Head gaskets are the most over stressed gaskets in an engine, because they have to deal with sealing a multitude of things at the same time. They simultaneously seal oil, coolant, and engine compression away from each other and the outside.
When do they blow, they can fail in a number of different ways, and each failure will result in different symptoms:
Coolant to outside leak
This one’s not as easy to spot as some of the others, especially if the leaking coolant dribbles to the ground while driving, or evaporates, and leaves little trace of where it is leaking. If a head gasket fails between a water passage and the outside of the engine, the cooling system will eventually run low on coolant. When the coolant level drops too low the engine will overheat.
Compression leak to oil galleries
If the head gasket fails between the cylinder and an oil gallery, compression will enter the oil system and pressurize the crankcase. Hot compression gasses will compromise the lubrication of vital bearings, such as the crankshaft main bearings.
The same failure will also allow the piston to suck oil into the cylinder, where it will be burned off as part of the combustion process. This will eventually cause a drop in engine oil levels, and will also cause blue smoke from the exhaust from burning oil.
Water to oil leak
If the head gasket fails between an oil gallery and a water passage it will allow the engine oil and coolant to mix. This will result in contaminated oil (the white milky sludge or “milk shake” associated with a blown head gasket) and a compromise the cooling system.
Compression leak between cylinders
If the gasket fails across the fire rings it can allow the compression to leak from one cylinder to the next. This causes a loss of compression, and sometimes allows exhaust gasses back into the intake, depending on cam timing. This sort of head gasket leak typically results in rough running, misfires, and a loss of power.
Compression to outside leak
This type of failure is not as common as the others, but some engines are prone to the head gasket leaking between the cylinder and the outside of the engine. This allows some of the compression to escape out of the engine, causing a ticking noise at idle that can sound like an exhaust leak. You will also experience rough running, and a loss of power.
Oil to outside leak
This one is fairly easy to spot, and if the head gasket fails between an oil gallery and the outside of the engine you will be able to see the oil leaking down the engine.
While not as immediately dangerous as other types of head gasket failure it will still require repair as it will only get worse over time. Depending on how the engine is designed, this sort of leak may result in less oil pressure getting to an overhead camshaft. Keep an eye on the engine oil level and don’t let it run low.
Compression leak to coolant
If the head gasket fails between the cylinder and a water passage it will allow exhaust gases to enter the cooling system, resulting a pressurized cooling system and overheating. Often times this presents itself as a radiator hose blowing off its fitting.
This, in turn, can result in a damaged radiator, hoses, and other cooling system components. This leak also results in a drop in coolant level, causing overheating.
The same failure will also allow coolant to get sucked into the cylinder, where it is then burnt off as steam, or white smoke out the tailpipe, and a sweet smell. The steam that is burnt off can also cause lead to damage in the catalytic converter as it exits through the exhaust system too.