Is the Cylinder Head Gasket Blown? 7 Symptoms, Causes, and Choosing the Right Gasket

The cylinder head gasket is the seal between the engine block and the cylinder head that keeps combustion pressure, coolant, and engine oil separated from one another. When a gasket "blows"—that is, when it is punctured at a point or its surface degrades—these three circuits begin to mix. A failure caught early can often be resolved by just replacing the gasket; if left too long, head warping, head cracking, and engine damage may occur.

7 signs of a blown cylinder head gasket

1. Heavy white exhaust smoke: Sweet-smelling white smoke that continues even after the engine has warmed up indicates that coolant is burning in the combustion chamber. Steam seen during a cold start that disappears shortly after is normal.
2. Coolant (antifreeze) loss without external traces: If the level in the expansion tank drops regularly but there are no leaks under the engine or in the hoses, the fluid is most likely burning internally.
3. Overheating: When combustion gases enter the cooling circuit, an air pocket forms, fluid circulation is disrupted, and the engine overheats.
4. Bubbles and excessive pressure in the expansion tank: Continuous bubbling in the tank while the engine is running, an excessively hard upper hose, or fluid spraying out from the cap indicates that exhaust gas is leaking into the cooling circuit.
5. Foam in oil, “mayonnaise” on the oil cap: A light brown, foamy layer on the dipstick or under the oil filler cap shows that water and oil are mixing. Condensation can produce a similar look in vehicles used for short distances, so evaluate this alongside other symptoms.
6. Compression loss and misfiring: When the section of the gasket between two adjacent cylinders blows, compression drops in both; the engine idles roughly and loses power.
7. External leak: Traces of oil or coolant at the junction of the block and the head may indicate that the gasket is leaking from its outer edge.

Why does a gasket blow?

• Overheating: This is the most common cause. The head expands due to a faulty thermostat, water pump, fan, or radiator, crushing the gasket surface and causing a loss of sealing.
• Improper installation: A dirty surface, incorrect tightening sequence, insufficient torque, or reusing stretch-to-yield head bolts prevents even compression of the gasket.
• Warped head or block surface: A warped surface on which a new gasket is installed without resurfacing will cause the same failure to repeat in a short time.
• Knocking and excessive combustion pressure: Incorrect ignition or injection timing, unsuitable fuel, and improperly tuned performance software put excessive stress on the gasket's fire ring.
• Incorrect gasket thickness: If the wrong thickness is chosen, especially in diesel engines where it must be selected based on piston protrusion, compression changes and the piston may approach the head.

How to confirm the failure?

Since symptoms can overlap with other failures, confirmation is recommended before replacement:

• Compression test: Low values in two adjacent cylinders together indicate a blown gasket.
• Leak-down test: Compressed air is fed into the cylinder; air escaping from the expansion tank indicates that the gasket is open to the cooling circuit.
• Exhaust gas (CO₂) test: Uses a color-changing test fluid to show whether there is exhaust gas in the air within the expansion tank.
• Cooling system pressure test: The system is pressurized, and the pressure is monitored to see if it drops over time.

Points to consider during replacement

1. Check the head and block surfaces with a straightedge and feeler gauge; if out of tolerance, have the head resurfaced.
2. Clean surfaces of gasket residue; do not use scrapers or abrasive discs that will scratch the surface.
3. Replace head bolts if the manufacturer specifies stretch-to-yield (torque-to-yield) bolts.
4. Follow the tightening sequence and torque/angle values according to the engine's service information.
5. In diesel engines, measure piston protrusion and select the gasket in the correct thickness category. The thickness class is usually indicated by the number of notches or holes on the gasket tab.
6. Choose a cylinder head gasket set to also renew the valve cover gasket, manifold gaskets, and valve seals that are opened when the head is removed.

How to choose the right cylinder head gasket?

The correct gasket is determined by the engine code, OEM (original part) number, and thickness class if required. The vehicle model alone is not sufficient; different engines and different gasket types may be used for the same model. Products in our cylinder head gaskets category are listed by engine code; you can search by product code (CH...) or OEM number. We explain how to find the OEM number in our finding the correct gasket with OEM number guide.

You can also check our complete engine gasket set installation article for details on the assembly steps.

Frequently asked questions

Can you drive a vehicle with a blown head gasket?

It is not recommended. Coolant loss and overheating can quickly lead to head warping, and water mixing with oil can lead to bearing damage.

Is the head always resurfaced when changing the gasket?

Not always; surface flatness is measured first. Resurfacing is required if the engine has overheated or if the measurement is out of tolerance.

Should the gasket be bought individually or as a set?

When the head is removed, other seals such as the valve cover, manifold, and valve seals are also opened. Therefore, in most cases, a cylinder head gasket set is the more economical and safer choice. We compared the differences in our gasket set vs. cylinder head gasket set article.