The positive crankcase ventilation system is responsible for removing gases that escape from the cylinders into the engine crankcase. Its main purpose is to stabilize the pressure inside the engine, reduce harmful emissions, and prevent oil leakage through the seals.
During engine operation, some of the exhaust gases seep past the piston rings into the crankcase. If they are not removed, the pressure will rise so high that it will begin to push oil through the seals and gaskets. That is why the PCV system creates a controlled path for these gases to be removed, returning them to the intake manifold to be burned again.
How does the crankcase ventilation system work?
To understand how crankcase ventilation works, you need to understand one simple fact: any engine, even a new one, always has a small amount of gases escaping past the piston rings. These are called crankcase gases and contain oil vapors, unburned fuel, water vapor, and combustion products. If these gases are left inside the crankcase, the pressure will begin to rise and literally “inflate” the engine from the inside.
The PCV system acts as a controlled “breathing apparatus” for the engine. It creates a safe path for these gases to escape from the crankcase and return to the intake tract, where they are burned again. The mechanism works as follows:
During idling or calm driving, a stable vacuum is formed in the intake manifold, causing the PCV valve to open wider and gases from the crankcase to be sucked into the intake. At the same time, the oil separator traps oil droplets to prevent them from entering the throttle or manifold. This is the most important mode, when the system actively “extracts” gases from the engine.
During acceleration or high load, the vacuum in the manifold disappears and the pressure in the intake increases. The PCV valve closes partially to prevent air from flowing back into the crankcase. At this point, the gases exit through another channel (called reverse ventilation), where the pressure is already safe. This prevents gas backfire and stabilizes the pressure in the engine.
In many engines, a membrane that reacts to pressure differences is used instead of a valve. When the vacuum is strong, the diaphragm flexes and opens the channel; when there is no pressure, it flattens and the channel narrows. The oil separator acts as a filter: it traps oil and returns clean gases back to the intake. This reduces carbon deposits, extends the life of the throttle, and prevents the engine from “gorging” on oil.
When the PCV system is working properly, the engine maintains a steady speed, does not smoke, does not consume excess oil, and does not emit odors into the cabin. When it fails, whistling, air suction, increased pressure, oil consumption, smoke from the throat, and unstable idling begin.
Typical PCV system failures
Due to constant contact with oil vapors and high temperatures, PCV components gradually lose their elasticity, become contaminated, or crack. The most common malfunctions are:
- A torn PCV valve membrane, leading to uncontrolled air intake and mixture depletion.
- A coked oil separator, through which lubricating vapors are not separated, and the engine begins to “eat” oil.
- Damaged hoses or connections cause the vacuum in the system to be lost and the pressure in the crankcase to increase.
- Worn ventilation valve seal. Leads to oil leakage and air suction.
- Contamination of channels. Due to carbon deposits and condensation, the valve stops responding to pressure and the system stops working.
Signs of crankcase ventilation system malfunction:
- unstable idle speed,
- smoke from the filler cap,
- oil leaking from under the valve cover,
- increased oil consumption,
- smell of exhaust gases in the engine compartment.
PCV repair kits from Klifex — the solution for accurate repairs
Klifex develops specialized PCV repair kits that allow you to completely restore the ventilation system without replacing the entire valve cover or manifold.
The repair kit usually includes:
- crankcase ventilation valve membrane;
- sealing ring;
- valve spring;
- housing cover;
- valve gasket.
All elements are made of heat-resistant materials (Viton, FKM, reinforced silicone) and are fully compatible with the original parts. The advantage of using Klifex crankcase ventilation system repair kits is not only in the accuracy of dimensions and complete tightness after installation, as a result of which the engine “breathes” stably again and the pressure in the system normalizes. By using a repair kit, you significantly save on the repair of the unit. Usually, the cost of a PCV repair kit is $20-30, while replacing the entire unit will cost dozens of times more.
The main components of the crankcase ventilation system that can be repaired with a repair kit:
Valve over Repair Kit
The valve cover in modern BMW, VW, Mercedes, and Audi engines combines several functions: sealing the cylinder head, holding the oil separator, and the PCV valve. When the membrane or valve fails, the entire cover is often replaced, although it is sufficient to install a valve cover repair kit. It includes a new ventilation membrane, membrane compartment cover, gasket, and retainer.
How to replace:
- Disconnect the PCV hoses from the valve cover.
- Remove the plastic cover of the membrane compartment.
- Remove the old membrane and spring.
- Clean the mounting surface and install the new parts from the repair kit.
- Make sure that the cover is tightly closed and does not leak air.
After replacement, the engine speed will stabilize and oil consumption will decrease.
Crankcase Ventilation Valve
The crankcase ventilation valve regulates the flow of gases between the crankcase and the intake manifold. When the valve is working properly, it opens only when a vacuum is created. When it breaks down, it is either constantly open (sucking in air) or constantly closed (increasing pressure in the crankcase).
Signs of malfunction:
- whistling or hissing sound during operation;
- smoke from the filler cap;
- oil in the intake tract.
How to replace:
- Disconnect the vacuum hose.
- Unscrew the valve from the cover or housing.
- Install a new part from the repair kit.
Membrane PCV
The membrane is the heart of the crankcase ventilation system, which regulates the flow of gases in response to pressure.
PCV membranes are usually made of silicone or fluororubber, but over time they lose their elasticity and crack.
Signs of a rupture:
- The engine “whistles” during operation.
- Fluctuating RPM.
- Smoke coming from the filler cap.
To replace the PCV membrane, disassemble the valve assembly, remove the old membrane, clean the seat, and insert a new one. Do not use sealant — only the sealing ring from the kit.
Valve Cover Oil Separator
The oil separator cleans the crankcase gases from lubricating vapors so that they do not enter the intake and create carbon deposits. Its failure causes the engine to start “drinking” oil, and a greasy film appears on the walls of the throttle and manifold.
To check the oil separator, disconnect the hose and look for oil. If there is liquid there, the separator is not working and you will need to use an oil separator repair kit , which includes a new partition, filter, and seal, to restore its performance.
Oil Seal Crankcase Ventilation Valve
If the seal is worn, the system loses vacuum and gases seep past the valve, creating a leak. It is very easy to replace — just remove the old ring and insert the oil seal from the crankcase ventilation valve repair kit. Viton material provides resistance to oil and temperatures above 150°C.
Intake Manifold Vacuum Valve Diaphragm
The intake manifold vacuum valve diaphragm regulates additional air flows in the intake manifold. In many BMW, VW, and Audi engines, it is connected to the PCV system. If the diaphragm loses its seal, the RPMs begin to fluctuate and the engine becomes unstable at idle. Klifex manufactures manifold vacuum valve repair kits to accurately restore function without replacing the entire assembly.
Diaphragm Vacuum Valve Actuator EGR
EGR (Exhaust Gas Recirculation) works in conjunction with PCV, returning part of the exhaust gases back to the intake. If the EGR valve diaphragm is damaged, the valve does not respond, the mixture becomes lean or oversaturated, and jerks appear. Klifex repair kits allow you to replace only the EGR vacuum actuator membrane, without purchasing a new valve.
PCV system FAQ
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Why does the engine start whistling after starting?
This is a typical symptom of a torn PCV diaphragm or a leaky valve cover. When the diaphragm ruptures, uncontrolled air suction occurs in the system and the intake begins to “whistle.” Replacing the diaphragm or cover solves the problem instantly. -
Why did the engine suddenly start consuming a lot of oil?
The reason is a faulty oil separator or PCV valve. When they are not working, the oil vapors are not separated — instead of returning to the crankcase, they are sucked into the intake. As a result, the oil burns along with the fuel. If there is a greasy film in the intake manifold, this confirms the diagnosis. -
How to check the crankcase ventilation valve without tools?
Remove the hose and try to create a vacuum with your finger. If the valve does not respond or hisses, it is faulty. Another way is to remove the oil filler cap while the engine is running: if thick smoke comes out of it, the PCV system is not removing the gases. -
Is it allowed to drive with a faulty crankcase ventilation system?
No. Increased pressure in the crankcase literally “pushes” oil through all weak points. First through the valve cover, then through the oil seals. In severe cases, this results in oil being released into the exhaust or drive belt. -
Why is the idle speed still unstable after replacing the diaphragm?
The reasons may be:
- The diaphragm cover is not sealed tightly.
- Air is being sucked in through a cracked hose.
- There are clogged channels in the manifold.
- The engine adaptations have not been reset. -
Why does smoke form in the engine when the filler cap is open?
This is not always a problem. A small amount of smoke is normal gas venting. But if the smoke is constant and thick, and the engine is also “spitting” oil, the crankcase ventilation is clogged or the PCV valve is not working. -
How does the membrane differ from the PCV valve itself?
A diaphragm is an elastic element that moves under pressure. A PCV valve is an entire mechanism: a body, a spring, a diaphragm, and a channel. In some engines (BMW, VW, Mercedes), the diaphragm is integrated into the valve cover, so it cannot be replaced without a repair kit. -
Can the ventilation valve be cleaned without disassembly?
No. Aerosol or cleaner will not restore the membrane or eliminate cracks. These methods are temporary cosmetic fixes that do not provide a lasting effect. The only real repair is to replace the membrane or the entire mechanism. -
Why is there an exhaust smell under the hood?
This usually means that one of the ventilation hoses is cracked. Hot gases escape through the crack and enter the engine compartment. If the smell is accompanied by increased oil consumption, the PCV is torn. -
How often should the PCV diaphragm be replaced?
Every 80–120 thousand km. On turbocharged engines — more often, because the pressure in the system is higher. Klifex diaphragms can withstand up to 150–200 thousand km thanks to Viton materials and reinforced silicone. -
Does a faulty PCV affect fuel consumption?
Yes. When the system is not working, the engine receives uncontrolled air intake. The control unit enriches the mixture — consumption increases by 1–2 liters. After repair, consumption returns to normal. -
How to understand that the oil separator is clogged?
Signs: - oil in the intake manifold, - greasy soot in the throttle valve, - whistling and unstable idle, - black smoke when accelerating. A clogged oil separator means that the engine is sucking in oil vapors. -
Is there a difference between PCV in diesel and gasoline engines?
Yes. In diesel engines, the system is more complex — it often includes cyclones, filter elements, check valves, and a separate oil separator housing. In gasoline engines, the emphasis is on the membrane and vacuum channels. -
How to check the tightness of the valve cover after replacing the membrane?
The best way is to use a smoke generator. If smoke comes out through the cover or PCV unit, the cover is not tight. If smoke comes out of the intake manifold, this is a sign of incorrect membrane installation. -
Why does the pressure in the system increase again after repair?
Perhaps it was not only the membrane that was faulty. Often, clogged channels in the valve cover or a cracked pipe restrict the outflow of gases. A complete diagnosis is mandatory. -
Why is there oil in the air filter?
This means that the gases cannot escape into the intake manifold and flow back into the intake before the turbine. This happens when the PCV is jammed or the oil separator is completely clogged. -
Can universal Chinese valves be installed?
It is not recommended. Their spring stiffness and pressure response do not meet OEM parameters. Instead of fixing the problem, they create chaos in the engine's operation. Klifex manufactures parts with precise dimensions and stiffness, and they are compatible with specific engine models. -
How to understand that the system is working properly after repair?
The engine is stable at idle.
There is no smoke from the throat, or it is minimal.
The intake pressure is stable.
The oil does not disappear.
LTFT/STFT fuel adaptations are normal. -
Where to buy crankcase ventilation repair kits?
On the official website Klifex.ua, they manufacture membranes, valves, oil separators, diaphragms for manifolds, and repair kits for EGR vacuum valves. The parts are tested for tightness and temperature resistance. -
Can faulty crankcase ventilation damage the turbine?
Yes. If there is excess pressure in the crankcase, oil is discharged into the intake. The turbine starts to run on a “rich” oil mixture, becomes covered with carbon deposits, the impeller becomes contaminated, and the efficiency of the supercharger drops. Repairing the PCV restores the turbine's operation without additional intervention.