Almost every gasket and seal in your engine is designed to do one very specific thing: to keep a single type of fluid or gas from leaking out of, or into the engine. Examples are the front cover gasket, cam seals, intake manifold gasket, thermostat housing gasket, etc.
The head gasket (HG for short) is a whole other animal. Not only is it responsible for keeping fluid where it belongs, but it also must prevent oil, water, and air from mixing inside the engine. You can think of it as dividing the engine block in half horizontally; so everything flowing around inside the engine must pass through the head gasket at some point. Because of this, a head gasket can fail in many different ways, with many different symptoms. The well-known "chocolate looking water in the oil" problem may or may not be due to a blown head gasket...and you may just as easily have a blown HG without any water in the oil at all.
There are many types of head gaskets, such as MLS (multi-layer steel), composite, copper, and even some more exotic types. Which is best for a particular application is beyond the scope of this article, but any of them can fail.
Note: I believe that different types would be more likely to fail in specific ways, but I have no hard evidence available for this type of comparison. If anybody does, please contact me.
So how can you tell if your head gasket is shot?
An HG failure can be caused by the gasket not sealing flat, a small section of one surface peeling away, a small tear, a gaping rip, or even parts of the gasket completely missing. To understand how this affects your engine, you need to think about the ways in which fluids and air can mix together where the head meets the block, and what the symptoms are.
There are some strange things that can happen when a head gasket bites the dust, but listed below are the 5 most common issues. The numbers on the picture represent possible locations for each type of failure described, but keep in mind that failures can happen in many other areas of the gasket as well. (Not sure whose picture this is; if you would rather I didn't use it for this article, please let me know. Otherwise...thanks!)
1. Water in the oil
This seems to be the most common and well-known type of head gasket failure, and is easily identified by the engine oil being a chocolate-tan color instead of the usual black or golden color. It will actually look kind of like a cappuccino or light chocolate milk.
This problem occurs when a pathway has opened in the HG that lets hot, high-pressure coolant find it's way into the oil passages. The coolant then mixes together with the oil and gets whipped up into a frothy or slightly foamy-looking brown fluid, the color of which varies depending on how much leakage there is. This is the nastiest single failure mode IMO, since there usually isn't any obvious warning before serious damage is done due to lack of lubrication, unless you happen to notice it when checking the oil. You may also see oil in the coolant, depending on which system is under the most pressure when the HG fails.
Note: Just because you have water in the oil, it doesn't necessarily mean a blown head gasket...but it's a likely cause, especially if the problem appears suddenly for no apparent reason.
2. "Pushing coolant", or coolant in the cylinders
If a break occurs in the head gasket between a cylinder sealing ring and a water passage, a couple of things can happen. Depending on the temperature and pressure in the coolant system, coolant can be drawn into the combustion chamber and burned, or pressure from the cylinders can leak into the coolant system...or both.
When coolant is drawn into the cylinders and burned, you will usually notice the distinctive "sweet" smell of burning anti-freeze from the exhaust, assuming you aren't running pure water. You can also smell this under the hood sometimes, but check for a radiator or coolant hose leak before assuming you have a blown HG. There may also be white smoke coming from the exhaust. (FYI – burning coolant produces white smoke, a rich fuel mixture produces black smoke, and burning oil usually produces blue or bluish-white smoke).
You'll also more than likely feel a difference in the car's power, although this can vary a great deal from hardly noticeable to extreme, again depending on the size of the leak. At the very least, you'll likely notice a miss or strange idle; at the most you could theoretically hydro lock the motor due to fluid in the cylinders, although I've never actually heard of this happening.
When it goes the other way and air escapes from the cylinders into the coolant, you'll get the often-mentioned "pushing coolant" scenario. What happens is the compression in the combustion chamber pressurizes the coolant system, causing it to bubble or flow out of the radiator overflow bottle. This type of failure will usually cause the car to run hot, and can quickly cause a cascade of other problems.
Although you can buy equipment to test for this, there is a cheaper method that has been used for years. Remove the thermostat (with the engine cold!), start the car, and observe the fluid flowing through the thermostat housing after the engine is warm. If you see it bubbling or the level bouncing around dramatically, you may have a pushing coolant issue.
Note: A few occasional bubbles in the coolant means that there is air in the system, but not necessarily from a head gasket failure. If this is the case, the air should work it's way out of the system within a few minutes. If you are pushing coolant, it won't stop and the bubbling will probably be more dramatic.
3. Oil or coolant coming from under the head
This one is a bit more obvious. If the outer perimeter of the head gasket develops a break at an oil passage, you'll usually notice it right away due to smoke or oil coming from the junction between the block and head. Note that you can also leak coolant from under the head.
Note: On DSM's it is far more likely for the exhaust manifold to be leaking oil in the front of the head than it is for a bad head gasket to be the culprit. Some of the stock heads have a couple of exhaust manifold mounting bolt holes drilled into oil passages, and oil can leak out around the mounting bolts and onto the exhaust manifold or head.
It's also possible to get oil in the cylinders due to a failed head gasket, but it's not all that likely. Oil in the cylinders (without other HG symptoms) is usually due to worn piston rings or leaking valve seals.
4. Cylinder pressure leaking from under the head
This type of HG failure is usually very obvious and can quickly become catastrophic. If pressure starts leaking from the cylinder to the outside of the head, it doesn't take long for it to get under the gasket and destroy it. Once this happens, you will probably see a combination of the other symptoms since at this point oil, coolant, and air can all get together and do really nasty stuff to your engine. You'll most likely see a sudden drop in power or the car will be hard to start, followed by white smoke in the engine bay and the smell of burning coolant.
This type of failure can also cause a rapid buildup of heat in the head, sometimes causing it to warp.
5. Cylinder pressure leaking to another cylinder
This is another common type of head gasket failure, although it's usually not disastrous...at least not at first. The main symptom is a loss of power and an overall drop in performance. It may show up in a compression test as two adjacent cylinders both reading significantly lower than the others and with fairly close numbers, but the best way of isolating this type of problem is with a leak-down test to see if air is escaping into an adjacent cylinder.
Besides the symptoms described above, a good way to isolate problems with the head gasket and other similar issues is to perform a leak-down test. This involves pressurizing the cylinders and then measuring the amount of pressure that is being lost, as a percentage. By listening for where the air is going, you can isolate problems such as leaky valve seals and piston rings, as well as HG issues.
The head gasket (HG for short) is a whole other animal. Not only is it responsible for keeping fluid where it belongs, but it also must prevent oil, water, and air from mixing inside the engine. You can think of it as dividing the engine block in half horizontally; so everything flowing around inside the engine must pass through the head gasket at some point. Because of this, a head gasket can fail in many different ways, with many different symptoms. The well-known "chocolate looking water in the oil" problem may or may not be due to a blown head gasket...and you may just as easily have a blown HG without any water in the oil at all.
There are many types of head gaskets, such as MLS (multi-layer steel), composite, copper, and even some more exotic types. Which is best for a particular application is beyond the scope of this article, but any of them can fail.
Note: I believe that different types would be more likely to fail in specific ways, but I have no hard evidence available for this type of comparison. If anybody does, please contact me.
So how can you tell if your head gasket is shot?
An HG failure can be caused by the gasket not sealing flat, a small section of one surface peeling away, a small tear, a gaping rip, or even parts of the gasket completely missing. To understand how this affects your engine, you need to think about the ways in which fluids and air can mix together where the head meets the block, and what the symptoms are.
There are some strange things that can happen when a head gasket bites the dust, but listed below are the 5 most common issues. The numbers on the picture represent possible locations for each type of failure described, but keep in mind that failures can happen in many other areas of the gasket as well. (Not sure whose picture this is; if you would rather I didn't use it for this article, please let me know. Otherwise...thanks!)
You must be logged in to view this image or video.
1. Water in the oil
This seems to be the most common and well-known type of head gasket failure, and is easily identified by the engine oil being a chocolate-tan color instead of the usual black or golden color. It will actually look kind of like a cappuccino or light chocolate milk.
This problem occurs when a pathway has opened in the HG that lets hot, high-pressure coolant find it's way into the oil passages. The coolant then mixes together with the oil and gets whipped up into a frothy or slightly foamy-looking brown fluid, the color of which varies depending on how much leakage there is. This is the nastiest single failure mode IMO, since there usually isn't any obvious warning before serious damage is done due to lack of lubrication, unless you happen to notice it when checking the oil. You may also see oil in the coolant, depending on which system is under the most pressure when the HG fails.
Note: Just because you have water in the oil, it doesn't necessarily mean a blown head gasket...but it's a likely cause, especially if the problem appears suddenly for no apparent reason.
2. "Pushing coolant", or coolant in the cylinders
If a break occurs in the head gasket between a cylinder sealing ring and a water passage, a couple of things can happen. Depending on the temperature and pressure in the coolant system, coolant can be drawn into the combustion chamber and burned, or pressure from the cylinders can leak into the coolant system...or both.
When coolant is drawn into the cylinders and burned, you will usually notice the distinctive "sweet" smell of burning anti-freeze from the exhaust, assuming you aren't running pure water. You can also smell this under the hood sometimes, but check for a radiator or coolant hose leak before assuming you have a blown HG. There may also be white smoke coming from the exhaust. (FYI – burning coolant produces white smoke, a rich fuel mixture produces black smoke, and burning oil usually produces blue or bluish-white smoke).
You'll also more than likely feel a difference in the car's power, although this can vary a great deal from hardly noticeable to extreme, again depending on the size of the leak. At the very least, you'll likely notice a miss or strange idle; at the most you could theoretically hydro lock the motor due to fluid in the cylinders, although I've never actually heard of this happening.
When it goes the other way and air escapes from the cylinders into the coolant, you'll get the often-mentioned "pushing coolant" scenario. What happens is the compression in the combustion chamber pressurizes the coolant system, causing it to bubble or flow out of the radiator overflow bottle. This type of failure will usually cause the car to run hot, and can quickly cause a cascade of other problems.
Although you can buy equipment to test for this, there is a cheaper method that has been used for years. Remove the thermostat (with the engine cold!), start the car, and observe the fluid flowing through the thermostat housing after the engine is warm. If you see it bubbling or the level bouncing around dramatically, you may have a pushing coolant issue.
Note: A few occasional bubbles in the coolant means that there is air in the system, but not necessarily from a head gasket failure. If this is the case, the air should work it's way out of the system within a few minutes. If you are pushing coolant, it won't stop and the bubbling will probably be more dramatic.
3. Oil or coolant coming from under the head
This one is a bit more obvious. If the outer perimeter of the head gasket develops a break at an oil passage, you'll usually notice it right away due to smoke or oil coming from the junction between the block and head. Note that you can also leak coolant from under the head.
Note: On DSM's it is far more likely for the exhaust manifold to be leaking oil in the front of the head than it is for a bad head gasket to be the culprit. Some of the stock heads have a couple of exhaust manifold mounting bolt holes drilled into oil passages, and oil can leak out around the mounting bolts and onto the exhaust manifold or head.
It's also possible to get oil in the cylinders due to a failed head gasket, but it's not all that likely. Oil in the cylinders (without other HG symptoms) is usually due to worn piston rings or leaking valve seals.
4. Cylinder pressure leaking from under the head
This type of HG failure is usually very obvious and can quickly become catastrophic. If pressure starts leaking from the cylinder to the outside of the head, it doesn't take long for it to get under the gasket and destroy it. Once this happens, you will probably see a combination of the other symptoms since at this point oil, coolant, and air can all get together and do really nasty stuff to your engine. You'll most likely see a sudden drop in power or the car will be hard to start, followed by white smoke in the engine bay and the smell of burning coolant.
This type of failure can also cause a rapid buildup of heat in the head, sometimes causing it to warp.
5. Cylinder pressure leaking to another cylinder
This is another common type of head gasket failure, although it's usually not disastrous...at least not at first. The main symptom is a loss of power and an overall drop in performance. It may show up in a compression test as two adjacent cylinders both reading significantly lower than the others and with fairly close numbers, but the best way of isolating this type of problem is with a leak-down test to see if air is escaping into an adjacent cylinder.
Besides the symptoms described above, a good way to isolate problems with the head gasket and other similar issues is to perform a leak-down test. This involves pressurizing the cylinders and then measuring the amount of pressure that is being lost, as a percentage. By listening for where the air is going, you can isolate problems such as leaky valve seals and piston rings, as well as HG issues.
Last edited by a moderator:
