Understanding The Job Of Your Car's Head Gasket
Your vehicle's head gasket sits between the engine block and cylinder head
, and functions as a sealing component. It seals the compression of each combustion chamber, and prevents coolant from mixing with the oil that circulates around your engine. If one or more of the combustion chambers loses compression, your engine's performance will suffer. If coolant mixes with the oil, the latter will become less able to manage the temperature surrounding the engine.
There are a number of reasons this component can fail. When it does, it must be replaced. The problem is, despite the part being relatively inexpensive, replacing it can cost several hundreds of dollars due to its location. Below, we'll describe the factors that contribute to a blown head gasket, and offer a few tips for troubleshooting the cause of a failure.
Why The Head Gasket Fails
One of the most common reasons the head gasket can lose its sealing capacity is because of thermal expansion. While you're driving, the temperature around your engine rises. It is kept in check by the coolant and oil. However, even under normal circumstances, the heat can cause severe wear on the component. This is due to the stress placed upon it by the engine block and cylinder head.
In today's vehicles, the engine block is made of iron, which, in the past, was also true of the cylinder head. Back then, when heat was generated, both components would expand at the same rate. The head is now typically made of aluminum, which expands more quickly than iron. The different rates of expansion between the block and head creates stress on the gasket. As the temperature around your engine climbs, the pressure weakens - and eventually crushes - the gasket.
Another common cause of failure is poor installation. The gasket is held down by a series of head bolts. These bolts must be tightened according to precise specifications (the specs are usually listed in your owner's manual). They need to be tightened with the proper torque setting, and in the proper order. A lot of mechanics all but ignore the specifications, which causes them to install the gasket incorrectly. The result? It fails prematurely.
Other factors that can cause the head gasket to fail include engine problems, such as detonation (also called spark knock) and overheating. From the above, the reasons may be intuitive. Detonation occurs when the air-fuel mixture in the cylinders explodes unpredictably. This allows the pistons to create excess stress against the gasket, which weakens it over time. Overheating leads to the aforementioned problem of varying thermal expansion rates between the head and block - again, this causes bimetal stress.
Troubleshooting A Failing Head Gasket
When the head gasket fails prematurely (outside reasons related to thermal expansion), it is critical to diagnose the root cause. Otherwise, you risk having a replacement gasket fail prematurely. Since overheating is a common cause, check your coolant level and inspect your radiator. If you suspect detonation is the cause, have your mechanic check the knock sensor and the EGR valve; both can affect the air-fuel mixture in the cylinders.
As a side note, you'll notice a difference in the gasket when it fails due to overheating versus detonation. In the former case, the component will have been crushed between the cylinder head and engine block. In the latter case, you'll notice cracking and burning near the edges that lie near the combustion chambers.
Unless you have plenty of experience working on your vehicle, a head gasket replacement is better left to a trained mechanic. The job is difficult, requires certain tools, and can take three or four hours to complete. Simply getting to the component is often challenging, depending on the placement of the exhaust manifold, head, and block. The most important thing is to identify the reasons the part failed in the first place so you can avoid a recurrence down the road.
by: George Rackley
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