First things first!
1. Are you losing coolant from the expansion bottle?
2. Can you find the source of the leak in the engine bay?
3. Is there a smell of petrol or exhaust gas in the coolant bottle?
4. Is the cooling system pressurising, can you see air bubbles entering the expansion bottle from the small diameter hoses?
5. Is there any oil in the coolant or coolant (mayonnaise) in the oil?
6. Leave the car to cool down overnight & then remove the plugs one by one & shine a bright torch into the plug hole, can you see any liquid in the bores? or firmly tape a cotton bud to long pencil or similar and carefully lower into the bore until it touches the piston head and check if it is wet when removed.
Note: Even if the head gasket has gone, it is not a difficult job to do it yourself with some basic tools and a common sense approach to the job, you do not have to be a mechanic to fix a 2.2 head gasket & it will cost in the region of £150 ota in parts plus approx. another £30 - £40 to get the head skimmed if that is necessary. Plus you will get all the help you need on here for free too!
