Hi there wounder if anyone could help i have a skoda fabia i have a small problem with the expansion tank cap,after the car as been used and left over night the cap  becomes loose so i tighten this back up before using the car and iam not sure why this happens,ive only noticed this when checking the water level

 

any info would be greatfull

There should a rubberized seal on it, sometimes they harden up and allow them to work loose in service, a new cap is about £8 from skoda dealer.

Moved to fabia section.

A whole new tank and lid is only about £15

i couldnt see any rubberized seal  so i will replace the cap thanks guys for your help