When hot, the coolant level in the header tank of our 58 plate 09 model year Corsa 1.3 ecoflex goes down ( I would expect it to expand when hot). When cold again, if pressure cap is removed, then level rises again (sometimes air can be seen bubbling up from pipe joining tank to radiator).

Garage supplying car from new has:

1. replaced thermostat, flushed out system. (Coolant then ok ie stayed at full mark in tank, no leaks) for a while.

2. 1st Service, ok.

3. When coolant again going down in tank, slight coolant loss, garage couldn't find leak, but flushed out system. Level then ok for several months, stayed at full level.

4. 3 months or so later, large loss of coolant, going into engine oil (oil too high on dipstick, sludge in oil filler cap). Garage replaced oil cooler and coolant, coolant still being psuhed down a bit in tank.

5. 2nd service, coolant level ok but now level being pushed down again and slight coolant loss.

There is one more year's warrenty to go.

Before it goes back to garage yet again, any suggestions please as to why the coolant is being "pushed down" in the header tank? The leaks may be unrelated to this or it could be the real reason for the other components failing. I've just been told by an engineer that the egr valves on small diesels are likely to fail, letting coolant escape into the exhaust.

There has never been any sign of an external water leak.

Thanks for any advice, despite having had many large and small car problems over many tyears, I'm really puzzled by this.