coolant slow leak, expansion tank pressured

[Alchemis]

Hi guys,

i need some ideas for what's going on to my car, it's suffering from coolant leak
- coolant leaks 1 cup every 2 weeks (weekly check), 500 to 600ml every 2 to 3 months (low coolant warning)
- mechanic checked no sign of leaking on the hoses
- mechanic checked pressure test of coolant, no drop

personal findings not sure if it's related to any issue, but certainly coolant related
- everytime the engine cools down next day, i try to open the expansion tank, it's pressurized. sound like opening a new bottle of coke.
- no sweet smell from exhaust, only condensation on cool morning.
- coolant surface doesn't have any oil sign.

i've did searches, but i doubt the mechanic checks up to that level, probably did not check the deeper side
- turbo coolant hose

coolant expansion tank become pressurized like a bottle of coke is my concern what could be the issue.

[Hxngs]

Leak in the tmic from what I have read Around here.

[jagus]

It could be the main radiator that leaks only under pressure.

Try to smell around the engine when the engine is hot

[ENDL3SS]

I had an internal leak in the TMIC that was replaced under warranty

[Alchemis]

it's not the TMIC... it's the main expansion tank that's water missing. and i can't smell anything from the engine bay... mechanic claimed they looked every hose.. no sign of leaking....

what's the probability the tiny leak at the turbo ? perhaps that area not easy to be spoted.

[mapster37]

Have an experienced bmw tech take a look at it

[TopJimmy]

If it’s the engine coolant reservoir you just need to search really good for leaks or white residue buildup. Sometimes the expansion tank can leak at the side seams. I haven’t heard of any other “common” issues. My tank is also under pressure when I open it cold, so that seems to be normal.

[Alchemis]

argh... found the issue.... radiator leak... it was hiding behind the plastic covers.... that explains the mechanic wasn't able to detect it at the first try.

replaced it... all good..

[jagus]

I already told you that it could be your radiator... Happy to hear that all is good now!