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Most installers should know the first symptoms of a head gasket that is failing are usually a slowloss of coolant with no visible leaks, engine overheating and/or aCheck Engine light with cylinder misfire codes. The Onboard DiagnosticII (OBD II) system should detect any misfires that occur due to a lossof compression or spark plugs fouled by coolant seeping into acombustion chamber. So if you find a cylinder misfire code, check thespark plug for coolant fouling, and do a compression or leak-down teston the cylinder to find out what’s going on. Internal coolant leakscaused by a leaky head gasket can usually be diagnosed by pressuretesting the cooling system.

Regardlessof the cause, a head gasket failure is bad news for the vehicle ownerbecause it will require replacing the head gasket, or if it is a warranty situation for one of your engines, you’ll get to do it again. Depending on theapplication and how much labor it requires, installing a new headgasket can easily cost up to $1,500 or more. And if coolant has dilutedthe oil and damaged the bearings inside the engine, your customer couldbe looking at a considerably higher repair bill.

What Happened?
Many motorists are puzzled by head gasket failures because they don’tunderstand the causes of gasket failure. Regular maintenance and oilchanges can’t prevent a high-mileage head gasket from failing, but itcan increase the odds of detecting other problems that may lead topremature head gasket failure.

For instance, one reason head gaskets fail is because of engineoverheating. If the engine gets too hot, the cylinder head can swell tothe point where it crushes the head gasket (usually between thecylinders because this is the thinnest point). The extruded materialand/or cracked combustion armor then provides a leak path for coolantand/or combustion gases.

Replacing the head gasket will cure the leak, but the underlying causethat made the engine overheat in the first place also needs to bediagnosed and repaired — otherwise, the newly installed gasket willsuffer the same fate the next time the engine overheats.

Thecooling system on many late-model imports is just barely adequate (toreduce weight and cost), so it doesn’t take much loss of cooling tomake the engine run hot and overheat. A low coolant level, a dirtycooling system, a cooling fan that isn’t working correctly, oroverloading or overworking the vehicle can all be contributing factors.

On 1998 to 2000 Volvo S40 models with the 1.9L turbo engine, the enginecooling fan resistor may fail, preventing the cooling fan from working.This may cause the engine to overheat at low vehicle speeds, resultingin head gasket damage if the engine gets really hot. Replacing the headgasket will fix the coolant leak, but it’s important to make sure thecooling fan is working correctly and comes on when needed so the enginedoes not overheat again. The fan resistor is located under the frontengine splashguard, just below the fan motor. A new resistor (P/N30644121) can be spliced into the wiring if the original resistor isdefective.

If a head gasket has failed because of overheating, be sure to checkthe coolant level and condition; the concentration of antifreeze in thecoolant (too much antifreeze in relation to water reduces the abilityto transfer heat); and the radiator (obstructions, leaks, etc.), hoses,thermostat, water pump, cooling fan, EGR system, belts and the exhaustsystem to determine why the engine overheated. In some cases, anexhaust restriction (clogged converter) can also make an engine run hot.

How can you tell if overheating caused the head gasket to fail? A headgasket that failed because of overheating or a hot spot will be crushedand measurably thinner in the damaged area when checked with amicrometer. By comparison, a gasket that has failed due to detonationor pre-ignition will usually have cracked armor around the combustionchamber, which leads to burn-through.

Detonation Damage
Another reason head gaskets fail is because of damage caused bydetonation (spark knock). Detonation causes a sharp spike in combustionchamber pressure, which, over time, can overload and crack the gasketarmor that surrounds the cylinder. This leads to burn through and lossof compression.

Detonationcan be caused by a variety of problems. One is an accumulation ofcarbon in the combustion chamber that increases compression. Manylate-model import engines run fairly high compression ratios, and somerequire premium octane fuel. If compression reaches a point where thefuel ignites spontaneously before the spark can set it off, the enginewill knock and ping under load.

Mislabeled fuel that does not have the octane rating claimed on thepump can also lead to trouble in high-compression or turbochargedengines, especially when the engine is working hard under load or highboost pressure. If there aren’t enough octane-boosting additives in thefuel, the engine may experience mild to severe detonation.

Other factors that may increase the risk of detonation include an EGRsystem that isn’t working (by diluting the air/fuel mixture slightlywith exhaust, EGR actually helps cool combustion temperatures underload). Over-advanced ignition timing can also cause detonation, as canan overly lean air/fuel mixture. Any problems in the cooling systemthat make the engine run hotter than normal will also increase thechance 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 getsso hot that it becomes a source of ignition instead of spark. The hotspot might be the exhaust valve, spark plug or a sharp edge in thecombustion chamber.

The underlying cause is often an overly lean air/fuel mixture or acooling problem. If pre-ignition occurs, it will ignite the air/fuelmixture before the spark plug fires, causing combustion pressure topeak too early on the compression stroke. This, in turn, can causedetonation that may damage the head gasket.

Weak Gasket Design
Another reason why head gaskets fail is because the original OEM gasketdesign is not robust enough to go the distance. Vehicle manufacturerssubject their engines to extensive durability testing, but sometimes aweak head gasket design isn’t discovered until an engine has been inservice for a number of years. If the head gasket fails while theengine is still under warranty, it becomes a warranty expense for thevehicle manufacturer. But, in most cases, head gaskets don’t fail untilthe engine is out of warranty and has a lot of miles on it.

Subaruhas had head gasket problems on some of its 1996-’99 Outback, LegacyGT, Forester and Impreza RS models with the 2.5L engine. In 1996,Subaru used a composite head gasket on these engines, and then changedto a stronger multi-layer steel (MLS) gasket with a graphite overlay in1999. Even so, some of these head gaskets develop leaks that allowcoolant and oil to mix. Symptoms include engine overheating due to lossof coolant, and an oily residue in the coolant overflow bottle. The fixis 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 2.5Lengines. The problem occurs on the left side of the engine and isusually an external oil and/or coolant leak. The cause may be batterycorrosion (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 headbolts because they are torque-to-yield (TTY) bolts. TTY head boltsstretch when tightened and should not be reused because they may breakor fail to maintain torque.

Some head gasket failures can be blamed on the design of the cylinderhead and the way it loads the gasket. The 1988-’95 Toyota 3VZE 3.0L V6light truck engine, and 1995-’98 Toyota 5VZFE 3.4L V6 in T100s, Tacomasand 4Runners are all examples of engines with hard-to-seal heads andfrequent head gasket failures.

In the case of the Mitsubishi 3.0L V6, the armor around the combustionchambers on the OEM gasket has a tendency to crack. The cracking occursas a result of metal fatigue caused by the head scrubbing back andforth on the block because the engine has aluminum heads and acast-iron block. Aluminum expands at a much higher rate than cast iron,which causes the head surface to move more than the block surface. Ifthe head gasket can’t accommodate the movement, or lacks any built-inlubricity (such as a non-stick coating), thermal expansion andcontraction can literally tear the head gasket apart as the milesaccumulate. The fix is to replace the OEM head gasket with an improvedand redesigned aftermarket head gasket that has stronger combustionarmor, better materials and an anti-friction coating that can handlethe movement.

On older Toyota 3.0L and 3.4L engines, excessive head motion is also aleading cause of gasket failure. Installing a redesigned aftermarketgasket with improved combustion armor and an anti-stick coating shouldprovide a long-lasting fix.

Some aftermarket gasket suppliers now offer MLS replacement headgaskets for older import engines that were originally equipped withcomposite or graphite head gaskets. The MLS head gaskets are made ofseveral layers of embossed stainless steel and are much stronger thanthe OEM gaskets that were originally used.

Gasket Replacement Tips
Replacing a head gasket is a big job for many installers and vehicle owners, so here are some tips to help avoid making any mistakes that could prevent the gasket from sealing properly and the engine coming back to you.

• After removing the old head gasket, carefully remove any residue fromthe head and block using a gasket removal chemical and scraper. Do notuse an abrasive pad in a drill to whiz off or clean the head or blocksurfaces because doing so may also remove metal and create shallowdepressions that can prevent a new head gasket from sealing.

• Before you install a new head gasket, use a straight edge and feelergauge to carefully check the flatness of both the cylinder head and theengine block. If flatness is not within specifications, the head orblock will have to be resurfaced. For engines with aluminum heads,flatness should be 0.002 in. (0.05 mm) in all directions.

• If the head and/or block are resurfaced, the surface finish must beto specifications. MLS head gaskets typically require a much smootherfinish (20 micro-inches or less) than composite head gaskets (which canhandle up to 50 micro-inches or more).

• Do not use any type of sealer on a head gasket unless theinstallation instructions that come with the gasket specifically say asealer is required. If so, use the type of sealer specified by themanufacturer and follow the application instructions to the letter.

• As mentioned earlier, do not reuse TTY head bolts. If the originalhead gasket is multi-layer steel, the engine usually has TTY headbolts. 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 boltsand discard any that are damaged or stretched. Then clean all of thebolt threads and lightly oil them with engine oil before installation.Dirt, thread damage and lack of lubrication can cause false torquereadings when the bolts are tightened.

• If the cylinder head has been resurfaced, check bolt lengths to makesure they don’t bottom out in blind holes. A bolt that bottoms out willapply little or no clamping force on the head, which may allow thegasket to leak. To compensate for resurfacing, you may have to installhardened steel washers under the bolts to raise them up, or use acopper 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), andfollow the recommended tightening sequence to make sure the head gasketis loaded evenly when you install the cylinder head. Mistakes here canlead to uneven loading that results in a poor seal and leaks.

• As a preventive measure, add a dose of cooling system sealer to thecoolant when you refill the cooling system. Also, make sure any bleedvalves are open while refilling the system so you don’t end up with airpockets that may cause the engine to overheat. You may have to startthe engine and allow it to warm up to operating temperature, then shutit off, allow it to cool and recheck the coolant level to make surethere are no air pockets.

•Finally, make sure any factors that may have caused the original headgasket to fail have been identified and corrected so the new headgasket won’t suffer the same fate. Make sure the cooling system isfunctioning normally and holds pressure. Make sure the engine doesn’tknock or ping under load. And make sure the vehicle owner is using thecorrect grade of gasoline (premium if required).

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