What does the head gasket do?
It seals the joint between the engine block and the cylinder head, keeping three circuits separate: combustion, oil and coolant. When it fails — most often after the engine overheats — those circuits begin to mix.
The symptoms
- Coolant loss with no visible external leak.
- Thick white smoke from the exhaust when the engine is up to temperature.
- Milky "mayonnaise" residue under the oil filler cap or on the dipstick.
- Repeated overheating and coolant bubbling in the expansion tank.
- Loss of power and misfires caused by compression loss.
The causes
The primary cause is overheating: a coolant leak, a faulty thermostat, a failed fan or a worn water pump. Repeated temperature fluctuations and an already-warped cylinder head are also contributing factors.
Can you keep driving?
No, or only very briefly. Coolant contaminating the oil destroys its lubricating properties and damages the bearings; overheating warps the cylinder head. Every kilometre driven adds to the repair bill.
Repair or replace?
If the engine has not overheated too severely, the gasket can be replaced — though a flatness check and cylinder head skim are always required. If the cylinder head is cracked or the oil has been contaminated for a prolonged period, a reconditioned engine on an exchange basis, supplied as a bare engine and warranted for up to 24 months, is often the safer option. In every case, fix the root cause of the overheating first.
