Crankcase Breather: What It Is, How It Works & Why It Matters for Car Maintenance Beginners

If you are just starting to get involved in car maintenance, you may be unfamiliar with the component "Crankcase Breather". It is not as conspicuous as the engine or the braking system, but it plays a crucial role inside the engine.

As a professional brand of automotive diagnostic equipment, ANCEL has long been concerned about such "easily overlooked yet crucial" system issues. Among a large number of user cases, we have found that many engine failures are often related to clogged ventilation systems or aging hoses.

To help car owners better understand and troubleshoot such issues, this article will take you through a comprehensive understanding of the function, types, fault manifestations and replacement methods of the crankcase ventilation system.

What Is a Crankcase Breather?

The crankcase breather is a ventilation component inside an engine that is used to discharge the high-temperature gases within the crankcase. When the engine is running, some combustion gases will leak into the crankcase (called blow-by gases). These gases must be discharged in a timely manner; otherwise, problems such as excessive pressure, oil contamination, and component corrosion will occur. It is usually composed of several parts including a breather hose, a ventilation valve (such as a PCV valve), a breather filter, and an interface connected to the intake manifold.

What Does a Crankcase Breather Do?

1. Relieve Pressure and Prevent Leaks

The breather maintains normal pressure inside the crankcase by expelling blow-by gases. This prevents excessive crankcase pressure that could cause oil leaks.

2. Purify Exhaust Gases and Protect Engine Oil

If blow-by gases remain in the crankcase for a long time, they mix with the engine oil, causing oil degradation and reduced lubrication performance. The breather expels these gases in a timely manner, helping keep the oil clean.

3. Control Emissions and Meet Environmental Standards

Closed - loop ventilation systems direct crankcase gases into the combustion chamber for re - combustion. This avoids releasing unprocessed blow-by gases into the atmosphere, helping vehicles meet environmental regulations.

4. Maintain Engine Efficiency and Longevity

A properly functioning breather stabilizes crankcase pressure and preserves oil quality, ensuring smooth engine operation.

Blow-by Gas: Why Crankcase Needs Ventilation

Blow-by gas refers to the phenomenon where, during the engine combustion process, a portion of high-pressure gases, either unburned or post-combustion, leaks from the cylinders into the crankcase through the gaps between the piston rings. These gases usually contain fuel vapors (unburned gasoline or diesel), water vapor, carbon particles, or trace amounts of engine oil. Blow-by gas is a normal byproduct of engine operation, especially in high-pressure and high-temperature working environments, and it cannot be completely avoided.

Why does Blow-by Gas Appear in the Crankcase? 

This is due to the sealing limitations of the piston rings. When the piston moves up and down inside the cylinder, even if the piston rings fit well against the cylinder wall, it is difficult to achieve 100% sealing. Therefore, some gases leak into the crankcase through the gaps in the piston rings, forming blow-by gas.

The crankcase ventilation system (such as the PCV valve) can guide the blow-by gas back into the intake system for re-combustion. This maintains the pressure balance inside the crankcase, prevents oil leakage, reduces oil contamination, and extends the engine's lifespan. This is also the reason why every engine must be equipped with a crankcase ventilation system.

In summary, blow-by gas is an inevitable byproduct of engine operation. It must be promptly discharged or recycled through the crankcase ventilation system. Otherwise, it will seriously affect the engine's sealing performance, lubrication quality, and lifespan.

Types of Crankcase Ventilation Systems

Ventilation systems can roughly be divided into two major categories:

What are the Signs of a Failing Crankcase Breather？

The failure of a crankcase breather might not trigger direct fault codes like sensors or electronic systems do, but it can have significant effects on engine pressure, lubrication, and combustion efficiency:

• 1. Unstable or Shaky Idling
  Malfunctioning ventilation can disrupt the air-fuel ratio, causing irregular idling, fluctuation, or even engine stalling.
• 2. Emulsified or Discolored Engine Oil
  When blow-by gases can't be expelled, moisture accumulates inside the engine, leading to emulsified oil (milky or foamy appearance), which reduces lubrication performance.
• 3. Oil Leaks or Seepage
  Excess pressure in the crankcase can blow out oil seals or gaskets, causing visible oil leakage—especially around valve covers and front/rear crankshaft seals.
• 4. Blue Exhaust Smoke or Abnormal Emissions
  Ventilation failure may allow oil to enter the intake manifold with gases and burn, producing blue exhaust smoke. This is common in blocked or broken PCV systems.
• 5. Check Engine Light (CEL)
  On certain vehicles, PCV system faults may trigger codes like P0171 (system too lean) or P0507 (idle control system RPM too high).
• 6. Unusual Suction Noise or "Whistling"
  A cracked or loose breather hose can let unmetered air into the intake system, causing high-pitched whistling or hissing and disturbing the air-fuel mix, lowering fuel efficiency.

Note: Even though the breather hose is often considered an auxiliary part, it works together with the breather system to form a complete ventilation loop. Its aging, clogging, or leakage can degrade the entire system's function and may even lead to misdiagnosis of other engine faults.

Real-World Insight:
Several car owners have reported in forums that symptoms caused by a damaged breather or hose are often misidentified as ignition or fuel system issues. The true cause wasn't found until the crankcase ventilation system was inspected.

This shows that when facing vague or overlapping symptoms, using diagnostic tools for systematic troubleshooting is far more efficient than relying solely on experience or trial-and-error.

To address this, ANCEL offers a range of OBD2 scanners that support reading fault codes related to the PCV valve and breather system. These tools help users accurately determine whether the ventilation system is functioning properly—avoiding misjudgment of fuel or ignition system issues.

What Happens When a Crankcase Breather Goes Bad? 

Just as we mentioned above, when the Crankcase Breather malfunctions, the engine will show a series of “warning signs” – such as unstable idling, emulsified engine oil, blue smoke in the exhaust, engine shaking, and so on. If these signs are ignored and the Crankcase Breather completely breaks down, the impact will be far more than just superficial problems like “blue smoke emission” or “unstable idling”. It can cause pressure disorder, lubrication failure, abnormal emissions, and may lead to the overall premature damage of the engine. 

What's Breather Hose, Filter and Valve?

How to Replace a Crankcase Breather?

Tools:

• Flathead or Phillips screwdriver (depending on the vehicle model)
• Gloves, rags
• New breather filter (original or compatible model is recommended)

Steps:

1. Turn off the engine & let it cool

Ensure the engine is turned off and completely cooled to avoid burns from high temperatures.

2. Locate the filter position

It is usually located above the valve cover; some are built into the cover, and others are in the middle of the ventilation pipeline. (Refer to the vehicle manual for more accuracy.)

3. Remove the old filter

Use a screwdriver to remove the fixing screws or clips, then carefully take off the plastic cover.

Take out the old filter and note its installation direction.

4. Clean the installation area

Wipe the inner wall of the housing clean with a rag.

5. Install the new filter

Fit the new filter into place in the correct direction, ensuring it is securely installed without looseness.

6. Reinstall the cover and tighten the screws

7. Start the vehicle for inspection

Start the engine and check for abnormalities such as unusual noises or idle shaking to ensure the filter is working properly.

What’s the Difference between Crankcase Breather vs PCB valve?

Both crankcase breather and pcv valve belong to the Crankcase Ventilation System, but there are multiple dimensional differences between the two:

Overall, one intake and one exhaust jointly maintain the internal pressure balance and cleanliness of the crankcase.

Is it safe to drive with a broken breather hose?

If the breather hose is damaged, the engine won’t shut down immediately or stop working completely, and the vehicle may still be driven for short distances. However, it’s best to avoid doing so. A broken hose can cause a series of hidden risks to gradually worsen, and ignoring the issue over time can directly damage the engine’s sealing system and intake system.

Therefore, if you notice the following abnormalities while driving—such as a strong smell of oil in the engine bay, unstable idling, oil stains in the engine compartment, or the activation of warning lights—it may indicate a cracked breather hose. In such cases, especially if there is obvious oil or gas leakage, you should stop the vehicle immediately for inspection or have it towed to a repair shop to prevent more severe mechanical failures or safety hazards.

Why Crankcase Ventilation Matters for Engine Longevity?

The crankcase ventilation system effectively expels exhaust gases, maintains internal pressure balance, and prevents damage to oil seals and gaskets. It also helps reduce carbon deposits and oil sludge, ensuring smooth engine operation. Poor ventilation can lead to oil leaks, increased carbon buildup, and impaired engine performance and lifespan. Clearly, the crankcase ventilation system is the "respiratory lifeline" of the engine, and its efficiency directly determines the maximum lifespan of the engine. Therefore, regular inspection and maintenance of the breather, PCV valve, and hoses are crucial for extending engine life.

ANCEL provides professional engine diagnostic solutions to help you detect and resolve ventilation system issues in a timely manner, protecting your vehicle's long-lasting power and performance.