The most common signs of a blown head gasket are white exhaust smoke with a sweet smell, coolant loss with no visible leak, overheating, milky oil, and bubbles in the coolant reservoir or radiator. None of these signs proves a failed gasket on its own, so a qualified shop should confirm it with tests such as a combustion-gas test, a cooling-system pressure test, or compression and leak-down tests before a repair is quoted.
What does a head gasket do?
The head gasket seals the joint between the cylinder head and the engine block. It keeps combustion pressure inside the cylinders and keeps coolant and oil in their own passages as they pass between the block and the head. When it fails, combustion gas, coolant and oil can leak into places they should not be, or pressure can leak between neighboring cylinders. The symptoms depend on where the breach is.
Common signs of a blown head gasket
- White exhaust smoke with a sweet smell. Coolant leaking into a cylinder burns off as white smoke. A little vapor on a cold morning is normal condensation; thick smoke that continues after the engine is warm is not.
- Unexplained coolant loss. The reservoir keeps dropping, but there is no puddle under the vehicle.
- Overheating. Combustion gas forced into the cooling system creates air pockets and excess pressure that interfere with cooling.
- Milky oil. A light-brown, milkshake-like look on the dipstick or a creamy residue under the oil fill cap can mean coolant in the oil. Short trips in cold weather can leave some condensation under the cap, so a shop should look at the whole picture.
- Bubbles in the coolant reservoir or radiator with the engine running, a sign of combustion gas entering the cooling system.
- Misfires and rough running, often worst on a cold start, if coolant is getting into a cylinder.
- Loss of compression in one cylinder, or in two cylinders next to each other, often felt as a rough idle and lost power.
- External leaks of oil or coolant at the seam between the head and the block.
How do shops confirm a blown head gasket?
- Combustion-gas test, also called a block test. A tester draws air from the radiator or reservoir neck through a test fluid that changes color if exhaust gases are present in the coolant.
- Cooling-system pressure test. The system is pressurized and watched for pressure loss, which shows whether coolant is escaping somewhere, inside or outside the engine.
- Compression test. A low reading on one cylinder, or on two neighboring cylinders, can point to a gasket breach.
- Leak-down test. Each cylinder is pressurized with shop air. Air bubbling into the coolant points to a head gasket, a cracked head or a cracked block.
These tests also help rule out problems that look similar, such as a leaking water pump, a leaking intake manifold gasket on some engines, or a cracked cylinder head.
Can you keep driving with a blown head gasket?
Driving on a failed head gasket can turn a gasket job into an engine replacement. Repeated overheating can warp the cylinder head or crack the head or block. Coolant mixed into the oil reduces its ability to lubricate, which can damage the crankshaft and rod bearings. Coolant burning in a cylinder can also harm the catalytic converter, and if enough coolant collects in a cylinder, the engine can hydrolock when you try to start it. If you suspect a blown head gasket, stop driving and have the engine diagnosed.
Head gasket repair or replacement engine?
A head gasket repair means removing the cylinder heads, checking the head and block surfaces for flatness and cracks, having them machined if needed, and reassembling with new gaskets and, on many engines, new head bolts. Labor is a large share of the cost, and on some engines the heads are hard to reach.
A replacement engine starts to make sense when testing finds a cracked block or head, bearing damage from coolant in the oil, or high overall mileage and wear. The main choices are a short block, which does not include cylinder heads, and a long block, which does; our short block vs long block guide explains the difference. Our engine replacement cost guide covers what goes into the total, and you can browse the replacement engines and blocks we carry on our engines page. Whichever route you take, follow an engine break-in procedure after new internal parts go in.
Related guides: compression and leak-down tests · coolant leak · engine overheating · white exhaust smoke · Subaru head gasket problems.
More guides: oil in coolant · milky engine oil · losing coolant but no leak · cracked block or warped head · low compression in one cylinder · Cummins head gasket · head studs vs head bolts.
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