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
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 show 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.
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.
Overheating from a blown head gasket
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. 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 multiple failure types to exist in an engine. This type of failure will give multiple system, depending on the combination of problems.
Hydrocarbon coolant testing
A handy procedure for identifying a blown head gasket is the hydrocarbon test. When combustion gasses leak into the coolant, we may detect the unburned hydrocarbons. We use a tool, made for the purpose, and special chemicals that change color when exposed to hydrocarbons.
To perform the hydrocarbon 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 hydrocarbons turns the chemical yellow, showing a problem.
A failure of the hydrocarbon test means there is a problem. Unfortunately, a lack of hydrocarbons does NOT show a healthy system. Many head gasket failure types will not show on the test and even hydrocarbon related problems do not always show. While the test is not perfect, it is very helpful.
Cracked cylinder heads
A hydrocarbon 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 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.
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
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.
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.
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.