The head gasket is an airtight seal between the engine block and the cylinder head. It provides a means of maintaining pressure in the engine, increasing the efficiency of the internal combustion system. Because the oil and coolant systems also run through the head gasket, the symptoms present when the gasket cracks or is otherwise damaged are variable. It is therefore possible to have a bad head gasket without experiencing all of the following symptoms.
Loss of Power
In order for an internal combustion engine to function at peak efficiency, the engine must be kept under pressure. The head gasket pressurises the engine, forcing more air into a tighter space. This allows more fuel to be burnt, releasing more power. Also, if the gasket is bad, some of the energy from fuel combustion can be released through the broken seal, causing further power loss.
Coolant in the Engine
The coolant pipes for the engine run through the head gasket to keep them under pressure. If the gasket cracks, coolant can leak through into the engine block. Visible symptoms of coolant in the engine are spark plugs that leak coolant when the engine in revved; white smoke; and the car using up more coolant than usual.
Water in the Oil
Short journeys can cause condensation in the oil without a fault in the car. However, water in the sump is a symptom of a bad head gasket when brown foam appears on the dipstick. This is caused by water from the coolant leaking into the engine and mixing with the oil. The water evaporates when the engine is running, but the emulsified oil remains in the dipstick pipe.
Oil in the Coolant
Pressurised exhaust gas can be forced into the coolant system through a cracked head gasket. When this happens, oil particles from the engine will often be blown in with the gas, causing pollution of the coolant system. The result is visible traces of oil in the coolant tank.