Coolant issue

[Duc]

Hi all need some advise re 2001 boxster S losing coolant with in the engine not in the oil lot of stream when first start up when at full temperature run ok have had a test carried out on head tank and found product of combustion in the coolant could it possibly be cylinder heads or liners have had issue with No5 re spark plug have to replace regular any ideas.
Many thanks Duc

[JFP in PA]

If you have combustion gasses in the coolant, either a head or cylinder liner is cracked, and the engine has to come out and apart.