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A blown head gasket may cause a number of different symptoms. Coolant in the engine oil, an engine misfire or oil leaking from the engine could all be a symptom. Each of these symptoms could also have other causes and an actual blown head gasket may show none of the above symptoms.

Diagnosing head gasket symptoms

some of the ways cylinder head gaskets fail

It is easy to be mislead, when trying to diagnose a bad head gasket. Symptoms will often vary considerably, depending on how the gasket fails. The term "blown head gasket" does NOT necessarily describe a single thing. Head gaskets may fail in several different ways. With each failure type different symptoms may result. The symptoms each person sees depend on how and where the head gasket fails.

To make diagnosis even more confusing, other things may cause the same symptoms as a head gasket failure. Another issue may be multiple failures which can cause more than one symptom. For example, a restricted radiator may cause an engine to overheat, very much the same as a head gasket failure. The farther we drive the vehicle, the more it may overheat. Often, intake gaskets cause coolant in the oil.  Coolant in the oil is often mistaken as a head gasket. Each of these symptoms may suggest a head gasket problem, but may have another explanation. Diagnosing the problem requires experience and a logical approach.

Consequences of not replacing a bad head gasket

Because of the difficulty in diagnosing and the expense of repairing a head gasket, we may be tempted to let it go. This is a big mistake. Depending on the type of failure, far more damage will soon occur.

If coolant enters the exhaust, through the combustion chamber, the catalytic converter(s) is often damaged. Coolant in the engine oil may destroy the engine, by breaking down lubrication. Combustion gases that leaks into the coolant usually results in continued overheating and more damage. Hydrocarbons entering the coolant will also greatly increase corrosion. This may destroy the radiator, heater core and other expensive components.

Types of head gasket failure

Adding to the problem of diagnosis may be other factors. For example, a warped or cracked cylinder head will produce the exact symptom of a blown head gasket. An external inspection will only determine the problem is head gasket related. This may mean a blown gasket, warped or cracked cylinder head or other problems in the area. For instance corrosion on the head gasket surface will cause leakage, though technically not a blown gasket. Removing the head and testing it is the only way to know the extent of the damage.

Engine misfire caused by a head gasket

A head gasket that fails between cylinders will generally cause a misfire and perhaps few other symptoms. With a failure between cylinders, compression from one cylinder leaks into another. Lowered compression results in a rough idling engine. Damage of this type may not cause overheating, coolant in the oil or any other outward sign.  Many people overlook this as the cause of engine misfires, because the blown head gasket does not also give the other more common symptoms.  When the head gasket fails between a cylinder and the coolant port, coolant may leak into the cylinder.  This often results in misfires on startup, especially after the engine is run, turned off and restarted.  This type of failure may not show with a compression test.  Pressuring the cooling system and then starting the engine, may make the misfire more noticeable.  Coolant on the tip of a spark plug is another dead give away.

Overheating from a blown head gasket

A blown head gasket sometimes gives an intermittent overheat condition

When a head gasket fails between a combustion chamber and the cooling system, a loss of coolant and overheating are often the result. This may be intermittent in nature. For instance the vehicle may only overheat after driving a distance. With short trips, symptoms may not show up. This type of failure is very damaging. Not only does overheating continue to get worse, combustion gasses are corrosive and may severely damage the cooling system. Other symptoms may include repeat failure of hoses, radiators and cooling system components. Pressure may build in the cooling system until the weakest link fails. Replacing the component causes the next weakest part to fail.

Frequently this type problem develops after an engine overheat caused by another reason. For example, a leaking water pump may cause the engine to over heat. We replace the pump, but the overheating continues. In other instances, the water pump replacement cures the overheat but it starts again several months later. The original overheat crushed the head gasket, which took time to show a problem. This can be confusing and often we have forgotten the original cause when symptoms return.

Oil in the coolant and coolant in the oil

Head gaskets may also fail between the coolant passages and the lubrication system. This type failure may show up as oil in the coolant or coolant in the oil.  Most often a swollen seal on the radiator cap is the first symptom.  Glycol in engine oil is much harder to detect.  Unlike pure water, glycol may not turn the oil milky.  Unfortunately it will destroy the ability of the oil to lubricate and change the viscosity of the oil.  An oil-testing laboratory can check an oil sample and identify glycol contamination.

An external oil leak

It is also common to find failures between an oil passage and the outside. Such damage results in an external oil leak, and possibly no other symptom.

An oil leak from a head gasket is not common and sometimes it is mis-diagnosed. For instance we may think oil leaking from the head gasket is leaking from a valve cover. Because valve cover leakage is far more common, this is an easy mistake to make. Other times the oil may drip from the area below the transmission. Gravity causes the oil to flow down and this is the lowest point. We may mistake this as a rear-main seal or oil-pan gasket leak, if we are not careful.

It is also possible for head gaskets to fail in multiple ways.  Multiple failures often produce combinations of the symptoms listed.

Testing coolant for carbon dioxide

A handy procedure for identifying a blown head gasket is the carbon dioxide test. When combustion gasses leak into the coolant, we may detect the carbon dioxide from the combustion process. We use a tool, made for the purpose, and special chemicals that change color when exposed to carbon dioxide.

Testing the cooling system for carbon dioxide

To perform the carbon dioxide test, the engine coolant level is slightly lowered in the radiator. This allows a temporary air space for testing. Fully warming the engine will help make the results more accurate. The testing apparatus draws a sample of fumes, from the cooling system and mixes them with the test chemical. The presence of carbon dioxide turns the chemical yellow, showing a problem.

A failure of the carbon dioxide test means there is a problem. Unfortunately, a lack of carbon dioxide does NOT show a healthy system. Many head gasket failure types will not show on the test and even carbon dioxide related problems do not always show. While the test is not perfect, it is very helpful.

Cracked cylinder heads

cylinder head with a large and noticeable crack

A carbon dioxide test may also not show a cylinder head with a crack. Despite not showing, a cracked head can give the same symptoms as a blown gasket. In the photo above, the crack is obvious. Often the crack is too small to see with the eye. Though they are small, a crack in a cylinder head will cause serious problems. Cylinder head shops use dye, pressure and other methods to find very small cracks. Sending cylinder heads to a professional, for testing, is a wise precaution.

Other cylinder head problems

cylinder head surface pitted by corrosion and no longer useable

Cylinder-head surface corrosion is similar to a crack, in the way it affects the gasket. A corroded cylinder head will not allow the gasket to seal.  Such corrosion occurs with improper cooling system service. If the corrosion is severe, we have to replace the cylinder head. With a less serious case, a cylinder head shop can restore the surface with machine work.

Warped cylinder heads 

Warping is another problem, particularly with aluminum cylinder heads. The surface of a cylinder head may warp and no longer provide a flat surface to hold the head gasket properly. We check a cylinder head for warp, with a straightedge and a feeler blade.  After we remove and clean the head, we use a straightedge and feeler blade to check the surface.  If a problem exists, the feeler blade will pass between the head and the straightedge.

checking a cylinder head for warp with a straight edge

The specification for an allowable warp varies from one engine to another.  The normal limit is .003 to .004 inches for four-cylinder, V6 and V8 engines. With some inline six-cylinder engines the tolerance may be slightly more.  Cylinder head specialty shops may straighten or machine a warped cylinder head to make it usable. They can even sometimes repair cracked cylinder heads.

The cost of head gasket replacement

head gasket replacement can be quite involved on modern engines

In most cases, replacement of cylinder heads and gaskets will be quite expensive.  This is due to their location on modern engines and the amount of labor required to replace them.

With over-head and double over-head cam engines, we have to remove the front and top of the engine gain access. This often includes removing the timing chain[s] or timing belt. In some vehicles we have to remove the engine from the vehicle to gain access.

Price quotes for head gasket replacement may vary from one repair shop to another. To effectively compare prices we must understand what each job includes. For instance, using a mechanical method to clean the head and block surface is quick. These methods are also risky and may permanently damage the head or engine block. They should never use a scraper or rotary disk to clean the surfaces.

Properly cleaning a head and engine block take time and patience.

It takes much longer to properly soak and clean the heads and engine block. This is tedious work, but adds greatly to a successful outcome. Incomplete cleaning or scratching the surfaces results in failure of the job. Properly cleaning all components adds costs but also greatly improves results.

Replacing the head bolts is another area of concern. Most head bolts today are torque to yield type. This means the repairer should replace them, each time they remove them. These fasteners also require a very specific tightening method. They may price a proper job, several hundred dollars more than someone prices a substandard repair. Before investing in head gasket replacement, investigate the reputation of the shop involved.

Other causes of head gasket failure

Design of the engine is often a factor in head gasket failure.  Some vehicles have more problems than others. This is due to engine design limitations. Preventing head gasket and cylinder head problems is always much easier with good cooling system service.  A well maintained cooling system greatly reduces the odds of head gasket and cylinder head problems.

Modern engines operate at very high temperature, often as high as 200 to 225 degrees Fahrenheit under normal conditions. When the temperature gauge shows hot, they are very hot and damage will quickly occur. When an engine exceeds 240 degrees Fahrenheit, damage can occur. When an engine exceeds 260 degrees Fahrenheit, damage is likely.

Fuel that has an octane rating too low for the engine may cause detonation and pinging. With detonation or pre-ignition, pressure and temperature in the cylinder may drastically rise.  Elevated cylinder head pressure may cause the gasket to fail.  Using the proper fuel can help prevent problems.

Preventing head gasket problems

 Replace engine coolant before the pH falls below 7.0

 Use distilled water and the proper coolant type only

 Always pre-mix coolant and water, before putting in the engine

 When an engine starts to overheat, shut it off, IMMEDIATELY

 Repair any over-heating conditions immediately

  Address any problem with pinging or detonation immediately

 When replacing a head gasket, replace head bolts and properly torque the cylinder heads

A cooling system specialist like AGCO Automotive can diagnose and resolve head gasket and cylinder head problems for you. Even better, we can normally prevent most problems. Quick action is the key! If your temperature gauge starts to climb, remember AGCO, it’s the place to go.

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