(+86 ) 138-1981-5069         sales@cnhenshine.com
Diesel common rail system parts
We sincerely welcome friends from domestic and abroad to visit and work together to create brilliant !
You are here: Home » Blogs » KNOWLEDGE » The difference between PCV valve, SCV valve and EGR valve

The difference between PCV valve, SCV valve and EGR valve

Author: Site Editor     Publish Time: 2021-07-23      Origin: Site

PCV Valve, SCV Valve, EGR valve, do you know the difference between them?


PCV valve meaning: Positive crankcase ventilation

SCV valve meaning: Suction control valve

EGR valve meaning: Exhaust gas recirculation valve

The difference between function and principle

PCV valve

The PCV valve consists of a valve body, valve, valve cover and spring and cannot be disassembled. The open system has been modified to form a closed crankcase ventilation system.

Open-ended PCV system: the fresh air from outside, under the action of intake vacuum, enters the crankcase through the same vent as the atmosphere at the oil filler cap, etc., and mixes with the crankcase fugitive air, then is sucked into the intake system through the PCV into the combustion chamber. This system is simple in structure, easy to install and maintain, but in some operating conditions can not be all crankcase fugitive gas into the intake system, especially under heavy load, crankcase fugitive gas increased, the intake pipe vacuum reduced can not be all fugitive gas into the intake system, part of the fugitive gas from the oil filler cap into the atmosphere, thus causing pollution of the environment.

Enclosed PVC system: this system is based on an open system with a sealed crankcase and a vent pipe connected to the crankcase between the air filter and the carburettor (throttle body for EFI engines). Fresh air first enters the crankcase through the air filter and vent pipe and mixes with the scavenge air, which enters the cylinder for combustion through the PCV valve under vacuum in the intake pipe. When the engine is under heavy load, the excess exhaust air enters the back of the air filter through the vent pipe and mixes with the engine intake air to enter the cylinder for combustion. The closed system does not allow the exhaust air to be vented into the atmosphere, but also allows the crankcase to be air-exchanged with fresh air.

PCV valve

SCV valve

The SCV is a mechatronic component. It controls the amount of fuel entering the fuel rail to meet the requirements of the common rail system pressure setting and together with the rail pressure sensor forms a closed loop control of the rail pressure. the SCV valve is controlled by the ECU which receives signals from the engine speed, water temperature, common rail pressure and suction pressure to derive a command current to the SCV valve to control the opening of the SCV valve. Under the control of the engine computer (ECU), high pressure fuel is generated and delivered to the common rail to generate pressure. The output fuel is controlled by the diesel fuel volume adjustment solenoid valve (SCV valve) on the diesel pump to achieve the purpose of regulating the common rail pressure.

SCV valve

EGR valve

The EGR valve is one of the main components that control the flow and recirculation of these exhaust gases. It is a mechatronic component and is divided into mechanical and electronically controlled types. When the valve is open, it allows exhaust gases to pass through the vehicle's EGR system to help control vehicle emissions. When there is a problem with the EGR valve it can cause problems with the flow and operation of the EGR system, which can lead to increased emissions and performance issues.

EGR valve

PCV Valve, SCV Valve and EGR valve

Mounting position



Consequences of failure

SCV Valve

Diesel common rail high-pressure oil pump on

Controls the amount of oil entering the oil rail to meet the common rail system pressure setting

Controlled by the ECU, the amount of fuel flow is controlled by the magnitude of the current pulses that change the area of the internal oil passage

Engine can not start

PCV Valve

Between the valve rocker cover and the intake manifold (between the ventilation system and the air intake system)




Avoids icing at the throttle and inadequate combustion.

Prevents the risk of crankcase explosion from engine backfire.


Controlled by vacuum to regulate the flow of soot from the crankcase ventilation system into the air intake system


Abnormal oil consumption

Increased fuel consumption, increased engine wear


EGR valve

On the short metal pipe leading to the exhaust manifold on diesel engines

The amount of exhaust gas entering the intake manifold is controlled so that a certain amount of exhaust gas flows into the intake manifold for recirculation

Controlled by a computer, some of the exhaust gas enters the intake manifold and enters the combustion chamber with the mixture to lower the combustion temperature and reduce the amount of NOx produced.

Substandard emissions, idling, engine popping, high temperatures, etc.

The PCV valve, SCV valve and EGR valve have different roles, but their failure can affect the engine and, in severe cases, lead to engine failure. So how can you recognize what is causing the fault?

The Differences of Faults

The Failure Phenomena of PVC Valve

  • The engine shakes, idles erratically and there is a lot of white smoke coming out of the exhaust pipe.

  • It is possible that the fugitive will stay in the crankcase, causing corrosion, accelerating wear and shortening engine life.

  • It can also cause engine failure such as difficult to start, idling shudder, weak acceleration and excessive oil loss.

  • It is easy to deposit sludge and blockage, after blockage caused by the crankcase pressure increase, crankcase oil and exhaust gas will reverse the flow into the air filter, the air filter element caused by pollution.

The Failure Phenomena of SCV Valve   

  • Sudden drop in engine power during operation, unstable idle speed, weak or black smoke from the vehicle

  • Works smoothly but the engine suddenly stalls.

  • It will not start, the engine will not start, but it will continue to work when the SCV valve is unplugged.

The Failure Phenomena of EGR Valve

  • Unstable idle speed, stalls easily

  • Poor acceleration, black smoke

  • Increased fuel consumption

  • Engine overheats, bursts into flames and exceeds emission standards

The Methods of Checking Fault

To determine if the PCV valve is faulty

Method 1: Make the engine idle, remove the PCV valve from the cylinder hood hose, check whether the PCV valve is blocked. If you put your hand on the PCV valve interface, your fingers can feel a strong vacuum suction.

Method 2: After the PCV valve is installed from the air filter to remove the crankcase intake tube, with a thin paper gently cover the mouth of the tube, to be crankcase pressure is reduced, should be obvious to see the thin paper was sucked into the mouth of the tube. In addition, stop the engine running, remove the PCV valve after shaking by hand to check, if you hear a "click tower" sound, that the PCV valve flexible available.

Determining if the SCV valve is faulty

A faulty SCV valve will cause the pressure in the common rail tube to be too high, which will cause the common rail pressure limiting valve to open and release pressure. Once the common rail pressure limiting valve has opened, the engine will enter the limp function, being limited in torque and exhibiting a weak driving fault.

Generally speaking, the coil resistance of the metering unit is between 2.6 and 3.15 ohms at a room temperature of 20°C. If the resistance value is not within this range, it can basically be judged to be damaged. The direction of inspection is based on the initial judgement of the driving data stream and fault codes.

Determine if the EGR valve is faulty

An engine light is another symptom of a faulty or failed EGR valve. If the computer detects a problem with the vehicle's EGR valve circuit or location, it will activate the check engine light to notify the driver of the problem. Remove the EGR valve and check that its passage is not clogged with dirt and that it is cleared or replaced if it is.

If you are unsure of what is causing the problem, you can scan the computer for fault codes to solve the problem.

Product Category

Recommended Products

Contact us
Contact us for more information


HENSHINE PRECISION Machinery Co.,Ltd.was established in 2012,and it is high-tech enterprise which is professional diesel engine Valve and industrial hydraulic valve research,development,
Manufacturing and sales.Themain research direction:Diesel common rail suction control valve(SCV),




   25 Zhongxinyungu 399# Juchao Road, Fenghua District, Ningbo, Zhejiang,China
   (+86) 138-1981-5069
Copyrights 2020 Ningbo Henshine Precision Machinery Co.,Ltd . Sitemap    Support by haipai