Boiling coolant in expansion tank

[mmayhem]

So last Thursday after an oil change I get home from the shop, park the car and get out to see a mess of fluid pour out from under the car. I pop the hood after seeing this and see steam and then notice that coolant is dripping (would pour out if I tried to remove the cap) out of the expansion tank from the vent tube underneath while it is literally boiling. After I let the car cool down I pressure tested the system and there were no leaks at all. I call the shop first thing in the morning and let them know whats going on, drop the car off for the day to let them try and find the problem. Surprise, surprise they can't replicate it no matter what they try. No codes thrown and the expansion tank and cap were replaced about 6 months ago, thermostat checked etc etc.

Fast forward to today and the coolant finally boils again, a full week without doing so and then it happens. Can anyone give me some ideas as to what could possibly be causing this.

2004 with 96.5k for reference.

[yung turbo]

Maybe air is in your system and it needs to be let out. When the car is warmed up, put the heat on max and open the tank and let the air come out for about 5 minutes. then close it, rev the motor a bit then SLOWLY open then tank again to let some more air bubble out for a few more minutes and then close it back and you should be good from there.

[B6JoeS4]

The only thing that can lower the boiling point of coolant is a leak, or too much water in the mix. I'm betting on a leak somewhere. A friend of mine has a c6 s6 that had this same issue. The car passed pressure tests with no leaks, but I finally realized that a small crack in the expansion tank was causing air to escape but only when things got nice and hot and toasty. The coolant expansion tank is one of the most common failure points in the cooling system on every Audi ever made.

Since you see it coming from the overflow outlet on the tank, I'd bet on a bad cap. The cap has a pressure regulator that opens up when things get too hot. Your might just be opening up too early, at normal operating temps.

Also, given that, I bet it isn't boiling. You are just seeing highly pressurized air bubbles make their way into the tank to escape.

[Dcass08]

The only thing that can lower the boiling point of coolant is a leak, or too much water in the mix. I'm betting on a leak somewhere. A friend of mine has a c6 s6 that had this same issue. The car passed pressure tests with no leaks, but I finally realized that a small crack in the expansion tank was causing air to escape but only when things got nice and hot and toasty. The coolant expansion tank is one of the most common failure points in the cooling system on every Audi ever made.

Since you see it coming from the overflow outlet on the tank, I'd bet on a bad cap. The cap has a pressure regulator that opens up when things get too hot. Your might just be opening up too early, at normal operating temps.

Also, given that, I bet it isn't boiling. You are just seeing highly pressurized air bubbles make their way into the tank to escape.

Is it an oem cap or one of those crap aftermarket ones? My tank had a crack last year too, so small I never knew it was there.

[Xyber]

I'd go for the cap as well, fairly common on many VAG cars

[rxbike]

The only thing that can lower the boiling point of coolant is a leak, or too much water in the mix. I'm betting on a leak somewhere. A friend of mine has a c6 s6 that had this same issue. The car passed pressure tests with no leaks, but I finally realized that a small crack in the expansion tank was causing air to escape but only when things got nice and hot and toasty. The coolant expansion tank is one of the most common failure points in the cooling system on every Audi ever made.

Since you see it coming from the overflow outlet on the tank, I'd bet on a bad cap. The cap has a pressure regulator that opens up when things get too hot. Your might just be opening up too early, at normal operating temps.

Also, given that, I bet it isn't boiling. You are just seeing highly pressurized air bubbles make their way into the tank to escape.

I disagree with the statement of lowering the boiling point, understand that what does the cooling is actually the water, not the "coolant"
the antifreze is just that, an agent similar to salt to lower the freezing point, and apply protection to the metal surfaces from oxidation and rusting. If you ran a 50/50 mix vs 70/30 mix, in a cold environment you are keeping it from freezing.

I will agree on the cap sometimes bad cap (did you replace it when you replace the tank) or I personally experienced when I did not screw on the cap tight enough causing leak/spray when boiling over under the hood.

[mmayhem]

So to provide closure to this I took it back to the shop last Friday and managed to get the "boiling effect" going on so they could see it actually going on since they couldn't replicate it for the day they had the car. The tech saw the "boiling" and then squeezed the main hose running to the radiator from the expansion tank and was able to collapse it. Turns out it was the cap as after they put a new one on and I drove home no "boiling" and then a full track day on Saturday with no issues.

[S-PHORIA]

Glad to hear you got it fixed, man!

Literally had the same exact thing happen to me last Friday. Turned out the gasket on the coolant tank cap had crapped out...Replaced it and good as new

[Greedo]

So to provide closure to this I took it back to the shop last Friday and managed to get the "boiling effect" going on so they could see it actually going on since they couldn't replicate it for the day they had the car. The tech saw the "boiling" and then squeezed the main hose running to the radiator from the expansion tank and was able to collapse it. Turns out it was the cap as after they put a new one on and I drove home no "boiling" and then a full track day on Saturday with no issues.

Blah blah.

Fall cruise on the 7th. Be there with you're red hot boiling car.

[B6JoeS4]

I win.

Touche on the water/boiling point thing. I didn't think that through I guess

[jaydogg]

I know this is an old thread, but I'm having similar issue minus the leaking. Noticed my temp gauge going over half way mark and when i looked under hood, saw bubbling coolant in reservoir. Also believe my fan is running more now than ever or I just never noticed it.

[2004B6S4]

check the coolant level when the engine is cold, it's probably low. I'd top it off and bleed out the air and then wait and see if the level goes down again after you drive it. I'm guessing you have a slow leak somewhere, could be one of the radiator hoses or an aux radiator which would drip onto the belly pan, if it's leaking slow enough it may never make it to the ground


i believe b6joes4 was right when he said that coolant only boils when there's a leak. antifreeze actually does increase the boiling point as well as decreasing the freezing point, but the reason a leak makes your coolant boil is because of the loss of pressure (higher pressure = higher boiling point). also if the coolant is low you may see a lot of air bubbles circulating that look like it's boiling

[jaydogg]

Thanks for this info. I'm loading up on as much info as I can bc I'm bringing car to Audi dealer and don't want to get hosed.

check the coolant level when the engine is cold, it's probably low. I'd top it off and bleed out the air and then wait and see if the level goes down again after you drive it. I'm guessing you have a slow leak somewhere, could be one of the radiator hoses or an aux radiator which would drip onto the belly pan, if it's leaking slow enough it may never make it to the ground


i believe b6joes4 was right when he said that coolant only boils when there's a leak. antifreeze actually does increase the boiling point as well as decreasing the freezing point, but the reason a leak makes your coolant boil is because of the loss of pressure (higher pressure = higher boiling point). also if the coolant is low you may see a lot of air bubbles circulating that look like it's boiling

[RPMtech147]

You guys want a mind blower? Have a shop pull a vacuum on the cooling system...If it holds they'll use the vacuum to fill the system and you shouldnt have an issue. If not, I prefer to use a smoke machine to pinpoint coolant leaks, less mess, but and old fashion pressure test works too.

[baurat88]

I think i have a leak now on my 07 S6 in the expansion tank reservoir. I noticed it crusting up on one of the corners so i ordered a new tank and i came today. For those of you that had to get a new tank, did you notice that there is something in there that sounds like a dessicant pack? when i shake it there seems to be some sort of powder on the left hand side of the tank (side closest to the outside of the car) I don't think its in there by mistake because its way in there and you cant see it or access that side of the tank. Just wondering if anyone knows what it is.

[beazman]

I am on the second expansion tank in my S6. I believe that because the engine gets so damned hot that with all that's going on, fine cracks develop in the expansion tank. Now the trick is, when the expansion tank was replaced, did you also replace the cap? I believe so, but it may just be a maintenance item that just gets done as regularly as every 30k due to the extreme heat of the load. Nevertheless, because the radiator temp has a variance of _+ 50 degrees, I am having the radiator replaced just to be safe. Dealer price on the part 500, online 290. I've found a VW spot in town that's about 1/2 the hourly rate of the only Audi dealership in about a 200 mile radius. Sucks to be me...

Anyway, found the part number for my expansion tank cap 8E0121321 which I'll just order a few off eBay and keep an extra handy.