Head Gasket Replacement – UnderhoodService

Head Gasket Replacement

During some point in their service lives, most import engines built with cast-iron cylinder blocks and aluminum cylinder heads will need a cylinder head gasket replacement. The vulnerability of aluminum-head engines to head gasket failure is caused by the aluminum cylinder head expanding approximately 1.5 times more than the cast-iron block. Although measured in thousandths of an inch, this small and seemingly insignificant difference between the expansion rates of aluminum and cast iron will eventually wear out the head gasket and cause failure.

Head gasket failure is also aggravated by design factors such as deficiencies in the number and placement of cylinder head bolts, the thickness of the cylinder head and cylinder block deck surfaces, and the addition of supercharging or turbocharging on the engine.

Last, neglected cooling system maintenance, and spark timing and exhaust valve gas recirculation (EGR) errors in the electronic engine management system, may cause cylinder detonation, which eventually destroys the metal head gasket “fire ring” sealing the cylinder from the cooling system itself.

Since head gasket failures are relatively common, a head gasket replacement offers numerous opportunities to sell new cylinder heads, machine shop services and complete engine assemblies as well as the belts, hoses, and cooling, ignition, fuel system and other related parts. In fact, cylinder head gasket failure may merely be symptomatic of the failure of any of these related parts, but more about that later.


In most cases, cylinder head gaskets seem to fail catastrophically because many vehicle owners don’t check engine oil and coolant levels at recommended intervals. In other cases, a cylinder head gasket fails because the engine has severely overheated because coolant is lost through a leaking water pump or coolant hose. If the engine is severely overheated, the aluminum cylinder head may warp, which usually crushes the head gasket at its ends and causes a loss of gasket compression at the center. While the head gasket may not be leaking at this point, it’s certainly conditioned to fail later on.

On post-OBD II engine systems equipped with cylinder misfire monitors, a misfire or P0300 diagnostic trouble code stored in the PCM’s diagnostic memory may be symptomatic of a leaking cylinder head gasket. When the spark plug is removed, the spark plug insulator may also show a distinctive darkening or antifreeze color when compared with other spark plugs in the engine.

Increasing levels of coolant consumption also may indicate imminent head gasket failure. Coolant consumption may increase unnoticed until low coolant levels cause an overheating condition. When the engine overheats, the leaking head gasket fails catastrophically due to increased mechanical and thermal stress.

Another symptom of a leaking head gasket is an “air-locked” heater core, which is caused by combustion gas accumulating in the heater core. Since hot coolant won’t circulate through an air-locked heater core, the initial complaint may be the lack of heat in the passenger compartment. On the other hand, combustion gas can also build up under the thermostat and prevent hot coolant from contacting the thermostat’s wax pellet. Since the thermostat won’t open until the wax pellet achieves operating temperature, a temporary overheating condition may result.

A leaking head gasket may also force coolant away from a closed thermostat and back into the coolant overflow tank. This condition may persist until the thermostat opens and vents the combustion gases through the top radiator hose, through the cap vent and into the coolant reservoir.

Eventually, the gasket will allow coolant to leak into the crankcase, causing a milky, thick-textured mixture of oil and coolant to accumulate under the oil fill cap. In many cases, the engine dipstick may also begin to rust and the engine oil will thicken as contamination increases. Since heavily contaminated oil won’t lubricate critical engine parts, the engine will suffer a mechanical failure.


Before we get into our diagnostic overview, it’s important to remember that a cylinder head gasket can leak intermittently in its earliest stages of failure. Since most intermittent leaks depend heavily upon engine temperature and load, diagnosis can be difficult, if not impossible.

Compression leakage and gas analysis are two basic methods that may be used to diagnose a leaking cylinder head gasket. Choosing a method usually has most to do with whether the spark plugs can easily be removed in order to attach compression testing or air pressurization hoses. If the spark plugs are easily accessed, either of these tests can provide accurate information. If access to the spark plugs is poor, then a gas analysis, which tests for the presence of exhaust gas in the cooling system, may offer the best proof of cylinder head gasket leakage.

Several different approaches may be used to determine if compression loss from an individual cylinder is venting into the cooling system during the compression stroke. Some technicians, for example, like to fill the radiator close to the filler neck and then attach a common cooling system-pressurizing tool to the radiator. With the ignition disabled, the engine is cranked while the technician looks for an increase in cooling system pressure. If an increase is noted, the engine has a leaking cylinder head gasket.

In another variation of the compression test, compressed air may be applied to the cylinder in question with the piston at “perfect” top dead center on compression stroke and with the radiator filled to the neck. When compressed air is applied to a cylinder with a bad head gasket, air bubbles may be seen venting from the radiator filler neck. The primary difficulty with this method is locating the piston exactly at TDC in the cylinder, especially on engines with limited access.

Testing for the presence of combustion gases in the cooling system can be done in one of two ways. Shops equipped with an exhaust gas analyzer, for example, may prefer to test for the presence of hydrocarbons in the cooling system by “sniffing” around the radiator filler neck or coolant reservoir with the analyzer probe. The downside of this method is that it’s easy to accidentally contaminate the analyzer with coolant.

A simpler, but less accurate method is to use a “block testing” kit to test for the presence of combustion gases in the radiator neck or coolant reservoir. The kit utilizes a tester that draws cooling system air through a coolant “sniffing” tube filled with a blue detection fluid. The presence of combustion gas is indicated if the blue fluid rapidly turns yellow.


The only way to ensure that the cylinder head gasket will last through a normal service life is to make sure that the cylinder head is machined to OEM specifications. Because only the most modern machine shops can straighten a warped cylinder head, correctly weld cracks and resurface the head to OEM specifications, repairing aluminum cylinder heads has become a specialty in today’s machine shop market.

Resurfacing the head is a critical operation because, as mentioned at the outset, the aluminum cylinder head must slip over the head gasket as it expands. Consequently, the cylinder head surface must be very smooth in order to avoid scuffing the head gasket. Although modern cylinder head gaskets made from graphite-based compounds will accommodate normal slippage, it’s also important to understand that roughly machined cylinder heads will ruin even the best-designed head gasket.

In addition, it’s also important to understand that modern head gaskets may be manufactured in different thickness to compensate for the removal of material from the head or block surface. In some applications, the head gasket manufacturer may supply a steel shim to accomplish the same task. The fact that a thicker gasket or steel shim isn’t supplied may indicate that the deck thickness of the head may not accommodate removal of material. In this case, the head should be replaced with a new OEM or aftermarket unit.

After a cylinder head gasket is replaced, the engine’s exhaust gas recirculation functions should be tested and base ignition timing, if applicable, should be adjusted to specification. If all repair precautions are taken, the new cylinder head gasket replacement will, with proper cooling system maintenance, enjoy a normal life expectancy.

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