Coolant Reservoir Cap

[DAC350]

Can someone please confirm on your S3 something for me. After driving to normal temps, turn off the engine and pop the hood. Does the cap on the coolant reservoir make a hissing sound? I know it's a pressurized system, but I didn't notice the noise until recently. It's not terribly loud, but you can definitely hear it when you put your ear to it.

Thanks.

[n8vs3]

There is a low level mechanical pumping sound after the engine is shut off, but it doesn't appear to be coming from the reservoir area

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[DAC350]

I hear that, but my reservoir cap definitely makes a hissing sound. Should it be releasing pressure from the cap? I would think that would lead to evaporation long term.

[esss 3]

it's a closed system, you should not be hearing a hiss, unless it is building too much pressure and releasing it, but that would be another issue in and of itself. check the O-ring gasket on the cap and see if it is compromised in any way. if it still continues take it to the dealer where they can test both the cap and cooling system. good luck and please come back and let us know what was the issue !!!

[DAC350]

The O-ring, that's a good idea. I actually took it to the dealer today for a pressure test, but no leaks were found. I've been losing coolant very slowly and now I'm thinking it's evaporating from the cap if that is possible. I just cleaned off the o-ring, next time I heat up the engine I'll see if it still makes a hissing noise.

[DAC350]

Well, I cleaned off the o-ring and so far the hissing sound has stopped. I'll know in a few days if the slow coolant loss is gone, thanks for the heads up.

[esss 3]

glad I could help. hope it solved your problem
you brought it into the dealer, they checked for leaks, found none which was fine, BUT they did not check the condition of the cap and O ring gasket...what's up with that ??? smh.
it might be possible to lose some coolant if the coolant tank was open to the air, but how much are you talking about and over how long a period of time ?
you know some S3's have been prone to having the coolant level drop on its own with no leaks, due to an air bubble within the cooling system that slowly(over thousands of miles) bleeds itself out. there is even a TSB for it !!!
so keep an eye on the level from time to time. the level drops so drastically that it may even trip the low coolant warning light.

[DAC350]

I'll keep an eye on it. It went from 1cm overfilled to below minimum in 3 weeks. I'm hoping it's just been venting from the cap the entire time since the dealer didn't find any leaks.