We have written a lot about the symptoms of a blown head gasket, and you can find our article here: Blown Head Gasket Symptoms. However, all of these symptoms are not always present or obvious; some may be present while others are not, and of course there are varying levels of severity and more subtle head gasket failures may present much less obvious signs. So how does your mechanic actually know if you have a blown head gasket, and how does he test for this?
Your mechanic will often use a diagnostic tool referred to as a block test, sometimes also called ‘sniff test’, combustion leak test, CO2 test, leakdown test or compression test.
A block tester consists of an aspirator bulb fitted on to a glass tube or beaker which is fitted to the top of the radiator. The bulb is then used to draw fumes out from the radiator and into the beaker through a gas-sensitive bromothymol blue chemical. If there is a leak in the head gasket and there are combustion gases (hydrocarbons) present in the cooling system this fluid will then undergo a chemical reaction turning usually green or yellow. If there is no damage then there will be no change in color.
Home use block testers can be bought quite cheaply nowadays, and a search of the internet will yield several results, or any good repair shop should be able to provide this type of diagnosis for a minimal fee.