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Blown Head Gasket Symptoms, Causes, & Cost

The head gasket plays a major role in internal engine combustion and the optimal performance of your vehicle. It is what seals coolant and oil in their respective reservoirs and helps maintain appropriate compression pressure necessary for engine power. Without it, fluid contamination is bound to happen, upsetting the closed nature of the engine, fuel, and cooling systems.

Recognizing blown head gasket symptoms is essential to avoid on-road complications and irreparable engine damage. Telltale signs such as engine overheating, visible leaks, misfiring error codes, and loss of power should be identified early on and immediately addressed.

Here is a list of the 11 most prevalent symptoms of a blown head gasket:

  1. Engine overheating
  2. Gradual coolant loss with no visible leaks
  3. A visible coolant leak in the exhaust
  4. Oily residue in the coolant overflow bottle
  5. Misfiring fault codes
  6. Poor fuel economy
  7. Milky oil
  8. Fouled spark plugs
  9. Rough idling and loss of power
  10. Unusual engine sounds
  11. Failed catalytic converter and emissions

Blown head gasket symptoms are easily recognizable but often indicate engine problems other than what its description implies. It also does not help that it takes about six hours to a few days to nip the causes of head gasket failure in the bud.

Given the painful wait, it would do well for car owners to identify “What causes a blown head gasket?” and its symptoms in the nick of time. If you like to do the same, stick around and continue reading today’s guide.

Common Symptoms of a Blown Head Gasket

Check Engine Light on Car Instrument Panel

1. Engine Overheating

An overheated engine is both a trigger and the aftermath of a blown head gasket. How? Well, engines that are too hot can cause the cylinder head to swell until it crushes the head gasket (usually at the thinnest point of the latter, between the cylinders).

The swollen cylinder head then creates a crack in the combustion armor that inadvertently turns into a path for coolant leaks and combustion gases to pass through. Once coolant escapes through this path, the vehicle’s cooling system becomes inadequate, causing engine overheating.

2. Gradual Coolant Loss With No Visible Leaks

Losing coolant but without signs of a leak anywhere is one indication of a blown head gasket. This mysterious loss occurs when there is a failing head gasket between one of your power mill’s combustion chambers and the coolant channel linking to it.

In this scenario, the coolant leaks through seams between the cylinder head and engine block. In turn, this leak causes your car’s engine to consume coolant.

This inadvertent consumption is the reason behind dipping coolant levels that do not leave any visible puddle. It also results in engine overheating.

3. A Visible Coolant Leak in the Exhaust

Another telltale sign of a head gasket blown out of spec is coolant leaking through the tailpipes in either liquid or vapor form. This scenario happens when coolant travels from below the intake or exhaust manifold and into the cylinders (as a result of the gasket-induced leak) and is only possible when your engine is completely warmed up.

What is tricky about blown head gasket symptoms like this is that they do not trace back to head gaskets alone. Depending on its placement in your engine bay and other coolant passages within close proximity, an exhaust leak may or may not be a definitive indicator.

Thankfully, you can add UV dye to the coolant during diagnosis. And with the use of a UV light, you would be able to positively identify whether or not the source of the leak is from the suspected head gasket.

4. Oily Residue in the Coolant Overflow Bottle

The scenario above is just one of the many blown head gasket symptoms relating to your vehicle’s cooling system. An addition to the ‘cooling-related’ list is spotting an oily or sludge-like residue in your coolant overflow bottle.

Coolant leaks resulting from thinned-out head gaskets are again to blame for this. And mind you, the residue is not limited to the overflow bottle.

You can also see it in the coolant reservoir or atop the radiator. Compromised head gaskets even allow coolant to find its way into the combustion chamber!

5. Misfiring Fault Codes

One of the more obvious signs that your head gasket might be compromised is an illuminated Check Engine Light accompanied by a DTC or fault code indicating a potential misfire.

Typically, misfires are linked to the ignition or fuel system. But because spark plugs can get fouled due to a blown head gasket, these codes should be part of what you need to look into.

A trusty OBD-II or DRB-III diagnostic tool like a FOXWELL GT75TS Bi-Directional Scan Tool (view on Amazon) should tell you the problem. However, it also pays to know possible gasket-related codes beforehand.

Misfiring Codes

Here are some of the misfiring codes your scanner may throw out while you diagnose the problem:

P0100—P0104Mass airflow sensor
P0171—P0172Lean or rich fuel mixture
P0200Fuel injector circuit malfunction
P0300Random/multiple cylinders — misfire detected
P0301Cylinder 1 — misfire detected
P0302Cylinder 2 — misfire detected
P0303Cylinder 3 — misfire detected
P0304Cylinder 4 — misfire detected
P0305Cylinder 5 — misfire detected
P0306Cylinder 6 — misfire detected
P0307Cylinder 7 — misfire detected
P0308Cylinder 8 — misfire detected

For any of these error codes, I highly recommend examining the spark plug for coolant fouling. Follow it through with a compression or leak-down test performed on the affected cylinder.

You may also ascertain whether or not a leaky head gasket is at fault by pressure testing the cooling system.

6. Poor Fuel Economy

Reduced gas mileage is a tertiary sign of a blown head gasket since it is caused by a fouled or defective spark plug. It is not easily noticeable, though, as fuel economy only reduces dramatically when car owners continue to drive with bad spark plugs against their better judgment.

7. Milky Oil

In this scenario, the head gasket blown is situated “between an oil gallery and a water passage.”

Contamination happens, too, except that coolant seeps its way through the oil reservoir and contaminates the latter. The result is a sludge formation that looks like a milkshake when inspected.

However, milky oil is not automatically indicative of a blown head gasket. Typically, the latter blows to the cylinder and manifests as a leak from the tailpipes or an overheating radiator.

It could also mean lots of condensation (usually from overnight temperature drops) has formed inside your power mill.

8. Fouled Spark Plugs

I extensively covered this topic in another article on bad spark plug symptoms. And wouldn’t you know it — it is related to blown head gaskets too!

Head gaskets leading to spark plug fouling are unsurprising at this point since coolant leaks that reach and burn inside the combustion chamber leave deposits on the electrode and around the plug’s ground strap.

Note, however, that this outcome is not exclusive to head gaskets. If anything, you will need to look more closely into the severity of plug contamination and consider other symptoms.

9. Rough Idling and Loss of Power

Somewhat related to misfiring and fouled spark plugs, these two scenarios could occur before or after getting relevant fault codes from your diagnostic scanner.

To be more specific, rough idling and power loss happen when blown head gaskets cause your engine to lose compression.

The bottom line is that blown head gaskets create gaps for coolant and combustion gases to seep through. These openings adversely affect your vehicle’s fuel, ignition, and cooling systems and result in fluid levels and cylinder compression to be out of spec.

10. Unusual Engine Sounds

Engine knocking results from detonation, one of the blown head gasket causes (more on this later). Coolant mixing with fuel and entering the combustion chamber creates surplus vapor inside the latter.

Customarily, pinging or knocking only becomes apparent during acceleration — when excess vapor catches fire and detonates.

11. Failed Catalytic Converter and Emissions

Like engine pinging, a failing catalytic converter is farther down the pipeline of blown head gasket symptoms. And the only time that circumstances would reach this extent is if early signs of head gasket failure were deliberately delayed or ignored.

It takes a while for a coolant-oil mixture to turn into goop and fill the honeycomb structure of the catalytic converter. The fact that it finally does only means that issues with coolant leaks and sludge buildup were addressed too late (if never addressed at all).

Seeing thick white, blue, or black smoke exiting your tailpipe only ascertains this case. And if this steam decides to become visible in the middle of your engine smog check, you are highly likely to fail your emissions test.

Causes of Head Gasket Failure

Cylinder Engine Block

Damaged Engine Block or Cylinder Head

A car with an unresolved overheating issue will likely have a warped block or head due to thermal stress. Likewise, the engine can also crack in freezing temperatures (especially true for vehicles with inadequate anti-freeze, left out of the garage overnight during winter).

In either scenario, excessive heat or cold creates a crevice in the engine block or cylinder head, consequently affecting the sealing capability of the head gasket. Experienced car owners know all too well that what comes after is leakage and head gasket replacement costs.

Detonation Damage

One thing that can lead to blown head gasket costs is pre-ignition or spark knock.

Several things can cause this phenomenon — the most common is a “sharp spike in combustion chamber pressure” that overloads and eventually damages the gasket armor surrounding the cylinder head (a result of carbon accumulation, burn-through, and loss of compression).

To avoid this situation, steer clear of fuel variants with mislabeled or incorrect Octane ratings (especially when your engine has a high compression ratio). Doing so will cause the fuel to ignite before spark ignition setting it off and leading to engine knocking or pinging.

Other detonation triggers include:

  • A non-working EGR system
  • Over-advanced ignition timing
  • Hot spotting (typically of an exhaust valve or spark plug)
  • An overly lean air-fuel mixture
  • Cooling system issues

Poor or Weak Gasket Design

A good example of this culprit is Subaru’s head gasket problems involving its 1996 to 1999 Forester, Impreza RS, Legacy GT, and Outback models with 2.5-L engines. The OEM fitted these vehicles with composite gaskets, which were not robust enough to prevent coolant and oil from mixing.

Situations like this do not automatically mean that the manufacturer is playing cheapskate. After all, vehicles freshly rolled off the line are subject to extensive durability tests before they make it to the showrooms.

Unfortunately, these tests are not fool-proof when assessing the durability of a head gasket design. Often, issues begin to surface only after engine components have been in use for several years.

When this happens, drivers can only hope that their engine is still under warranty. Otherwise, it will be a guaranteed expense.

Cylinder Head Construction

Another probable design flaw leading to a blown head gasket traces back to the cylinder head itself. Sometimes, cylinder heads are just not flush in loading the gasket. This hard-to-seal attribute is observed in some Toyota Tacomas and 4Runners.

In other cases, the armor around the combustion chamber is at fault. This scenario is usually true for engines with an aluminum head and cast-iron block.

Because the composition of the connecting parts is not the same, the back-and-forth scrubbing of the cylinder head against the engine block results in metal fatigue (weakening of metal parts in machines, vehicles, or structures due to repeated stresses).

A head gasket with a stronger combustion armor and built-in lubricity (non-stick, anti-friction coating) like this aftermarket FEL-PRO HS 9227 PT-1 Head Gasket Set (view on Amazon) is needed for power mills with similar construction to mitigate metal fatigue caused by thermal expansion of aluminum parts.

Excessive Head Motion

Four-wheelers with older 3.0-L and 3.4-L power mills are prone to head gasket failure due to excessive head motion — mainly due to a weaker combustion armor. It also does not help that these vehicles mostly have composite gaskets instead of multi-layer steel.

The fix for this is similar to the one above — procure an aftermarket option with improved combustion armor and non-stick coating.

Battery Corrosion

If you discover a head gasket blown to only one side of your vehicle, chances are you have something going on that side of the engine.

Naturally, your power mill is surrounded by other components. And if those components go bad or develop corrosion, the resulting chemicals can attack the part of the head gasket closest to it.

Inadequate Cooling System

A cooling system not up to snuff is both a cause and an indicator of a compromised head gasket. As a trigger, low coolant levels aid in engine overheating, dampening the sealing capability of gaskets.

As an aftermath, a head gasket blown due to any of the causes mentioned here will consequently affect the performance of that vehicle’s cooling system.

These scenarios aside, cooling inadequacies can also be attributed to a faulty thermostat and non-adherence to scheduled maintenance and negligence.

A filthy cooling system (or one with a damaged fan, insufficient anti-freeze, or low coolant levels) is bound to malfunction, jeoparding other car parts and components that rely on it, such as the head gasket.

How Long Do Head Gaskets Last?

Red Jeep Wrangler Driving on the Road

The timing of a blown head gasket is primarily contingent on the gasket material. Depending on its inherent properties, one type of gasket may last longer or shorter than others.

External factors like cylinder head/engine condition, temperature/pressure fluctuations, and driving behaviors also affect the longevity of head gaskets. So does the use of high-quality or sub-par fuel and lubrication.

It would serve you well to get acquainted with the different head gasket types. Knowing which one your car uses should give you a pretty good idea of how long it would last and what factors would adversely affect its service life.

Different Head Gaskets

Multi-layer Steel or MLS

This type has two to five layers of thin, spring stainless steel sheets, joined together by an elastomer or strong adhesive and small brass rivets. Its surface is coated in high-performance compounds for added protection and improved sealing.

Multi-layer variants are the strongest and most common head gaskets, withstanding considerable pressure from the engine block and cylinder head.


Instead of steel, these gaskets are cut from solid copper sheets. However, the trade-off for this material is a more tedious installation process requiring special equipment called an O-ringing machine.

Nonetheless, copper head gaskets are well-loved for their superior strength, malleability, and leakage protection — making them a top choice for performance vehicles.


This gasket variant is constructed from graphite composite laminates, widely used in pre-1990 car models. It is compressible, oil- and coolant-resistant, and perfect for nitrous and turbo applications.

Head gaskets of this type are difficult to deform but can be easily trimmed to fit modified parts.

While its composition is strong, it does not provide the same level of leakage protection as the first two gasket types. Because of this flaw, modern four-wheelers rarely use head gaskets of this material.


Made from elastomer-based materials like cellular urethane, rubber, and silicone foam, these head gaskets are highly compressible but capable of springing back into shape without damage to their elasticity. They are also less prone to becoming cracked or brittle regardless of exposure to volatile temperatures.

Compared to the other gasket types, they offer enhanced sealing and improved thermal conductivity while exhibiting strong resistance to acidic or corrosive fuels and chemicals. On the downside, they are not as durable as other gasket variants despite their appropriateness for many applications.

Ideally, head gaskets should last up to 200,000 miles or the lifetime of your engine — provided proper upkeep and service schedule are stringently observed. Otherwise, poor driving practices and haphazard gasket installation will shorten your head gasket lifespan.

Blown Head Gasket Replacement Cost

On average, replacing a blown head gasket would cost between $1,200 and $2,000 (including parts and labor). These values may still go up, depending on application and severity of coolant contamination — if it has been mixed with oil and damaged engine bearings.

head gasket sealer (view on Amazon) is an inexpensive alternative to a full-blown head gasket replacement (provided the issue is spotted in its early stages).

It is a liquid sealer poured into the radiator and used to patch minor leaks, thus extending the life of your head gasket. However, it is more of a preventive than a remedial measure — meaning it would not work if the head gasket is already damaged.

Replacement Tips

Never reuse the original head bolts when replacing gaskets. They are TTY (torque-to-yield) head bolts incapable of maintaining torque, as they stretch when tightened.

When installing a blown head gasket replacement, carefully check the flatness of the cylinder head and the engine block with a straight edge and feeler gauge (view on Amazon).

Engines with aluminum heads should have a flatness of 0.002 inches (0.05 mm) in all directions. If out of spec, the head or block will require resurfacing.

If the engine block or cylinder head has been resurfaced, ensure the surface finish is up to spec. Remember that MLS gaskets work only on surfaces with a smoother finish (approx. 20 micro-inches), while composite gaskets can tolerate up to 50 micro-inches (or more).

Also, ascertain that the head bolt lengths do not bottom out in blind holes, as this will yield little to no clamping force on the gasket head, resulting in a leak.

You may need to install hardened steel washers under the head bolts or use a copper head gasket shim to compensate for resurfacing.

Do not use any head gasket sealer unless recommended by the gasket manufacturer. If OEM-advised, follow application instructions to the T.

Engines with conventional head bolts require cleaning and light oiling before installation. Otherwise, you will get false torque readings once the bolts are tightened.

Ensure the head gasket replacement is loaded evenly with an accurate torque wrench and angle gauge (when applicable). Skipping this step is costly since uneven loading may lead to poor sealing and leaks.

Lastly, add ample cooling system sealer to the coolant and open any bleed valves when refilling the cooling system. Doing so helps ensure there are no air pockets to cause engine overheating.

Conclusion — 11 Blown Head Gasket Symptoms, Causes, & Cost

To recap, here are the 11 most common blown head gasket symptoms:

  1. Engine overheating
  2. Gradual coolant loss with no visible leaks
  3. A visible coolant leak in the exhaust
  4. Oily residue in the coolant overflow bottle
  5. Misfiring fault codes
  6. Poor fuel economy
  7. Milky oil
  8. Fouled spark plugs
  9. Rough idling and loss of power
  10. Unusual engine sounds
  11. Failed catalytic converter and emissions

Ultimately, it is ill-advised to continue driving with a blown head gasket. Being a closed system, your power mill requires well-functioning gaskets to run optimally.

Your vehicle may manage to tread a few more miles when pushed beyond its limits — but at the expense of your wallet and potential engine damage.

A well-maintained cooling system will go a long way in preventing these symptoms from surfacing, and so would regular inspection of relevant vehicle components. However, these good practices would not be able to keep these troubles at bay forever.

Like any other component, your head gasket will eventually deteriorate. And when it does, go to the nearest auto repair shop or do the gasket replacement yourself.