One of the most critical seals inside any car engine is the head gasket. This junction between the engine block and cylinder must contain the force of combustion inside the cylinder. It also seals a number of passages for engine coolant and undergoes wild temperature swings as the engine warms and cools. If the head gasket is blown it can result in minor annoyances or large problems that risk failure of the engine. Most symptoms of a blown head gasket can be diagnosed through careful observation, and specialised tools can assist in confirming problems before time-consuming and costly repair work.
Coolant in the Cylinders
One area of failure is the seal between an engine cylinder and a coolant passage. As the coolant heats up, and pressure rises, it will be forced into the cylinder during the intake stroke. This coolant will flash into steam during combustion and will be visible as a white cloud coming from the tailpipe. This steam cloud will be much thicker than normally seen during cold weather, and also have a sweet smell matching that of antifreeze. To determine the exact cylinder, there two tests can be performed. Visually inspecting the spark plugs will likely turn up one that is visibly wet or distinctively cleaner than the rest. A compression gauge will show lower pressure in the affected cylinder.
Air in the Coolant System
Like the problem of coolant in the cylinders, air in the coolant is also caused by a failure of the head gasket between a cylinder and coolant passage. Usually this will be happening at the same time as a coolant leak into the cylinders. The quickest symptoms to look for are overheating and low coolant level with no obvious leak. These symptoms also occur for other reasons other than a blown head gasket, however, and other tests are needed to confirm the problem. Test kits are available from many auto part stores that will show if exhaust gasses are present in the coolant of gasoline engines, and compression testing may show lower pressure in the leaking cylinder.
Oil in the Coolant
Sometimes a blown head gasket will occur in ways that allow engine oil to enter into the cooling system. Because this will rarely affect performance, symptoms can go undetected for a long time. Regular inspection of the coolant is the best way to catch this problem before further engine damage occurs. If any oil has mixed with the coolant, it will have changed in colour to a light tan, close to the colour of coffee with a lot of milk added.
Coolant in the Oil
Coolant entering the oiling system is one of the more common head gasket problems, and potentially one of the most damaging to your engine. As with oil in the coolant, you may notice no changes in the engine performance, and the blown head gasket can go undetected for quite awhile. It is most often discovered during oil changes. More severe cases cause the oil to become very thick and clumpy, with a light tan colour. However, more minor problems can be trickier to find. Symptoms occur much sooner at the rocker arms. Look at the bottom side of the oil cap and down into the oil filler neck for the same thick and tan oil mixture.
You may also see a blown head gasket that causes only a coolant leak. Early symptoms are overheating and visible puddles of coolant on the ground. Close inspection of the outside of the engine can show the location of the leak. However, there are many other areas on an engine that more commonly leak coolant, and it can be difficult to determine the source. This is particularly true on older engines that have accumulated a lot of grime and dirt. An ultraviolet dye test kit can help better locate the source of some leaks, and these are available at most auto parts stores.
Sometimes a gasket can fail and cause a cylinder to only lose compression. The first indication of the blown head gasket is an audible hissing or tapping sound, which can be difficult to locate, as it sounds like a bad lifter or rocker arm. More severe cases can see a noticeable loss of power from the engine. Depending on the location, you may see the air leak on the engine. The most accurate test for this problem is a compression gauge.