What Is a Head Gasket?
The head gasket is a sealing layer sandwiched between the engine block (the lower section of the engine containing the cylinders and pistons) and the cylinder head (the upper section containing the valves and combustion chambers). Its job is to seal three separate systems from each other:
- Combustion chambers — where fuel and air are ignited at high pressure
- Coolant passages — which run through both the block and head to manage temperature
- Oil passages — which lubricate the valvetrain above and the crankshaft below
Head gaskets are made from multi-layer steel (MLS) or composite materials engineered to withstand extreme pressure and temperature cycles. When one fails, the consequences range from inconvenient to catastrophic.
Why Do Head Gaskets Fail?
The most common causes in NZ vehicles:
- Overheating: The single most common cause. Aluminium cylinder heads expand at a different rate than cast iron engine blocks. If the engine overheats, the head warps — breaking the gasket seal. Many NZ drivers are unaware their engine is overheating until it's too late.
- Age and high mileage: Gaskets fatigue over time. Vehicles over 150,000 km are at greater risk.
- Pre-detonation (knock): Repeated engine knock creates excess pressure that can damage the gasket.
- Poor maintenance: Neglected coolant changes lead to acidic coolant that degrades gasket material.
- Subaru flat-four engines: The EJ-series Subaru engine (found in Imprezas and Foresters) has a well-documented head gasket weakness, particularly the 2.5-litre SOHC version. Many NZ mechanics are experienced with this specific repair.
Symptoms of a Blown Head Gasket
These are the warning signs — some are clear, others are subtle:
| Symptom | What It Indicates |
|---|---|
| White or grey steam from the exhaust (especially when warm) | Coolant burning in the combustion chamber |
| Milky, creamy residue on the oil filler cap | Coolant mixing with engine oil |
| Coolant loss with no visible external leak | Coolant leaking internally into a cylinder |
| Engine overheating repeatedly | Loss of coolant capacity; combustion gases entering the cooling system |
| Bubbles in the radiator or overflow tank | Combustion gases pressurising the cooling system |
| Loss of power or misfires | Compression escaping between cylinders |
| Excessive white smoke from the exhaust on a warm day | Coolant being burned — not just morning condensation |
| Sweet smell from the exhaust or engine bay | Burning coolant (ethylene glycol) |
Important distinction: White exhaust vapour on a cold morning is normal condensation. White smoke that continues after the engine is fully warm, especially with a sweet smell, is a serious warning sign.
How Is a Blown Head Gasket Diagnosed?
A mechanic will typically:
- Check for combustion gases in the cooling system using a block tester (chemical test) — the most reliable non-invasive test
- Compression test — to identify low compression in one or more cylinders
- Coolant analysis — check for oil contamination
- Cylinder head pressure test — after removing the head
- Visual inspection of the head gasket once the head is removed
How Serious Is It?
Very. A blown head gasket left unaddressed will cause:
- Hydrolock: Enough coolant entering a cylinder can bend a connecting rod when the piston tries to compress the liquid — this can destroy an engine
- Bearing damage: Coolant in the oil strips lubrication from the crankshaft and rod bearings
- Warped cylinder head: Overheating combined with a blown gasket often warps the aluminium head — requiring machining or replacement
- Total engine failure: In severe cases, the engine may need to be replaced entirely
Do not drive a car you suspect has a blown head gasket. The risk of catastrophic engine damage increases with every kilometre driven.
What Does Repair Involve?
Head gasket replacement is major surgery:
- Engine is partially disassembled — intake manifold, exhaust manifold, valve cover, timing components removed
- Cylinder head is removed and sent for machining (resurfacing) to restore a flat sealing surface
- New head gasket installed with new head bolts (most manufacturers require new bolts — they're torque-to-yield and shouldn't be reused)
- Engine reassembled with new coolant, fresh oil, and new timing components if needed
The entire job typically takes 1–3 days at a workshop.
NZ Cost Estimates
Head gasket repairs are expensive primarily because of the labour involved.
| Vehicle Type | Estimated Cost (NZD) |
|---|---|
| Small 4-cyl (e.g. Toyota Corolla, Mazda Demio) | $1,500–$2,500 |
| Subaru EJ-series flat-four | $2,000–$3,500 |
| V6 engine (both heads) | $3,000–$5,000+ |
| If head requires machining or replacement | Add $300–$800 |
| If engine damage (bearings, pistons) found | Potentially $5,000–$10,000+ |
For older, high-mileage vehicles, the repair cost can exceed the car's value. It's worth getting a quote and comparing it to the vehicle's trade-in value before proceeding.
Can Head Gasket Sealant Fix It?
Products like "Steel Seal" or "Bar's Leaks" head gasket sealer are sometimes used as a temporary measure. They can work for minor, pinhole-type leaks on older vehicles — but they are not a permanent fix and do not work for significant failures. A mechanic can advise whether a sealer is a viable short-term option for your specific situation.
WoF Implications
A blown head gasket can cause a WoF failure via:
- Excessive exhaust smoke (white smoke from coolant burning)
- Oil leaks (coolant/oil mixing can cause seals to fail)
- Coolant loss affecting the cooling system's ability to keep the engine safe
When to Book a Mechanic
Book a mechanic immediately if you notice:
- White steam/smoke from the exhaust when warm
- Milky residue inside the oil filler cap
- Your temperature gauge rising above normal
- Coolant level dropping without any visible external leak
- Bubbles in the radiator overflow tank
Do not continue driving and do not ignore these signs.