HI all

I have a Audi A4 1.9 TDI 2005 , and would like some advice please with issues im having .

Mileage 146K

Basically appx 1 year ago noticed the water level in the expansion tank was low , so i topped it up and thought nothing of it . I have owned the car for 5+ years and have never ever needed to top the water or oil in between services . Its been a amazing car .

Then a month later the coolant light came on and i had to top the coolant up again . It seemed strange , i got the car checked out and no mechanic could find any visible leak . I have changed the cap and expansion bottle to rule them out. Also, I have noticed that even after not using the car for a day, when I remove the coolant cap, there is a lot of pressure in the system.

Anyway fast forward I've been laboring on with it , and now i have to top up every other day .

Also now today i started the car this morning and it ran really rough . i gave it a rev and eventually it revved up and billowed a big white cloud of smoke .

Now emission sensor light is on and car is in limp mode permanently .

Any ideas what issue i might have ??