Does any one know how to actually test to see if head gasket is bad? I know what symptoms look like but I want to know how does a shop test it.

 

run the car and pop the rad cap. you'll see bubbling in the radiator fluid if there is a bad head gasket.

 

do a compression test or a leakdown test

 

There is no one test that will find all blown head gaskets. There are a couple of different tests you can do. You can do a compression and leakdown test if the engine is running rough and you suspect that it has a blown head gasket because you have oil in the coolant or vice-versa. But head gaskets don't always blow that way. Sometimes it is blown between a coolant passage and a cylinder, sometimes its an oil passage and a cylinder, sometime a coolant or oil passage can blow to the outside of the engine and you have a leak onto the ground. Using a cooling system pressure tester you can put the system under pressure, pull the plugs and see if coolant leaks into any cylinders. You can put compressed air into the cylinders and then look in the radiator to see if there are bubbles. You can use a four-gas analyzer to see if there are hydrocarbons in the coolant. You can use a block tester to see if there are hydrocarbons in the coolant also. It all depends on the situation and what you suspect.

Is there a specific reason you suspect you have a blown head gasket?

 

x2 tucker. excellent response. I find the most common leak to be from combustion chamber to cooant passage. Usually from overheated condition. Of course after shutoff the process reverses direction from pressurized cooant system into cyl. After someone works on a head any of the others are just as likely.