To answer that question, we must first address what “blown” means in head gasket terms. A blown head gasket is one that has failed or suffered damage to any of the separating portions of the gasket. This can mean that any area of the gasket, including the combustion chamber seals or the coolant and oil separating channels, have separated. This can allow oil and coolant to mix, leak out of the engine, or enter the combustion chamber, the combustion chamber itself can leak, or a combination of these issues may be present.
But what causes a blown head gasket? Though it’s possible for a head gasket to fail over time due to age and general wear in high-mileage engines, the most common cause of a blown head gasket is engine overheating. Overheating may be caused by a lack of coolant in the engine and radiator, a failing cooling fan, or by an obstruction in the cooling system, such as a thermostat that is stuck closed. This causes the engine to not only heat up rapidly, but also to stay hot for a long period of time because of a lack of coolant circulation. An engine that has reached unsafe operating temperatures transfers that heat to the head gasket, which can weaken the gasket material or cause it to expand. Once this occurs, pressure from the engine can push outward on the now weakened gasket and result in a blowout. The best way to prevent a blown head gasket is to never drive the vehicle when it is overheating. If your temperature gauge begins to rise beyond its normal level, pull the vehicle over in a safe place and shut it off until you can address the source of the cooling system problem.
Blown head gaskets are an issue that has plagued the automobile as long as most of us can remember. It seems like every inexpensive potential project car online includes a disclaimer that the head gasket is blown, and the phenomenon is even familiar enough outside of the car community to spark a common phrase, “don’t blow a head gasket!”. So while many of us are probably familiar with the terminology, it may be less clear what the head gasket actually does, what causes head gaskets to fail, and what happens when they do. Throughout this article, we hope to answer all those questions and help you prevent your own head gasket failures, or assist you in determining if your head gasket has blown.
What Is a Head Gasket and What Does a Head Gasket Do?
The head gasket is a thick gasket installed between the engine block and the cylinder head, and is designed to prevent coolant or oil leaks and cross-contamination of engine fluids. It also seals the combustion chamber to prevent air or exhaust leaks from the cylinder. Sealing the combustion chamber is necessary to produce the proper compression for the fuel mixture to ignite and power the engine. The remaining passages in the head gasket are designed to allow oil and coolant to circulate throughout the engine while keeping these fluid channels separated.
Head gaskets on modern vehicles have to withstand a lot of abuse, including high heat and pressure resulting from compression, and have to be built to last. Head gaskets are typically made of layered steel with elastomers for durability and long life. Older vehicles may have been equipped with head gaskets made of graphite or asbestos, which, while strong, pale in comparison to modern head gasket designs.
When your head gasket blows, a few different things can happen depending on what part of the gasket has been damaged. A head gasket that blows between two of the cylinders can cause compression leaks from one cylinder to the next, which can result in misfires and a lack of power. If the external oil or coolant passage seal ruptures, you may notice coolant or oil leaks from the cylinder head, and if the internal passage fails, oil or coolant can enter the cylinder. Finally, if the area between the coolant and oil passages fails, coolant may be present in the engine oil, or oil may contaminate the engine coolant.
Now that we know what happens when your head gasket blows, we can discuss the symptoms you might experience during a head gasket failure. Though you may only experience one of these signs when your head gasket fails, it is possible that multiple symptoms may present simultaneously.
Misfires
Misfires from your engine can be caused by a variety of factors, but a blown head gasket can allow the cylinders to leak and prevent combustion from occurring. To determine if your misfire is caused by low compression or leaks, a compression tester can be used to take readings of the pressures in each cylinder. Though other factors could cause misfires, a head gasket failure may also demonstrate other symptoms that will help you narrow down your search. For more information on what may cause misfires, check out our misfire article.
Leaks or Low Oil and Coolant Levels
Oil and coolant leaks near the cylinder head should not be ignored, as these can quickly lead to low oil or coolant levels in the engine. This could eventually cause overheating or low oil pressure and result in further engine damage if levels are not monitored and topped off regularly. You may also experience coolant loss without seeing any leaks on your driveway, indicating that the coolant is being burned off inside the engine or has entered the crankcase and mixed with oil.
Overheating, Bubbling, and Sludge in the Radiator or Coolant Reservoir
When a head gasket fails, it can allow coolant to escape the engine in the form of leaks, or it may leak internally and allow coolant into the combustion chamber. Low levels of coolant can result in rapid overheating and may cause additional engine damage. Oil may also mix with the coolant and result in a sludgy brown coolant mixture that resembles an unappetizing chocolate milkshake. Because of oil buildup in the radiator or cooling system, you may experience clogged passages that will prevent coolant from circulating properly. You may also notice your coolant system bubbling due to pressure from the combustion chamber being forced into the cooling system.
White Exhaust Smoke
Coolant leaking into the cylinder can also burn during combustion, and the vapors from the coolant can cause your exhaust smoke to turn white. There are a few other reasons for smoke from the exhaust that are discussed more fully in our article on exhaust smoke coloration.
Low Oil Pressure, Milky Oil, and Engine Noise
Not only can oil leaks cause low oil pressure and engine damage, but a buildup of coolant in the oil could also thin the oil significantly. This reduces the oil’s ability to lubricate and protect your engine and can reduce oil pressure. You may also hear clunking, ticking, or rattling noises from the engine caused by excess friction between metal components and bearing surfaces internally. If you check your oil and it seems thin and looks milky, this could indicate that coolant is leaking through the head gasket into the oil supply. Yellow buildup or froth under your oil filler cap can also be indicative of coolant in the oil, as the coolant can vaporize while the engine is running and the gas may condense on the fill cap, PCV valve, and more.
If you’re wondering if you can continue driving your vehicle with a blown head gasket, the short answer is “no”. All of the problems caused by head gasket failure are potentially dangerous to the engine and could result in further damage if not addressed promptly. It is usually best to park the vehicle until you can address the problem yourself or get your car into a repair shop for professional service. Continuing to drive with coolant in the oil, oil in the coolant, or the vehicle misfiring on one or more cylinders can lead to wear, overheating, and engine failure more quickly than you might expect.
Once you’ve determined that your head gasket is the source of your problem, you may have a couple of options. Head gasket leak sealers or stop-leak products are designed to be poured into the radiator and circulated throughout the engine while it is running. The chemical compounds in the head gasket sealer are designed to react to heat and plug any small holes or gaps in the head gasket, sealing the gasket permanently if the damage is not especially severe. More extensive head gasket damage may only permit the head gasket sealer to work temporarily, and the problem may return over time.
The only way to know you’ve completely repaired your head gasket is to replace it. This involves extensive engine disassembly, usually requiring the intake manifold to be removed to access the cylinder head(s), which will also need to be taken off to get to the head gasket. It is usually a good idea to have your heads inspected while they are off the engine for a head gasket repair, as the same extreme heat that damaged the head gasket could also cause the heads to warp. If this occurs they may need to be machined or replaced to ensure that the new gasket seals properly. If your cylinder heads require milling or valve service during your head gasket repair, check to see if there is an O'Reilly Machine Shop near you.
It is also important to remember that head gaskets, unlike some cooling and oil delivery system gaskets, do not require any sealant. Head gaskets are designed with sealing channels that only require proper torque to seat and seal the gasket. Be sure that you know the proper torque specifications and procedure for your specific vehicle and engine, as most cylinder head bolts have to not only be torqued to the appropriate specification but also in a specific order. Typically, it is recommended to replace head bolts whenever a cylinder head is removed.
The only way to know you’ve completely repaired your head gasket is to replace it. This involves extensive engine disassembly, usually requiring the intake manifold to be removed to access the cylinder head(s), which will also need to be taken off to get to the head gasket. It is usually a good idea to have your heads inspected while they are off the engine for a head gasket repair, as the same extreme heat that damaged the head gasket could also cause the heads to warp. If this occurs they may need to be machined or replaced to ensure that the new gasket seals properly. If your cylinder heads require milling or valve service during your head gasket repair, check to see if there is an O'Reilly Machine Shop near you.
It is also important to remember that head gaskets, unlike some cooling and oil delivery system gaskets, do not require any sealant. Head gaskets are designed with sealing channels that only require proper torque to seat and seal the gasket. Be sure that you know the proper torque specifications and procedure for your specific vehicle and engine, as most cylinder head bolts have to not only be torqued to the appropriate specification but also in a specific order. Typically, it is recommended to replace head bolts whenever a cylinder head is removed.
We hope this has helped you better understand how your vehicle’s head gasket works, what happens when a head gasket blows, and how to repair your engine’s head gasket yourself. If you have reviewed the procedure for your vehicle and aren’t equipped to complete the repair yourself, we would be happy to help you Find a Repair Shop near you that can help you with your head gasket repair. O’Reilly Auto Parts also carries the head gaskets, cylinder head bolt kits, torque wrenches, and more that you might need to complete this repair properly.