
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Carbon, I believe a small amount of venting is normal when it gets really cold out. The reason being, as the temperature drops more and more at night, the coolant contracts more as well, sucking more air in though the cap. When the engine warms up there is now more air in the system compared to when the car sits over night in warmer weather and the pressure rises enough to vent the excess.

I am using the Viton o-ring still and I noticed the same thing last week when it got bitter cold at night; my tank vent had condensation in it. I dried the tank throat, the vent and the underside of the cap, and since temps have been a little more reasonable the past week or so I have yet to see any more signs of venting.

Once the weather warms up a little in your area, try the same (take cap off and dry everything). If there's still venting going on with temps hovering just below freezing you may need a new o-ring. Note also that the VAST majority of people with a cap venting issue have coolant odors that enter the car while driving. If you are not smelling coolant in the car your cap and o-ring are likely doing OK.

If you have slow steady coolant loss over time the water pump is the #1 suspect. As Rob has pointed out before, a small amount of coolant weeping from the pump is considered normal, but shouldn't require frequent coolant top-ups. Maybe once/twice a year at most, down one bar or so. If you are seeing coolant residue behind the pulley it might need changing.
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In the description of the viton ring listed on the oring store page, it says that it is not resistant to glycol based fluids. Does this include coolant? I always thought it was a glycol based fluid, or at least polyethylene glycol. Are those similar chemicals or different?
Here's a link to the product page:

4mm X 23mm V75 Viton O-ring Black [V4.00X023] : The O-Ring Store LLC, We make getting O-Rings easy!

It states that the Viton o-rings are not compatible with glycol BRAKE fluids.

Here's a link to their fluid compatibility page:

Fluid Compatibility Chart : The O-Ring Store LLC, We make getting O-Rings easy!

If you scroll down the list to Glycols you'll notice that Viton (V) gets a "1" rating, or "Excellent", indicating less than 10% swell.

Also consider that the o-ring isn't really submerged and in constant contact with the coolant, more so with water vapor that condenses at the top of the tank (the actual glycol in the coolant doesn't evaporate much).
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Great info Singray! A couple questions:

1. Did you notice any change in how the car runs, particularly under boost in high ambient temps?
2. Did your check the ignition maps to see if there are any temperature related changes as a result of slightly lowering the coolant temperature?

The reason I ask is, I've noticed my car runs very well right up until about the point where the engine temp maxes out (a minute or two after the gauge gets into its happy place). After that, when it's warm out, I notice some slight inconsistency in power delivery, and only from time to time. The feeling is that the ignition advance is fluctuating, and I know there's lots of talk about "phantom knock" where the engine just seems to pull timing on a regular basis under load. Just wondering if dropping the peak coolant temps slightly has any effect on this?

If there's coolant boiling somewhere, particularly in the head, this could be a source of knock.

I also noticed there are two positions for the relief spring in the coolant cap, the lighter of the two being where it's set on our cars. I wonder if stepping up to the higher spring load in the cap would keep the coolant from boiling (if that's the issue)? I hesitate to simply try it out since I'm not sure what the higher setting is on the cap. Stock is 20 psi, if the higher setting is only 22-25 psi or thereabouts it's probably OK, but if it's higher than that I wouldn't want to subject the cooling system to those high pressures.

If you have another thread where this is being discussed, please direct me to it and we can continue there.

EDIT: Found it:
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