The head gasket may be leaking coolant into a cylinder. Check all the spark plugs and look for the one plug(s) that appear greenish or wet. That will narrow it down a little for you. Also, keep in mind, it could also be the cylinder head has cracked between a water jacket and a combustion chamber (cylinder). Has this car overheated, or been low on coolant recently?Both are major repairs since the cylinder head needs to be removed, which also includes the timing belt, manifolds etc. Part costs should be approximately $300 for a new cylinder head and $50 for a new gasket. Labor costs should be about $400 to $600 at a regular shop and about double at the stealership (parts double too).