Signs of a Blown Head Gasket

diesel engine at cross section

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Outside of an engine repair shop or an auto parts catalog, you’re unlikely to see a head gasket. Though well-hidden, the head gasket, one in an i4 or two in a V6 or V8, performs several critical functions. Because they’re not meant to be removed, head gaskets are extremely resilient, coping with fluctuations in temperature and pressure for hundreds of thousands of miles. If the head gasket fails, usually referred to as a “blown head gasket,” it can result in coolant leaks, oil leaks, or cylinder leaks. Depending on the severity, the results may simply be an annoyance or may prevent the engine from running efficiently, if at all.

What Does the Head Gasket Do?

diesel engine at cross section

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The head gasket is mounted between the engine block, containing the crankshaft and pistons, and the cylinder head, containing the camshafts and valves. Most modern engines use multi-layer steel (MLS) head gaskets, while most older engines used composite asbestos or graphite head gaskets. Some engines may use solid copper head gaskets. No matter what the material, head gaskets perform basically three main functions:

  • Cylinder Sealing – While the engine is running, gasoline cylinder pressures can easily exceed 700 psi, and diesel cylinder pressures can top 2,000 psi. Clamped between the cylinder head and engine block, the head gasket contains those pressures for efficient cylinder compression and expansion. MLS and composite head gaskets usually have a special sealing ring built in specifically for this purpose, while copper head gaskets may require engine block machining to install such a ring.
  • Conducts Fluids – Both engine coolant and engine oil are needed in the engine block and cylinder head, and ports in the head gasket allow for fluid flow between the two. In the cylinder head, engine coolant maintains valve and spark plug temperature, while engine oil lubricates and provide hydraulic pressure, such as for variable valve timing.
  • Contains Fluids – At the same time, engine coolant and engine oil passages may only be separated by a few millimeters. The head gasket prevents these fluids from mixing. Also, combustion gases are prevented from passing into nearby engine coolant and engine oil passages, and vice versa. Depending on the design, the head gasket may have O-rings to improve sealing in certain areas.

Seven Signs of a Blown Head Gasket

a blown head gasket highlighting burn-through between two cylinders

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If a head gasket fails in one of these three functions, the results may or may not be obvious, depending on exactly how the head gasket has failed. Here are several symptoms of a blown head gasket and how you can check them:

  • Cylinder Misfire – This is usually what most people think of when referring to a blown head gasket. In this case, the sealing ring around the cylinder has failed. Because the cylinder can’t build enough pressure, it may fire weakly, if at all. If there are two cylinder-misfires in adjacent cylinders, this usually means that the head gasket is leaking between two adjacent sealing rings. More common with composite head gaskets, it’s not unheard of with MLS or copper head gaskets.
  • External Fluid Leaks – External fluid leaks can be obvious, as you’ll notice low engine oil level or coolant level, as well as oil or coolant residue on the engine, emanating from the seam between the engine block and cylinder head. Barring leaks coming from above this point, such as on the cylinder head or the valve cover, a blown head gasket is a likely cause.
  • Internal Leaks – This may be harder to notice and even harder to pinpoint since you can’t see what’s going on with the naked eye.
    • If oil is forced into coolant passages, you may notice a film that looks much like mayonnaise, deposited on the radiator cap or in the coolant overflow reservoir.
    • On the other hand, if coolant is forced into the oil passages, you may note a frothy substance in the valve covers or the underside of the oil fill cap.
    • If cylinder gases being forced into the cooling system, this can lead air pockets and engine overheating. Bubbling in the coolant overflow reservoir is a sure sign of a blown head gasket.
    • Coolant forced into the cylinder may result in white smoke in the exhaust, usually on startup, because coolant under pressure leaks into the cylinder when the engine is shut down.
    • In extreme cases, enough coolant can force its way into the cylinder to cause a hydrostatic lock, or “hydro-lock,” and permanent engine damage.

Diagnosing and Repairing a Blown Head Gasket

Mechanic checking engine

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If you or your technician suspect a blown head gasket, diagnosis may be time-consuming, as other faults may exhibit similar symptoms. A compression test, leakdown test, and block test may be required to determine if the head gasket is at fault or if it’s being caused by some other fault, such as a cracked block, fuel injection, ignition, valve, or piston ring problem.

While a head gasket kit alone is inexpensive, replacement costs may seem steep, but it requires nearly complete disassembly of the engine, including timing components, intake and exhaust, cylinder head components, and the cylinder head. Machining may be necessary if overheating caused cylinder head warping, adding to the cost of repair. All things considered, it may be worth the cost to resurrect an engine to last another 100,000 miles or more.