When Does EGR Valve Open for Diesel? (Purpose + Solution)

when does egr valve open diesel

The EGR (Exhaust Gas Recirculation) valve is a critical component in diesel engines, responsible for reducing emissions and improving fuel efficiency.

With the increasing pressure to lower emissions and comply with stricter regulations, understanding the role of the EGR valve, its functionality, and troubleshooting techniques is essential for diesel engine maintenance.

In this comprehensive guide, we will dive deep into the diesel EGR valve, discussing when it opens, how it works, and how to troubleshoot problems associated with it.

The Purpose of the Diesel EGR Valve

The primary function of the EGR valve in diesel engines is to lower exhaust emissions, specifically nitrogen oxides (NOx). By recirculating a portion of the exhaust gases back into the combustion chamber, the EGR valve reduces the amount of oxygen available for combustion, leading to a lower combustion temperature.

Lower combustion temperatures result in decreased NOx emissions, making the EGR valve an essential component for meeting emission standards.

In addition, the EGR valve plays a role in improving fuel efficiency and engine performance. By controlling the amount of exhaust gas recirculated into the combustion chamber, the EGR system helps maintain the optimal air-fuel mixture for efficient combustion, reducing fuel consumption and improving overall engine performance.

When Does the Diesel EGR Valve Open?

The EGR valve operates in a dynamic manner, adjusting its position based on various factors such as engine load, speed, and temperature. It primarily functions in two settings: open and closed. However, the valve can vary between these two positions based on the engine’s requirements.

The EGR valve opens for diesel during idle and low-speed operations. While the engine requires less power, and thus, less oxygen for combustion, the EGR valve gradually opens, potentially reaching up to 90% open at idle.

By reintroducing exhaust gases into the combustion chamber, the valve reduces the oxygen level, leading to a slower burn and lower combustion temperatures.

On the other hand, when the engine demands more power, such as during full acceleration or heavy load operations, the EGR valve closes to ensure maximum oxygen intake for combustion. This results in increased power and torque output.

In summary, the EGR valve continuously adjusts its position based on the engine’s needs, maintaining optimal combustion conditions for improved performance and reduced emissions.

Read also >> EGR Valve System Malfunction (Here Is How To Fix)

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Types of Diesel EGR Valves

There are several types of EGR valves used in diesel engines, with the main distinction being between vacuum-operated and electronically-controlled valves. Let’s explore the primary types of diesel EGR valves:

Diesel High-Pressure EGR Valves

In high-pressure EGR systems, the exhaust gas is diverted before it enters the diesel particulate filter (DPF), where it carries high soot concentrations. The gas is then passed back to the intake manifold, either through a pipe or internal drillings in the cylinder head.

A secondary valve is also used to create a vacuum in the intake manifold, as diesel engines do not naturally produce a vacuum.

Diesel Low-Pressure EGR Valves

Low-pressure EGR systems divert exhaust gas after it has passed through the DPF, resulting in a lower flow but cleaner gas, free of soot particles. The gas is then returned to the intake manifold via a pipe.

Gasoline EGR Valves

Gasoline EGR valves operate similarly to high-pressure diesel EGR valves. The vacuum created by cylinder depression draws the exhaust gases in, and the flow is regulated by the opening and closing of the EGR valve.

Vacuum-Operated EGR Valves

Vacuum-operated EGR valves rely on a vacuum solenoid to control the valve’s diaphragm, opening and closing the valve. Some of these valves also include a feedback sensor to inform the engine control unit (ECU) of the valve’s position.

Digital EGR Valves

Digital EGR valves feature a solenoid or stepper motor and often include a feedback sensor. These valves receive a pulse-width modulated signal from the ECU to regulate exhaust gas flow accurately.

Causes of Diesel EGR Valve Failure

EGR valves are exposed to harsh operating conditions and are prone to wear and tear. The most common cause of EGR valve failure is the buildup of carbon particles from the exhaust gases along the EGR and intake system passages.

Over time, this buildup can clog tubes, exhaust gas channels, and eventually the valve’s plunger mechanism, causing it to stick in either the open or closed position. Other potential causes of failure include ruptured or leaking valve diaphragms.

Symptoms of Diesel EGR Valve Failure

EGR valve failure can manifest in various ways, with symptoms often resembling those of other engine management component issues. Some common signs of a failing EGR valve include:

  • Check engine light
  • Engine performance issues, such as reduced power, poor acceleration, and rough idle
  • Increased NOx emissions
  • Engine knock or knocking noises in the engine

Troubleshooting Diesel EGR Valves

Performing diagnostics on EGR valves involves following the service manual’s troubleshooting procedures specific to the type of valve in question. However, some general steps can help in diagnosing EGR valve issues:

  1. Read fault codes using a diagnostic scan tool capable of reading fault codes in the EGR system.
  2. Check that all vacuum lines and electrical connections are connected and positioned correctly.
  3. Use a vacuum gauge to check the vacuum supply hose for vacuum at 2000 to 2500 rpm. No vacuum at normal operating temperatures would suggest a loose hose, a blocked or faulty ported vacuum switch or solenoid, or a faulty vacuum amplifier/pump.
  4. Check the vacuum solenoid while the engine is running. On electronically-controlled EGR valves, activate the solenoid with a scan tool and check the vacuum at the end of the pipe. If the solenoid does not open when energized, is stuck in the open or closed position, or has a corroded electrical connection, loose wire or bad ground, EGR operation will be affected.
  5. If possible, check the movement of the valve stem at 1500 to 2000 rpm. The valve stem should move if the valve is functioning correctly. If not, and there’s vacuum, there’s a fault.
  6. Apply vacuum directly to the EGR valve using either a hand vacuum pump or scan tool depending on the type of EGR valve. If there is no change in idle quality, then either the EGR valve is faulty or the passages are completely restricted. If the engine idles rough or stalls, the problem is being caused by a malfunctioning control system.
  7. Remove the EGR valve and check for carbon buildup. Where possible, remove any carbon, being careful not to contaminate the diaphragm.
  8. Inspect the EGR passageway in the manifold for clogging and clean if required.

The Impact of EGR on Diesel Combustion Chambers

The EGR valve plays a crucial role in maintaining optimal combustion conditions within diesel combustion chambers.

By controlling the amount of exhaust gas recirculated into the chamber, the EGR system ensures the correct balance of heat, oxygen, and fuel for efficient combustion.

However, if the EGR valve malfunctions or becomes clogged, it can disrupt this balance, leading to performance issues and increased emissions.

EGR and Fuel Efficiency: A Complex Relationship

While the EGR system aims to improve fuel efficiency by reducing the oxygen level in the combustion chamber, it can also create some challenges.

The cooled exhaust gases introduced by the EGR valve can lower combustion temperatures, potentially leading to incomplete combustion and reduced fuel efficiency. However, the EGR valve’s dynamic operation ensures that when maximum power is needed, the valve closes to provide the necessary oxygen for combustion, maintaining optimal fuel efficiency and performance.

EGR Cooler and Its Impact on Combustion

The EGR cooler plays a crucial role in the EGR system by cooling the exhaust gases before they are reintroduced into the combustion chamber.

If the EGR cooler becomes clogged or fails to receive adequate coolant flow, it can return excessively hot exhaust gases to the combustion chamber, resulting in overheating and reduced fuel efficiency.

In extreme cases, a malfunctioning EGR cooler can also lead to engine damage, emphasizing the importance of proper maintenance and diagnostics.

Oil Leaks and Combustion Chamber Concerns

Oil leaks into the combustion chamber can cause a range of issues, from increased soot and sludge buildup to reduced combustion efficiency and engine performance.

If left unaddressed, these issues can lead to more severe engine damage, making it essential to diagnose and address oil leaks promptly.

EGR and Diesel Knock: A Delicate Balance

A properly functioning EGR system can help reduce diesel knock by maintaining optimal combustion conditions. However, a clogged EGR valve that remains open can lead to increased diesel knock during engine startup and idle.

Monitoring and addressing EGR valve issues is crucial for maintaining engine performance and avoiding potential damage.


The diesel EGR valve is a critical component in modern diesel engines, responsible for reducing emissions and improving fuel efficiency.

With its dynamic operation and impact on combustion conditions, understanding the EGR valve’s functionality and troubleshooting techniques is essential for maintaining optimal engine performance.

By staying informed about the role of the EGR valve and addressing potential issues promptly, diesel engine operators can ensure their engines continue to run efficiently and meet emission standards.

Stephan K.

I have been working as a mechanic for over 13 years and have been deeper into advanced automotive diagnostics for some years.

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