Hi I have 2015 1.0 ecoboost fiesta ,just noticed when I switched engine off, the expansion tank was boiling like a kettle, but there is no sight of  overheating,  water looks normal in tank no top up etc, oil is fine and nothing horrible coming out exaust,  runs  perfectly fine ,is thus normal dies anyone know  cheers 😀 

Any signs of damage or cracking to the expansion tank?

Is the expansion tank filler cap making a good seal, it’s possible the inner workings of the cap could have failed. 

The coolent can be pumped around the engine for several minutes after engine switch off so the engine esp turbo cool in a controlled manner esp after a spirited drive. Are you sure it's boiling and not just being pumped around the system

Hi mate , not sure  it looks like when a kettle  is about to  boil

On 9/10/2021 at 8:37 AM, Cobcarl said:

Hi I have 2015 1.0 ecoboost fiesta ,just noticed when I switched engine off, the expansion tank was boiling like a kettle, but there is no sight of  overheating,  water looks normal in tank no top up etc, oil is fine and nothing horrible coming out exaust,  runs  perfectly fine ,is thus normal dies anyone know  cheers 😀 

Hiya

I have seen what I think you're referring to in my engine bay having just driven in my mk7.5 ecoboost.  I can't speak for your car 100% but on mine it's the coolant being sucked/pulled through the cooling system to bring the engine temp down after use. Or after 'spirited' use as cjay1 described it.

If you watch it for a minute or 2 it will stop, suddenly, indicating not a gradual decrease in temp (as it would if boiling) but that the cooling mechanism has ceased and that engine temp is now within 'stopping' range.

If you check your dash, you'll see engine temp is indicated to be within normal guidelines throughout.

Sorry if my powers of description are not good, I've not long got up and my brain's not functioning at usual capacity 🤪

Thanks mate  I totally get what your saying  ,I appreciate your  reply ...

Perfectly normal. 👍🏻

I saw this a few days ago for the first time. I lifted the bonnet to check a noise and i to assumed the coolant was boiling. I took car to local dealers and was told they could find nothing amiss. Thanks for the post it has set my mind at peace.