Every standard piston/crankshaft car made today has an engine block and a cylinder head which bolts on top. Between these elements is the infamous head gasket. The function of the head gasket is to seal both the compression of the cylinders and the coolant which flows between the block and the head.
Sometimes the compression from the pistons blows a "hole" in the gasket allowing the compression to leak into the cooling system and coolant to leak back into the cylinder. There are a few symptoms of a blown head gasket. The first one is loss of coolant. Coolant can be lost from the leak in the gasket into the cylinder. From there it can go past the piston rings and into the crankcase or it can be forced out of the exhaust system by the action of the piston coming up on the exhaust stroke.
If the coolant enters the crankcase it mixes with engine oil. Of course oil and water don't mix very well but the presence of ethylene glycol (anti freeze) and the agitation caused by the crankshaft and other moving parts can whip the mixture of coolant and oil into a milkshake.
If the coolant is forced out of the exhaust system it is usually heated by the hot exhaust manifold and the rest of the exhaust system. This results in a sweet smelling steam emanating from the tailpipe. The steam is rather persistent, that is it will hover in the air and not dissipate like the normal steam that comes from the combustion process.
Testing an automobile engine for a blown head gasket is pretty straightforward. The first check is to see if there are combustion gasses getting into the cooling system. Take a sample of the coolant and go to the local radiator shop and request that they analyze the coolant for the presence of hydrocarbons. If they are present then it is most likely that you have a blown head gasket.
Another check is to look at the oil of a warmed up engine. The antifreeze in the oil will whip it up into a frothy brown mixture that looks like a milk shake. If it looks like chocolate mousse then you probably have a blown head gasket.
Another check is to fill the cooling system to the brim and remove the radiator cap. Do this when the engine is cool. Start the engine and race it. If there is a compression leak and the gasses are going into the cooling jacket then the gasses will displace the coolant. Since the cap is off the coolant will be pushed out of the radiator. You probably have a blown head gasket.
OK, so we are pretty sure there is a blown head gasket and if we have a straight cylinder engine then there is only one head to remove, but what if it is a V-8 or V-6? How can you isolate the cylinder that is leaking? Remove all the spark plugs. Get a cooling system pressure tester and pressurize the cooling system to specification, normally about 20 PSI. Leave the system pressurized for a few hours. Now lay down some clean paper towel along the cylinder heads where the spark plugs were. Crank the engine over for a few seconds. Since the spark plugs are not in place any antifreeze that leaked into the cylinder will be blown out onto the clean paper towel. Examine the paper towel for the telltale signs of antifreeze and you will be able to tell which is the guilty cylinder.
You can also look at the condition of the spark plugs. Any spark plug that looks different from the rest should be suspect. Of course if you can see antifreeze on a plug then you know that is the bad guy. If the plug looks cleaner than the rest or has a white powder on it then suspect that one.
Most blown head gaskets will require a new gasket be installed. There are very few which will be fixed by addition of a cooling system sealer - you can try it, but be prepared to spring for the big bucks and have the gasket replaced.
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