A cylinder head is the “top end” of the engine. It mates to the engine block to seal off the combustion chamber, which is where the engine burns fuel to make power.
Cylinder heads also house the valves that direct airflow. Intake valves draw in fresh air, combustion occurs, then exhaust valves allow the burnt air fuel mixture to exit the combustion chamber. An internal combustion engine is basically just a fancy air pump.
Under extreme conditions, a cylinder head can actually crack. A cracked cylinder head may yield similar symptoms to a blown head gasket or a cracked block, but is more expensive to fix than a blown head gasket. The solution is usually to buy a new cylinder head.
What Causes a Cracked Cylinder Head?
Internal combustion engines heat up very quickly. As the name implies, they are designed to contain small, controlled explosions.
Much of the energy in the combustion process is actually lost to thermal energy, instead of being converted into kinetic energy to power the vehicle. This is a byproduct of their design that can be mitigated, but never eliminated entirely.
The number one cause of a cracked cylinder head is overheating. When an engine overheats, its components may be stressed far beyond the heat threshold it was designed to withstand.
Since most heads are made out of aluminum, they may warp or crack when the engine gets hot enough.
An engine may overheat from a number of reasons, most of which are due to a failed component in the cooling system. Some possible causes are outlined below.
2) Air in the Cooling System
Air in the cooling system may allow hot spots to form, where one part of the engine is much hotter than the portion measured by the temperature sensor. This happens because air cannot transfer heat as effectively as a liquid can.
If air is trapped in one spot away from the coolant temperature sensor, the temperature sensor may not pick up the hot spot. You may not know your car is running hot until damage is already done.
Additionally, an air leak in the cooling system may reduce the pressure in the cooling system. Liquid coolant is more likely to boil at a lower pressure.
3) Failed Water Pump
If the water pump fails, coolant won’t be able to circulate through the engine properly. The coolant in the radiator may be nice and cold, but the coolant surrounding the water jackets in the head and block will be extremely hot.
Without a pump, coolant will only be able to circulate via the natural convection process, which is much too slow to remove excess heat before the engine overheats.
4) Failed Thermostat
Combustion engines are most efficient when they are running at the operating temperature they were designed for.
An engine’s normal operating temperature is still plenty hot enough to scald you – typically around 190 to 210 degrees Fahrenheit (85 to 99 degrees Celsius). Running the engine below this temperature increases fuel consumption, emissions, and wear.
A thermostat blocks off coolant from the radiator until the coolant in the engine reaches the target temperature. Once the target temperature is reached, the thermostat opens to allow coolant at ambient temperature to enter the engine, cooling it until the thermostat is cold enough to close. The cycle continues as the engine runs.
If the thermostat is stuck open, your car will probably run pretty cold. This is because the coolant from the entire system is cycling through the radiator 100% of the time and rarely gets a chance to fully warm up.
If the thermostat is stuck closed, your engine will overheat pretty easily. The hot coolant has nowhere to go to cool off. In a pinch, some engines allow you to use the heater as a tiny radiator.
The effectiveness of this method depends on the size of the heater core and how the cooling system is routed.
5) Coolant Leak
If too much coolant leaks out, eventually you won’t have enough to properly cool the engine. You can lose a decent amount of coolant before overheating, but it’s a good idea to regularly check the overflow reservoir and radiator to make sure you’re topped off.
Cracked Cylinder Head Symptoms
A minor crack in the cylinder head may go unnoticed for some time, but the symptoms will be pretty obvious in severe cases. Below are five common symptoms to watch out for in the event that this happens.
1) Oil Leak
A cylinder head contains oil in it. If you have a cracked head, you can expect oil to leak out of it. You should notice the oil light on your dashboard turning on when this happens indicating a low oil pressure.
If you see this light on, open your hood and try to see if there’s engine oil near the cylinder head.
2) Coolant Leak
Although a coolant leak can cause a cracked head, it is also a symptom. If the cylinder head is severely cracked, then you’ll probably have coolant leaking out of it in addition to oil.
This will cause your engine to overheat, which your car will warn you about on your dashboard. Sometimes coolant leaks are internal. Internal leaks may leak into the oil passages or the combustion chamber.
Either way, check your coolant level and do not continue to drive when you have a visible coolant leak or notice the coolant level quickly dropping over time.
3) Poor Engine Performance
The first symptom you might experience is a noticeable loss in engine power. If the cylinder head has a crack, that means compressed air is escaping from the combustion chamber.
You will notice the engine running a lot slower or awkwardly once this happens.
4) Smoke From Engine
Although this is rare, big cracks in the cylinder heads will allow coolant to leak into the combustion chamber and cause white smoke to come from the engine.
In addition, leaking oil can make contact with hot engine components to produce this smoke. This is clearly a sign that a cracked cylinder head may be at fault.
5) Engine Misfire
This relates to number three. If the cracked cylinder head is severe, then the mixture in the combustion chamber will cause a misfire. This means the mixture won’t burn like it should.
Either that, or your engine will simply die as you’re driving and you will have to repeatedly start it back up.
Cracked Cylinder Head Repair Cost
The cost of a cracked cylinder head repair job will vary depending on the make and model of the vehicle that it’s in. You can be sure that it will cost at least $500, which includes labor and parts costs.
If you were to replace the entire cylinder head, it would only cost $200 to $300 on average for parts. With labor at about $90 to $100 per hour, this comes out to roughly $500 for the job. However, this is assuming the cylinder head is made of aluminum like they are in most cars these days.
If the cylinder heads are hard to access (as they are on Subarus, since they use a boxer engine layout), labor costs may be significantly higher.
If you have an older vehicle or more expensive vehicle, you probably have a cylinder head made of iron. Since iron is a more expensive material than aluminum, you might be looking at $500 just for the parts.
The labor may also require more hours for this repair job to be done correctly. Therefore, you could be looking at $1,000 to repair a cracked cylinder head made of iron.