Coolant Boiling Normal?

Hello Everyone,

Tried searching but couldn't really find anything on 2019 Signature CX-5 model. I recently went on a roadtrip up to Mammoth Lakes. During the trip I drove to several national parks and climbed a lot of hills. Average speed, no stop and go, but car had 5 adults. After about 2 hours of driving up a mountain and pulling over for some site seeing, I heard some major boiling under the hood. I popped open the hood and saw that the coolant was boiling enough to cook some noodles. I left the hood open to cool it off while looking around and when I got back the boiling stopped so we continued on our trip. I did some google searching and believe it may be due to higher elevation but wanted to check with the group here. Never experienced this in my previous turbo SUV and I'm planning another roadtrip next month. TIA!
 
:
13' CX-5 and 16' Mazda6 both Touring w/Tech/Bose
Hello Everyone,

Tried searching but couldn't really find anything on 2019 Signature CX-5 model. I recently went on a roadtrip up to Mammoth Lakes. During the trip I drove to several national parks and climbed a lot of hills. Average speed, no stop and go, but car had 5 adults. After about 2 hours of driving up a mountain and pulling over for some site seeing, I heard some major boiling under the hood. I popped open the hood and saw that the coolant was boiling enough to cook some noodles. I left the hood open to cool it off while looking around and when I got back the boiling stopped so we continued on our trip. I did some google searching and believe it may be due to higher elevation but wanted to check with the group here. Never experienced this in my previous turbo SUV and I'm planning another roadtrip next month. TIA!

Did you temp gauge tell you anything?
 

rsvinylgraphics

2008.5 MGM MS3
Contributor
:
2008.5 MS3
Was the boiling in the coolant reservoir tank? I don't think that is normal I would visit the dealer and get it checked out since this is a brand new vehicle and under warranty.
 

shadonoz

SkyActiv Member
Contributor
:
State of Jefferson
:
2017 CX-5 GT AWD+
Water does boil at a lower temperature at higher elevations [lower pressure].

But boiling in the reservoir doesn't sound good at all. Dealer for sure.
 
Temp gauge showed normal operating temps which is why I continued to drive the vehicle. The boiling was happening inside the coolant reservoir. Doesn't sound like that should be happening so I'll schedule a visit before my road trip.
 
:
13' CX-5 and 16' Mazda6 both Touring w/Tech/Bose
OP should check coolant level while engine is cold. To ensure there's enough coolant to keep temps in check.
 

yrwei52

2016 Mazda CX-5 GT AWD w/Tech Pkg
Contributor
:
Plano, Texas, USA
check if the cap on the coolan reservoir is tight. Dont see what else could be on a brand new car.
Yes, a loose cap would do it.
No, the coolant overflow reservoir is not a sealed container. The cap is not sealed tight like radiator cap. It doesnt matter if the cap of coolant overflow reservoir is loose or not.

OPs coolant boiling problem is almost like those overheating problem happened on some Mazda SkyActiv-D diesels in some countries. No warnings, but the coolant is boiling from reservoir. And eventually cylinder head gasket is gone or the head is wrapped.
 

yugrus

Contributor
:
CX-5 GT
No, the coolant overflow reservoir is not a sealed container. The cap is not sealed tight like radiator cap. It doesnt matter if the cap of coolant overflow reservoir is loose or not.

How's that possible? I mean, how do you connect the overflow tank that is not sealed to the rest of the coolant system that is supposed to be quite sealed indeed?
It is sealed for sure...
 

shadonoz

SkyActiv Member
Contributor
:
State of Jefferson
:
2017 CX-5 GT AWD+
How's that possible? I mean, how do you connect the overflow tank that is not sealed to the rest of the coolant system that is supposed to be quite sealed indeed?
It is sealed for sure...

The pressure cap on the radiator can release excess pressure to the reservoir. When it's not doing that, it is sealed. The reservoir isn't sealed; you just flip the little lid. But the rest of the system is sealed.
 

Avoidin Deer

Zoom Zoom, baby
Contributor
:
Central Virginia
:
2019 CX-5 Reserve
How's that possible? I mean, how do you connect the overflow tank that is not sealed to the rest of the coolant system that is supposed to be quite sealed indeed?
It is sealed for sure...

The outlet for overflow tank is in the radiator neck above where the cap seats.

See it stick out to the right in this pic:

 

shadonoz

SkyActiv Member
Contributor
:
State of Jefferson
:
2017 CX-5 GT AWD+
That was a good read.

But kinda like going on WebMD. None of those things seemed like good news.

Sure is odd for a brand new car to do this. especially with no overheating symptoms.

That's why he needs to take it to the dealer!
 
:
2010 Mazda 5 Sport
How's that possible? I mean, how do you connect the overflow tank that is not sealed to the rest of the coolant system that is supposed to be quite sealed indeed?
It is sealed for sure...

The reservoir is not pressurized.

If the coolant inside the engine/radiator expands past a certain point, the excess pressure/coolant pushes past the radiator cap seal and is vented into the little tube on the neck of the radiator to the non pressurized reservoir.

When the system cools, the reverse happens and coolant sucks into the radiator from the reservoir. This is why you never really need to add coolant to the radiator...just the reservoir. And because the reservoir is not pressurized like the radiator, this is why you can remove the cap and add coolant with the engine being hot.

So the radiator cap, being loose, could cause this...but not the reservoir cap.
 
Last edited:

yrwei52

2016 Mazda CX-5 GT AWD w/Tech Pkg
Contributor
:
Plano, Texas, USA
How's that possible? I mean, how do you connect the overflow tank that is not sealed to the rest of the coolant system that is supposed to be quite sealed indeed?
It is sealed for sure...
shadonoz and Avoidin Deer have explained. And I might add, when pressurized cooling system is heating up and reaches to a certain pressure point, the outer rubber seal on radiator cap with preset spring-loaded pressure value gets pushed up and let expanding coolant escape to the overflow reservoir via the small escape outlet shown on the radiator pictured. When engine is cooled, coolant contracts and the vacuum in cooling system sucking the coolant in the overflow reservoir back through another one-way valve of the radiator cap. The small rubber hose from escape outlet on the radiator filler neck through overflow reservior cap have to be long enough to be submerged under the coolant to absorb the coolant back into the radiator.
 

yrwei52

2016 Mazda CX-5 GT AWD w/Tech Pkg
Contributor
:
Plano, Texas, USA
That was a good read.

But kinda like going on WebMD. None of those things seemed like good news.

Sure is odd for a brand new car to do this. especially with no overheating symptoms.
That's why he needs to take it to the dealer!
OP does need to take his CX-5 to the Mazda dealer. But unless he took a video showing the coolant is boiling, and the coolant temperature gauge was showing normal with no other warnings, otherwise itd be hard to diagnose. Those CX-5 owners with SkyActiv-D diesel in Taiwan had been fighting this similar issue for a couple of years until recently the government forced Mazda in Taiwan to fix the problem for free even if the vehicle is out of warranty.
 

yugrus

Contributor
:
CX-5 GT
Never paid attention to the radiator set-up in Mazda, always thought it was the same as our other car. Took a peek under the hood and yeah, I'm sorry for my confusion!
 

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