The first symptoms of a head gasket that is failing are usually a slow loss of coolant with no visible leaks, engine overheating and/or a Check Engine light with cylinder misfire codes. The Onboard Diagnostic II (OBD II) system should detect any misfires that occur due to a loss of compression or spark plugs fouled by coolant seeping into a combustion chamber. So if you find a cylinder misfire code, check the spark plug for coolant fouling, and do a compression or leak-down test on the cylinder to find out what’s going on. Internal coolant leaks caused by a leaky head gasket can usually be diagnosed by pressure testing the cooling system.
Regardless of the cause, a head gasket failure is bad news for the vehicle owner because it will require replacing the head gasket. Depending on the application and how much labor it requires, installing a new head gasket can easily cost up to $1,500 or more. And if coolant has diluted the oil and damaged the bearings inside the engine, your customer could be looking at a considerably higher repair bill.
Many motorists are puzzled by head gasket failures because they don’t understand the causes. Regular maintenance and oil changes can’t prevent a high-mileage head gasket from failing, but it can increase the odds of detecting other problems that may lead to premature head gasket failure.
For instance, one reason head gaskets fail is because of engine overheating. If the engine gets too hot, the cylinder head can swell to the point where it crushes the head gasket (usually between the cylinders because this is the thinnest point). The extruded material and/or cracked combustion armor then provides a leak path for coolant and/or combustion gases.
Replacing the head gasket will cure the leak, but the underlying cause that made the engine overheat in the first place also needs to be diagnosed and repaired otherwise, the newly installed gasket will suffer the same fate the next time the engine overheats.
The cooling system on many late-model imports is just barely adequate (to reduce weight and cost), so it doesn’t take much loss of cooling to make the engine run hot and overheat. A low coolant level, a dirty cooling system, a cooling fan that isn’t working correctly, or overloading or overworking the vehicle can all be contributing factors.
On 1998 to 2000 Volvo S40 models with the 1.9L turbo engine, the engine cooling fan resistor may fail, preventing the cooling fan from working. This may cause the engine to overheat at low vehicle speeds, resulting in head gasket damage if the engine gets really hot. Replacing the head gasket will fix the coolant leak, but it’s important to make sure the cooling fan is working correctly and comes on when needed so the engine does not overheat again. The fan resistor is located under the front engine splashguard, just below the fan motor. A new resistor (P/N 30644121) can be spliced into the wiring if the original resistor is defective.
If a head gasket has failed because of overheating, be sure to check the coolant level and condition; the concentration of antifreeze in the coolant (too much antifreeze in relation to water reduces the ability to transfer heat); and the radiator (obstructions, leaks, etc.), hoses, thermostat, water pump, cooling fan, EGR system, belts and the exhaust system to determine why the engine overheated. In some cases, an exhaust restriction (clogged converter) also can make an engine run hot.
How can you tell if overheating caused the head gasket to fail? A head gasket that failed because of overheating or a hot spot will be crushed and measurably thinner in the damaged area when checked with a micrometer. By comparison, a gasket that has failed due to detonation or pre-ignition will usually have cracked armor around the combustion chamber, which leads to burn-through.
Another reason head gaskets fail is because of damage caused by detonation (spark knock). Detonation causes a sharp spike in combustion chamber pressure, which, over time, can overload and crack the gasket armor that surrounds the cylinder. This leads to burn-through and loss of compression.
Detonation can be caused by a variety of problems. One is an accumulation of carbon in the combustion chamber that increases compression. Many late-model import engines run fairly high compression ratios, and some require premium octane fuel. If compression reaches a point where the fuel ignites spontaneously before the spark can set it off, the engine will knock and ping under load.
Mislabeled fuel that does not have the octane rating claimed on the pump can also lead to trouble in high-compression or turbocharged engines, especially when the engine is working hard under load or high boost pressure. If there aren’t enough octane-boosting additives in the fuel, the engine may experience mild to severe detonation.
Other factors that may increase the risk of detonation include an EGR system that isn’t working (by diluting the air/fuel mixture slightly with exhaust, EGR actually helps cool combustion temperatures under load). Over-advanced ignition timing can also cause detonation, as can an overly lean air/fuel mixture. Any problems in the cooling system that make the engine run hotter than normal will also increase the chance that detonation may occur.
Pre-ignition is a related problem that can also cause detonation. Pre-ignition occurs when a surface inside the combustion chamber gets so hot that it becomes a source of ignition instead of spark. The hot spot might be the exhaust valve, spark plug or a sharp edge in the combustion chamber.
The underlying cause is often an overly lean air/fuel mixture or a cooling problem. If pre-ignition occurs, it will ignite the air/fuel mixture before the spark plug fires, causing combustion pressure to peak too early on the compression stroke. This, in turn, can cause detonation that may damage the head gasket.
Weak Gasket Design
Another reason why head gaskets fail is because the original OEM gasket design is not robust enough to go the distance. Vehicle manufacturers subject their engines to extensive durability testing, but sometimes a weak head gasket design isn’t discovered until an engine has been in service for a number of years. If the head gasket fails while the engine is still under warranty, it becomes a warranty expense for the vehicle manufacturer. But, in most cases, head gaskets don’t fail until the engine is out of warranty and has a lot of miles on it.
Subaru has had head gasket problems on some of its 1996-’99 Outback, Legacy GT, Forester and Impreza RS models with the 2.5L engine. In 1996, Subaru used a composite head gasket on these engines, and then changed to a stronger multi-layer steel (MLS) gasket with a graphite overlay in 1999. Even so, some of these head gaskets develop leaks that allow coolant and oil to mix. Symptoms include engine overheating due to loss of coolant, and an oily residue in the coolant overflow bottle. The fix is to replace the head gasket.
Subaru has also had some head gasket leakage problems on 1998 Impreza, and 2000 and newer Outback and Legacy models with 2.2L and 2.5L engines. The problem occurs on the left side of the engine and is usually an external oil and/or coolant leak. The cause may be battery corrosion (the battery is located above the left side of the engine) that attacks the outer edge of the head gasket.
If you end up replacing the head gasket, don’t reuse the original head bolts because they are torque-to-yield (TTY) bolts. TTY head bolts stretch when tightened and should not be reused because they may break or fail to maintain torque.
Some head gasket failures can be blamed on the design of the cylinder head and the way it loads the gasket. The 1988-’95 Toyota 3VZE 3.0L V6 light truck engine, and 1995-’98 Toyota 5VZFE 3.4L V6 in T100s, Tacomas and 4Runners are all examples of engines with hard-to-seal heads and frequent head gasket failures.
In the case of the Mitsubishi 3.0L V6, the armor around the combustion chambers on the OEM gasket has a tendency to crack. The cracking occurs as a result of metal fatigue caused by the head scrubbing back and forth on the block because the engine has aluminum heads and a cast-iron block. Aluminum expands at a much higher rate than cast iron, which causes the head surface to move more than the block surface. If the head gasket can’t accommodate the movement, or lacks any built-in lubricity (such as a non-stick coating), thermal expansion and contraction can literally tear the head gasket apart as the miles accumulate. The fix is to replace the OEM head gasket with an improved and redesigned aftermarket head gasket that has stronger combustion armor, better materials and an anti-friction coating that can handle the movement.
On older Toyota 3.0L and 3.4L engines, excessive head motion is also a leading cause of gasket failure. Installing a redesigned aftermarket gasket with improved combustion armor and an anti-stick coating should provide a long-lasting fix.
Some aftermarket gasket suppliers now offer MLS replacement head gaskets for older import engines that were originally equipped with composite or graphite head gaskets. The MLS head gaskets are made of several layers of embossed stainless steel and are much stronger than the OEM gaskets that were originally used.
Gasket Replacement Tips
Replacing a head gasket is a big job, so avoid making any mistakes that could prevent the gasket from sealing properly.
After removing the old head gasket, carefully remove any residue from the head and block using a gasket removal chemical and scraper. Do not use an abrasive pad in a drill to whiz off or clean the head or block surfaces because doing so may also remove metal and create shallow depressions that can prevent a new head gasket from sealing.
Before you install a new head gasket, use a straight edge and feeler gauge to carefully check the flatness of both the cylinder head and the engine block. If flatness is not within specifications, the head or block will have to be resurfaced. For engines with aluminum heads, flatness should be 0.002 inches (0.05 mm) in all directions.
If the head and/or block are resurfaced, the surface finish must be to specifications. MLS head gaskets typically require a much smoother finish (20 micro-inches or less) than composite head gaskets (which can handle up to 50 micro-inches or more).
Do not use any type of sealer on a head gasket unless the installation instructions that come with the gasket specifically say a sealer is required. If so, use the type of sealer specified by the manufacturer and follow the application instructions to the letter.
As mentioned earlier, do not reuse TTY head bolts. If the original head gasket is multi-layer steel, the engine usually has TTY head bolts. If new bolts are not included with the replacement head gasket, don’t be tempted to reuse the old bolts.
On engines that have conventional head bolts, inspect the head bolts and discard any that are damaged or stretched. Then clean all of the bolt threads and lightly oil them with engine oil before installation. Dirt, thread damage and lack of lubrication can cause false torque readings when the bolts are tightened.
If the cylinder head has been resurfaced, check bolt lengths to make sure they don’t bottom out in blind holes. A bolt that bottoms out will apply little or no clamping force on the head, which may allow the gasket to leak. To compensate for resurfacing, you may have to install hardened steel washers under the bolts to raise them up, or use a copper head gasket shim to restore proper head height.
Look up the latest head bolt tightening specifications and procedures, as service procedures may have changed or been revised.
Use an accurate torque wrench and angle gauge (if required), and follow the recommended tightening sequence to make sure the head gasket is loaded evenly when you install the cylinder head. Mistakes here can lead to uneven loading that results in a poor seal and leaks.
As a preventive measure, add a dose of cooling system sealer to the coolant when you refill the cooling system. Also, make sure any bleed valves are open while refilling the system so you don’t end up with air pockets that may cause the engine to overheat. You may have to start the engine and allow it to warm up to operating temperature, then shut it off, allow it to cool and recheck the coolant level to make sure there are no air pockets.
Finally, make sure any factors that may have caused the original head gasket to fail have been identified and corrected so the new head gasket won’t suffer the same fate. Make sure the cooling system is functioning normally and holds pressure. Make sure the engine doesn’t knock or ping under load. And make sure the vehicle owner is using the correct grade of gasoline (premium if required).