Air In Coolant System?

[JAPCAR]

Avensis 2006,2.0, D-4D, 85000miles.

I red several threads about HG issues, that is why I check the coolant level from time to time. Usually the level was around FULL mark. Few weeks ago I saw it was around LOW. Next day again around FULL. I started to keep an eye everyday: Recently level sometimes is 2mm above LOW sometimes it is 2mm above FULL, never on middle levels.I know level should be checked at cold engine but cold/warm engine gives no more 2-3mm difference...(L-F is about 15mm)

My first hypothesis is there is an air in the system and sometimes it is above coolant in the expansion vessel , sometimes is somewhere inside in any air pocket. I can't explain how the air moves...

I am scare to think about second hypothesis- very small HG gas leaking which makes air(gas) pocket somewhere and level goes up. When gas escapes over cap, level drops until new gas pocket forms...

I red threads about coolant bleeding but seems no air vents to drive away the air. How to try expel air?

Any suggestion how to check both hypothesis? I don't think level change is normal.

[Druid Boy]

If you aren't losing coolant I wouldn't worry too much. Keep an eye on it all the same but try not to worry too much at this stage.

[mcorban]

If your concerned about exhaust gases getting into the coolant system, get it checked with an exhaust gas analyser, at the coolant bottle. A garage could do this or you can get a small tester in Halfords.

Normally, you would have a major buildup of pressure in the coolant bottle if it is and it would blow the coolant out of the system when you open the rad cap.

As above though, i wouldn't worry a whole lot about it.

[JAPCAR]

No visible signs of blown coolant but obviously real level is "L". No big pressure in the system- when open rad cap (warm) just short "FSSSS"... I will keep an eye...

Would you give me a link for "small tester in Halfords", I didn't find.