Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.
Symptoms of a Blown Head Gasket
Symptoms of a blown head gasket vary depending on how a gasket fails, but common signs include:
- Engine oil with a milky appearance
- Coolant with an opaque, brownish appearance
- Bubbles in the coolant
- Engine performance issues
- White smoke through the tailpipe
- Bluish smoke through the tailpipe
A blown head gasket may allow:
- Coolant or engine oil to enter one or more cylinders
- Oil to enter the coolant system
- Coolant to enter the lubrication system
- Combustion gases to contaminate and compromise the cooling or lubrication system
For the most part, a head gasket fails due to overheating, wear, or corrosion. Although a failed gasket is not too common, knowing the symptoms helps you prevent further and more expensive damage if you heed the warnings.
A faulty head gasket may share symptoms with other components like a thermal vacuum valve (TVV) or switch (TVS), for example. The TVV or TVS is part of the exhaust gas recirculation (EGR) system in some vehicles.
The EGR system uses the TVS to turn off exhaust gas recirculation when engine temperature is too low. Check the TVV, TVS or EGR system in your vehicle, if necessary. You’ll find some help in the Resources section at the bottom of this post, and your vehicle repair manual.
If you do not have the Haynes manual yet, you can buy a relatively inexpensive copy through Amazon. These manuals include:
- Step-by-step procedures
- Electrical diagrams
- Suggested maintenance schedule to prevent unnecessary break downs
- Diagnostic procedures
- Location and replacement of components
In a short period, you will recoup your small investment.
Answering Head Gasket Questions
- What Is the Purpose of a Head Gasket?
- What Are the Signs of a Blown Head Gasket?
- What Causes a Blown Head Gasket?
- How To Diagnose a Blown Head Gasket
- Diagnostic Results
- How Can I Prevent a Blown Head Gasket?
Read More from AxleAddict
1. What is the Purpose of a Head Gasket?
A head gasket plays a key role:
- Seals the combustion chambers.
- Allows engine oil and coolant to flow across the cylinder head and block without letting them mix together or enter the cylinders.
- Prevents combustion gasses from contaminating the oil or cooling systems.
To do its job, a head gasket should withstand normal combustion pressure and engine operating temperatures.
2. What Are the Signs of a Blown Head Gasket?
Structurally, a blown head gasket has stopped functioning properly.
Depending on how severe the initial damage is, you begin to notice one or more of the following signs.
Sign #1: Engine Overheating
Head gasket failure usually manifests through engine overheating.
Overheating may occur because one or more components wear down, a system malfunctions, or a component breaks down:
Overheating starts with one or more engine system faults:
- Clogged radiator
- Stuck-closed thermostat
- Leaking or damaged cooling system hose
- Failed radiator cap
- Bad water pump
- Timing belt or drive belt failure
- External or internal coolant leaks
Most head gaskets can stand some overheating. However, severe overheating will cause a head gasket to crush, which may result in coolant, oil, or combustion gas leaks.
If your car overheats on the highway: this pattern often comes from a small coolant leak that speeds up with engine speed and load. It's possible coolant is leaking into a cylinder. When the engine overheats, you may see coolant boiling in the overflow tank.
Sign #2: Frequently Topping Up the Radiator with Coolant
Coolant leaks are not uncommon, especially on older vehicle models. Still, small, exterior leaks are hard to detect. And internal coolant leaks are even harder to diagnose, especially when coolant seeps into one or more cylinders. You may become aware of the problem when topping up the radiator or coolant reservoir more often.
You may also notice:
- Poor heating
- Idle fluctuations
Furthermore, coolant leaking into a cylinder can lead to hydrolock. Hydrolock occurs when a piston seizes in place in its upward stroke because water in the cylinder won't let it travel any further. At this point, you won't be able to crank or start the engine.
Hydrolock may cause engine mechanical damage, though.
If possible, inspect the cylinders:
- Remove the spark plugs.
- Have an assistant try to crank the engine.
- Watch for coolant coming out of the spark plug holes.
Sign # 3: Bluish or White Smoke Coming Out Tailpipe
A failed head gasket may allow engine oil or coolant into the combustion chamber.
Engine oil leaking into a cylinder will cause a bluish smoke to come out the tailpipe.
Coolant leaking into a cylinder will cause a stream of white smoke to come out the tail pipe.
But don’t confuse a coolant leak with water condensation. Condensation during the cold months of winter can produce a bit of a white cloud through the tailpipe of your car.
If you find yourself frequently topping up the system with new engine oil or coolant, a blown head gasket or cracked cylinder head could be the problem.
Sign #4: Bubbles in the Coolant or Crankcase Under Pressure
A failed head gasket can contaminate the cooling system with combustion gasses.
Combustion gases under pressure will form bubbles in the coolant, which you may see in the coolant reservoir or by removing the radiator cap.
Also, combustion gases may find their way into the lubrication system and the crankcase. You won't be able to see the effect directly. However, the engine oil dipstick may pop out when pushed in into place while the engine is running.
Sign #5: Engine Oil with a White Discoloration
When coolant mixes with the oil in the lubrication system, the engine oil will turn into a milkshake-type substance.
You can check engine oil condition by pulling the engine oil dipstick or looking under the oil filler cup.
If you notice this typical oil discoloration, you may be dealing with a blown head gasket.
Sign #6: External Coolant or Engine Oil Leaks
A failed head gasket can create a path for coolant or engine oil to leak down the outside of the engine block.
Look for signs of coolant or oil on the floor under the engine. These dirty spots on the floor can help you trace a leak back to its source.
However, you may need to wash clean the engine first before you can detect or follow the path of the leak.
Sign #7: Engine Misfires
A failed gasket can open a path into a combustion chamber, allowing coolant, oil, into a cylinder; or combustion gasses to escape into adjacent cylinders. Any of these conditions may cause a cylinder to misfire.
Sign #8: Loss of Engine Power or Poor Engine Performance
When a head gasket snaps, coolant may leak into a cylinder and find its way out through the exhaust. As a result, coolant additives like silicate and phosphate can block the oxygen sensor (O2S), preventing sensor proper operation.
A blocked O2 sensor can’t read oxygen content and signals the car’s computer the air-fuel mixture is too rich. Then, the wrong signal from the sensor prompts the computer to lean the mixture. You may notice one or more of the following conditions:
- Rough idle
- Poor acceleration
If necessary, check the oxygen sensor in your vehicle. You will find some help in the Resources section at the bottom of this post. Consult your vehicle repair manual as well.
3. What Causes a Blown Head Gasket?
Broadly speaking, a normal temperature range for a car engine oscillates between 195 and 225F (90 to 107C) degrees. 260F (126C) degrees or more may cause severe damage to the head gasket and other internal components.
The temperature gauge on your dashboard will warn you of dangerous engine overheating. However, there’s no gauge to warn you of dangerous engine pressure. But overheating or too much pressure can destroy an engine if given a chance.
Various abnormal engine conditions can lead to overheating, excessive pressure, and a blown head gasket.
Cause #1: Faulty Cooling System
A faulty cooling system is a common source of engine overheating issues. System problems may include one or more of the following issues:
- Bad water pump
- Stuck-closed thermostat
- Clogged radiator
- Damaged or leaking hoses
- Old coolant
- Dirty or obstructed cooling system
If excessive overheating occurs, you may end up with a blown head gasket, a cracked engine block or cylinder head.
Cause #2: Excessive Combustion Pressure
Excessive combustion pressure may come from detonation or pre-ignition. Detonation may occur in late model vehicles that require premium octane fuel when the wrong type of fuel is used.
In most vehicles, pre-ignition can also occur with the following conditions:
- EGR system restrictions
- Over-advanced ignition timing
- Lean air-fuel mixture (dirty injectors, low fuel pressure)
- Cooling system problems
- Exhaust restrictions
Cylinder pre-ignition combined with regular combustion will cause knock or ping under load. That’s because the first explosion literally collides with the second combustion front and the upward moving piston.
These powerful explosions damage the fire rings (the head gasket metal rings surrounding the cylinders), allowing combustion pressure to escape the cylinder.
You may notice:
- Rough start
- Rough idle
Cause #3: Using the Wrong Type of Fuel
Adding gasoline to the fuel tank of a diesel engine can destroy a head gasket and the engine.
Since gasoline ignites faster compared to diesel fuel, the engine's temperature will soar in a matter of seconds. Overheating then readies to destroy not only the head gasket but other components as well:
- Glow plugs
- Other major components
Checking for Use of Wrong Fuel Type
Sometimes, checking for use of wrong fuel type takes a few easy steps:
- Remove the fuel filler cap
- Smell the fuel at the filler neck
- Does it smell like gasoline?
Remove and thoroughly clean the tank.
If you are not sure but suspect there's gasoline in the tank, you may need to have a shop test for proper API gravity using a sample.
Alternatively, you may decide to just drop the tank and clean it.
Cause #4: Plugged PCV System
A clogged positive crankcase ventilation (PCV) system will cause pressure inside the crankcase.
Too much pressure will destroy sealers and gaskets:
- Oil pan gasket
- Valve cover gasket
- Timing cover gasket
- Main oil seals
- Head gasket
Cause #5: Engine Modifications
Car engines can benefit from new aftermarket components and systems. Accessories change a particular style or help engine performance.
However, better performance may come with unwanted results. Take, for example, swapping a stock exhaust system with a high-performance unit that gives your engine increased fuel efficiency and power.
But increased exhaust flow can upset valve operation on some models equipped with a back pressure vacuum operated EGR valve.
More pressure in the combustion chamber can lead to severe spark knock and damage a piston or blow a head gasket.
Cause #6: Type of Engine Model
During the 1980s and '90s, some Honda Civic, Toyota, and Mitsubishi engines were facing unusual head gasket failures.
In many of these cases, small engine design in tight spaces under tight tolerances was leading to poor heating management.
- Aluminum heads would expand and contract under a range of operating temperatures, crushing the gasket.
- Other cylinder heads just could not seal right, leading to leaks.
Aftermarket suppliers solved some of these issues with special head gaskets with a strong design to stand cylinder head movement and better sealing properties.
You can check online for special bulletins put out by your vehicle manufacturer on your particular model. All you need is your vehicle identification number (VIN).
4. How to Diagnose a Blown Head Gasket
Diagnosing for a blown head gasket requires more than just looking under the hood. Actually, you may have to conduct more than one test.
The following tests can provide the evidence you are looking for. However, not all of these tests may apply to your particular case; sometimes, it depends on how a head gasket has failed. Follow the logic provided by the symptom and choose the test accordingly.
For example, an excess of white smoke coming out the tailpipe may call for a compression and combustion leak test.
Combine both the symptoms and your test results to reach a diagnostic.
Test 1: Looking for Bubbles in the Coolant
Bubbles in the coolant point to trouble under the cylinder head. This is what makes this test one of the simplest and favorites among home (and shop) mechanics.
This test applies to engines with a water pump run by a drive belt, not a timing belt.
- Make sure the engine is cold.
- Remove the drive belt that runs the water pump.
- Unscrew and take off the radiator cap.
- Set the transmission to Park (automatic) or Neutral (manual).
- Engage the emergency brake.
- Start and let the engine idle.
- Take a look at the coolant through the radiator filler neck.
If you see bubbles in the coolant before it heats up, either you have a defective head gasket or a cracked cylinder head.
Test 2: Pressure Test
Frequently topping up the cooling system with new coolant is a clear sign of a system leak. The leak may point to a blown head gasket with an external or internal leak.
An external coolant system leak may reveal itself by leaving behind a wet path on its way down from the head gasket area.
You may need to wash clean the engine. It is almost impossible to find a leak when working on an engine covered by layers of grease and dirt.
With the engine cleaned, do first a simple visual inspection around the head gasket area. Then, move to the rest of the cooling system areas.
If you still find no evidence of a leak, use a cooling system pressure tester. You may borrow a pressure tester from your local auto parts store. Follow the instructions that come with the tool.
Once the system is under pressure, examine the gasket area first, and then the rest of the cooling system areas:
- Radiator seams around tanks
- Timing cover bottom for water pump leaks
- Thermostat housing
- Radiator and heater core hoses
- Heater core
To look for an internal coolant leak, follow the instructions for the combustion leak test described next.
Test 3: Combustion Leak Test
A blown head gasket may allow combustion gases to leak into the cooling system. You can diagnose this type of leak with a combustion leak tester.
The combustion leak detector uses a container with a special chemical in a blue liquid. This blue liquid turns yellow if combustion gases are seeping into the cooling system. Check the video below to see how to perform this test.
You can perform this test at home:
- Remove the radiator cap.
- Insert the tip of the tool into the radiator neck, without touching the coolant.
- Draw air into the tool.
Follow the instructions that come with the tool.
You can find relatively inexpensive combustion leak detectors online. However, for one-time use, you may borrow this tool from your local auto parts store.
Test 4: Vacuum Test
The engine pistons create manifold vacuum when moving downward during the intake stroke as they pull the air-fuel charge into the cylinder.
With a blown head gasket, though, a cylinder may draw more than air and fuel. To diagnose this problem, use a vacuum gauge tester.
Pay attention to the gauge needle. A leaking head gasket will cause the needle in the vacuum gauge to vibrate as it moves past the normal vacuum range, up and down.
Needle movement patterns reveal different engine mechanical problems. A vacuum test can point to ignition timing problems, valves condition, and other mechanical issues.
You can conduct this vacuum test at home. If you don't have a vacuum gauge, you may loan one from your local auto parts store. Also, see the Resources section at the bottom of this post, and consult your vehicle repair manual as well.
Performing a Combustion Leak Test
Test 5: Exhaust Gas Analysis
As another option to the previous combustion leak test, you can take your vehicle to an emission test site in your area for an exhaust gas analysis.
An exhaust gas analyzer performs the same basic function of a combustion leak detector; it searches for the presence of hydrocarbons (HC) in the cooling system. If HC gases are present, there's a leak in the combustion chamber.
You can test your vehicle for free or pay up to $60.00 dollars, depending on where you live. Search for emission testing locations in your area.
Test 6: Compression Test
You can do a compression test to double check the results of a previous test, like a vacuum test for example.
Normally, all the cylinders in your car’s engine should have about the same compression, according to the manufacturer specifications.
A significant difference in pressure between one or more cylinders indicates a leak. The presence of a leak may come from a failed head gasket, but it’s not the only possibility.
Other potential leak sources include:
- Worn or broken piston rings.
- Buildup or excessive wear on valves.
You can do a compression test at home using a compression gauge. If you don't have this tool, your local auto parts store may loan you one.
For more help, check the Resources section at the bottom of this post. Your vehicle repair manual should list the correct compression specification and allowable variations for your particular model.
Test 7: Cylinder Leakage Test
A cylinder leakage test (or leak-down test) is one of the most accurate tools when evaluating overall engine condition. This test can detect a leaking head gasket, cylinder head, piston ring, and exhaust or intake valves.
Conducting this test in a shop, a technician tests each cylinder by injecting pressurized air. A leak of 20% is not cause for alarm; around 30% indicates an engine in poor state; over 30% will indicate a serious problem.
5. Diagnostic Results
One or more of these tests will help you reveal the condition of your engine’s head gasket and, possibly, the cause of the problem.
However, a test doesn’t guarantee a clear result. A test result can be ambiguous or may not you much about the source of the problem. A reputable shop can help you with clarify these issues.
Once you have confirmed that your head gasket has failed, make sure you know what caused the problem as well. Knowing the source of the problem will determine how you need to proceed with the repairs.
Overheating, for example, is the most common cause behind a failed head gasket. In this case, before you install a new head gasket, make sure to do the following:
- Check the gasket sealing surfaces for signs of warp.
- Inspect castings for cracks.
- Examine the cooling system and radiator fins for restrictions.
- Make sure the cooling fan is working right.
- Check the EGR system for restrictions, carbon buildup, and good valve operation.
It is important to have a proper diagnostic. A head gasket replacement job is not cheap by any means, especially if you have to turn to a shop to have it repaired. However, postponing the repair or replacing a gasket without finding the source of the problem may expose your engine to further and more expensive repairs in the future.
A Cheap Fix
If you know you have a minor head-gasket leak, you may be able to postpone repairs temporarily by using a stop-leak aftermarket product. But avoid putting off repairs for a long time, especially if you have not found the source of the problem.
Disassembling an Overheated Cylinder Head
If the head gasket failed because the engine overheated, and you want to remove the head yourself for repairs, follow this tip for better results.
When disassembling the cylinder head, avoid loosening the head bolts at the center and working your way out, which is the standard procedure. This could exacerbate the problem since warping usually happens at the center.
Instead, work your way backwards by starting with the bolts located towards the outside of the head, and work your way towards the center. In other words, follow the bolt-loosening sequence stated in your vehicle repair manual in the reverse order.
This will prevent further distortion of the head, and possibly save you from replacing the head.
6. How Can I Prevent a Blown Head Gasket?
Preventing overheating helps you prevent many of the problems associated with a blown head gasket and other issues.
First of all, follow the maintenance schedule for the cooling, lubrication, and ignition timing systems suggested by your car manufacturer.
A car’s manufacturer schedule appears in the vehicle repair manual for your particular vehicle make and model.
Here are a few important pointers you can follow to prevent the most common causes of head gasket failure:
- Change the coolant at the appropriate intervals.
- Add a cleaner or additive to the cooling system to prevent buildup.
- Check coolant condition and level periodically.
- Examine the condition of drive and timing belts and replace them according to schedule.
- Remove debris from the radiator fins and check hoses periodically.
- Check the thermostat housing regularly, and replace the thermostat every three or four years.
- Investigate and fix misfire or detonation issues as soon as they start.
- Check the condition and level of the engine oil and replace it according to schedule.
You can perform many other maintenance tasks that can help you prevent pre-ignition, oil and coolant leaks, and restrictions in the EGR and PCV systems, blockages and many other problems associated with overheating.
And the manual lists many other tasks that will help you maintain your engine’s overall health.
- Use a Vacuum Gauge to Troubleshoot Your Car's Mechanical Problems
Use a vacuum gauge to find car mechanical problems faster at a low cost.
- How to Check an Oxygen Sensor
Learn how to check an oxygen sensor and eliminate hit-and-miss repairs using a digital multimeter.
- How to Test an EGR Valve: A Simple Procedure
Learn how to test an EGR valve in a few minutes to confirm you actually need a new replacement.
- How to Do an Engine Compression Test
This engine compression test will reveal your gasoline or diesel engine's mechanical condition. It's fast, cheap, and reliable.
- Why Does My Engine Knock?
Diagnose engine knocking with the help of this simple guide before your engine suffers costly damage.
- What Causes a Car to Overheat?
What causes a car to overheat? This diagnostic-based guide will help you fix your car sooner.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2022 Dan Ferrell