Exhaust Gas Recirculation

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By Reinier (Contact Me)
Last Updated 2022-12-24
Automobile Repair Shop Owner

What Does the Exhaust Gas Recirculation Do?

The function of an EGR (Exhaust Gas Recirculation) valve is to allow small, but metered amounts of exhaust gas to enter the cylinders to eliminate or reduce the formation of NOx, which is a harmful component of exhaust gas, under some operating conditions.

Why is the Exhaust Gas Recirculation Needed?

SPECIAL NOTE: Engines that are fitted with either variable valve timing or variable camshaft timing systems do not have exhaust gas recirculation systems because the quenching effect that exhaust gas has on the combustion temperature is accomplished by manipulating the valve timing. END OF SPECIAL NOTE. 

Although nitrogen and oxygen exist naturally in the atmosphere, these two gases combine to form toxic oxides of nitrogen (aka NOx) during the combustion of petroleum-based fuels in internal combustion engines.

NOx forms during a complex series of interactions between nitrogen and oxygen molecules when the combustion temperature in an engine’s cylinders reaches about 2500 deg F. In most cases, the amount of NOx that forms can be predicted with a fair degree of accuracy, but it should be noted that at temperatures higher than 2500 degrees F, NOx forms not only earlier in the combustion process, but also, that more nitrogen molecules combine with oxygen molecules to form larger volumes of NOx during each combustion event.

As a practical matter, the formation of large volumes of NOx in modern engines is a direct consequence of efforts to make engines more efficient by a) increasing compression ratios, and b) and developing combustion chamber and piston designs that increase the efficiency of the combustion process. Collectively, these two developments are the single biggest contributors to the increase in combustion temperatures to the point where the formation of toxic NOx is a natural and inevitable consequence of the combustion of petroleum-based fuels during normal engine operation.

Thus, to prevent or reduce the formation of NOx, the PCM activates a valve, known as the EGR valve to open and admit a small, metered amount of exhaust gas to enter the cylinders. The presence of exhaust gas has a profound quenching effect on the combustion process, and in most cases, the exhaust gas lowers combustion temperatures to well below the temperature that allows the formation of NOx.

However, the presence of exhaust gas during the combustion process also has a significant negative effect on the efficiency of the combustion process, so the PCM will not allow the introduction of exhaust gas into the engine under some operating conditions. For instance, since combustion is generally poor in cold engines, the PCM will only activate the EGR system when the engine coolant temperature is above about 100 deg F.

Similarly, the PCM will also not activate the EGR system under WOT (Wide Open Throttle) conditions to prevent the loss of engine power when maximum power delivery is required. Note, though, that this is not always the case, since the EGR system on diesel engines in many vehicle makes and models is activated during WOT conditions, regardless of the calculated engine load.

How Does the Exhaust Gas Recirculation Work?

Although most, if not all early iterations of EGR valves were typically controlled by the engine vacuum, modern versions of EGR valves are controlled in various ways. So, while some EGR valves are still vacuum controlled, many others are controlled by a combination of engine vacuum and electric solenoids, or more commonly, by small DC motors known as “stepper motors” that open and close the EGR valve in precisely calculated and controlled steps.

Regardless of how any given EGR valve is controlled and managed, though, all EGR valves have the same function, which is to allow for the introduction of exhaust gas into the engine’s cylinders.

In terms of operating principles, and assuming that an EGR valve is in good working order, the PCM will calculate when and if the formation of NOx in the cylinder is likely to occur based on inputs from a variety of sensors. If the PCM deems the current operating conditions to be suitable for the introduction of exhaust gas, it will command the valve to open by a calculated amount to deliver a metered quantity of exhaust gas to the cylinders.

As a practical matter, the PCM cannot monitor the flow of exhaust gas directly, so to compensate for this, the PCM uses input data from the EGR valve’s pintle (as reported by a dedicated position sensor) to infer the volume of exhaust gas that flows through the EGR valve. In a fully functional EGR system, the PCM will only allow exhaust gas to enter the cylinders for as long as the possibility to create NOx exists; once this possibility is reduced to below a predefined probability threshold, the PCM will deactivate or de-energize the EGR valve to stop the flow of exhaust gas into the cylinders.

Note that the conditions under which NOx will form are largely engine-specific. Some engines may reach the enabling temperature under much smaller engine loads than others because they may have higher compression ratios or more efficient combustion-enabling technologies than other engines.

In practice, these differences mean that some engines may never, or seldom, operate under conditions or loads that require the activation of the EGR system. Therefore, the mere fact that an EGR valve or system does not appear to operate as often as others is not necessarily an indication that the EGR system is somehow defective.

Where is the Exhaust Gas Recirculation Located on the Engine?

This image shows the location (circled) of the EGR valve on a 2009 Kia Sorento that is fitted with a 2.5L diesel engine.

Note though, that the location of EGR valves varies greatly between different vehicle makes and models, meaning that it is not possible to provide even a broad description of typical locations here beyond saying that EGR valves are usually, but not always located directly on intake manifolds.

Based on the above, we highly recommend that you research the location of the EGR valve on your specific vehicle by referring to reliable service information, or by doing an online search for the relevant information. Note that official repair manuals sometimes do not provide the locations of some categories of components, including EGR valves because the layout and configuration of engines often change before existing manuals can be updated.

What Does the Exhaust Gas Recirculation Look Like?

This image shows an example of an EGR valve, but it should be noted that neither the overall appearance nor the construction of this example is representative of all EGR valve designs. Note, also, that even in cases where two EGR valves appear to be identical in all respects, the two valves may be calibrated differently.

In practice, EGR valves are almost always application specific, meaning that these components are rarely interchangeable with units that may appear to be identical to the suspect/defective/ failed EGR valve.

What are the Symptoms that the Exhaust Gas Recirculation is Bad?

SPECIAL NOTES: The most common symptoms of failed, defective, or malfunctioning EGR valves are not only many and varied; the severity and prevalence of common symptoms also vary greatly between vehicle makes and models.

The main reason for the variance in both symptoms and the severity of symptoms on different vehicles involves the fact that an EGR valve can fail in one of two ways; an AGR valve can become stuck in the open position, or it can become stuck in the closed position. The image above shows an excessive build-up of carbon in an EGR valve, which is arguably the most common reason why EGR valves can become stuck in either the open or the closed position since an excessive carbon build-up prevents or inhibits the free movement of the valve pintle.  

Note that an excessive build-up of carbon frequently extends into the passages that allow the flow of exhaust gas into the cylinders. On some vehicles, these passages run through the intake manifold while in other vehicles, the passages run through the cylinder head and terminate directly in the combustion chambers.

Thus, if these passages become clogged or blocked by carbon deposits, it becomes necessary to remove the intake manifold and/or the cylinder head(s) to clear the passages. Be aware though, that in some cases, it may impossible to clear the passages due to the design f the manifold or cylinder head(s), meaning that in these the only remedy to restore the functionality of the EGR system is the replacement of the manifold and/or cylinder head(s). END OF SPECIAL NOTES.

Nonetheless, some common symptoms of an EGR valve that is stuck in the open position could include one or more of the following-

  • One or more stored trouble codes and an illuminated warning light, which may be red and might be flashing- this is an indication of a serious condition that must be attended to immediately
  • The vehicle may be locked into a fail-safe or limp mode that will persist until the fault is found and corrected
  • The idling quality may be rough or poor especially when the engine is cold
  • The engine may stall unexpectedly or repeatedly at low engine speeds
  • Varying degrees of power loss may be present at some, or all engine speeds and load
  • The engine may exhibit misfire-like symptoms at some or all engine speeds and loads
  • The engine may be hard to start at all temperatures
  • Fuel consumption may increase noticeably
  • Potentially fatal damage to oxygen sensors and catalytic converters may occur if the problem is allowed to persist for an extended period
  • A strong odor of fuel and/or exhaust gas may be present when the engine is running at idle or at low speeds
  • The vehicle will fail an emissions test

Common symptoms of an EGR valve that is stuck in the closed position could include one or more of the following-

  • One or more stored trouble codes and an illuminated warning light, which may be red and might be flashing- this is also an indication of a serious condition that must be attended to immediately
  • Varying degrees of engine knock may be present, which is caused by the premature ignition of fuel as a result of excessively high temperatures in the cylinders
  • Detonation, aka uncontrolled combustion, can occur; be aware that severe detonation can cause fatal engine damage
  • The vehicle will fail an emissions test

NOTE: It should be noted, though, that most, if not always, all, of the symptoms listed above can also be caused by other issues that do not involve the EGR system. These could include one or more of the following-

  • Worn or damaged spark plugs
  • Wiring issues in the ignition system, or defective ignition driver circuits in the PCM and/or ignition control module
  • Damaged or malfunctioning ignition coils
  • Engine vacuum leaks
  • A clogged or restricted fuel filter that causes an insufficient fuel pressure
  • A damaged or malfunctioning fuel pump that causes an insufficient fuel pressure
  • A damaged or malfunctioning fuel pressure regulator
  • One or more defective or malfunctioning engine sensors, of which there are at least 10 on most modern engines, but note that multiple sensor failures almost always involve defects or wiring issues in shared reference voltage circuits

How do you test the Exhaust Gas Recirculation?

As mentioned in the previous section, symptoms that mimic those of a failed, defective, or malfunctioning EGR valve can be caused by many other failures and/or defects that do not involve the EGR valve or larger EGR system in any way. This could make diagnosing and/or testing EGR valves very difficult, but to make things a bit clearer, faults, failures, or malfunctions of EGR valves will always be indicated by one or more of the (generic) trouble codes listed below-

  • P0400: EGR flow malfunction
  • P0401: EGR insufficient flow detected
  • P0402: EGR excessive flow detected
  • P0403: EGR circuit malfunction
  • P0404: EGR circuit range/performance
  • P0405: EGR sensor A circuit low
  • P0406: EGR sensor A circuit high
  • P0407: EGR sensor B circuit low
  • P0408: EGR sensor B circuit high
  • P1403: EGR solenoid low
  • P1404: EGR system – closed valve pintle error
  • P1405: EGR solenoid high
  • P1406: EGR system pintle position error

Thus, if none of the above codes is present, the observed symptoms are likely not caused by issues that affect or influence the operation of the EGR valve or the larger EGR system. However, be aware that on some high-end applications that use advanced EGR systems/technology, the faults and defects listed here may be accompanied by one or more manufacturer-specific trouble codes that may only be accessible with manufacturer-specific scan tools and/or test equipment. Based on this, we highly recommend that you research all manufacturer-specific EGR-related trouble codes that apply to your specific vehicle if you suspect that you might be dealing with a defective EGR valve.

Nonetheless, it is possible to test the functionality or operation of a suspect EGR valve in one of two ways, these ways being-

Performing a vacuum test

On EGR valves that are vacuum-operated, it is possible to apply a vacuum (that is specified by the vehicle manufacturer) to the EGR valve with a hand-held vacuum pump. If the vacuum holds steady, the valve pintle should move perceptibly but note that the valve pintle is not visible on all EGR valves. In these cases, it is necessary to remove the EGR valve from the engine to observe the movement or, the lack of movement, as the case may be, of the valve pintle.

Performing a bi-directional test

Note that this test requires the use of a suitable scan tool that can perform bi-directional tests. In the case of EGR valves, the scan tool will mimic a command from the PCM, meaning that if the test is performed correctly, the EGR valve will react (or not, as the case may be) to the commands it receives from the scan tool.

Note that such tests will also assess the operation of the valve pintle’s position sensor. This sensor plays a critical role in both the control and management of the EGR valve, in the sense that the PCM infers the rate of exhaust gas flow through the EGR valve based on the position of the valve pintle as reported by the pintle’s position sensor.

In practice though, both of the above tests often produce inconclusive or even misleading results because the EGR valve might be clogged up with excessive carbon deposits. Removing the carbon deposits may sometimes restore the valve’s functionality, but be aware that this is not guaranteed to be an effective or reliable remedy.

From a reliability perspective, the best course of action to follow when an EGR valve is clogged with carbon is to replace the valve with an OEM or at least an OEM-equivalent part. However, given the wide range of possible causes and symptoms of failed or defective EGR valves, the quickest and most cost-effective option to resolve issues would be to seek professional assistance with diagnosing and repairing suspected EGR valve-related issues and problems.

How do you replace the Exhaust Gas Recirculation?

Be aware that replacing an EGR valve is rarely as easy as removing some bolts, removing the old valve, and installing a replacement valve in the reverse order of removing the old valve.

Gaining access to EGR valves on most modern engines requires the removal or partial disassembly of some major, but unrelated engine components, which means that the possibility of making mistakes or causing damage to critical engine components is directly proportional to the difficulty in reaching the EGR valve.

Based on this fact, we do not recommend that novice non-professional mechanics attempt an EGR valve replacement on any engine, simply because this process requires technical skills, technical knowledge, an extensive set of tools, and other, often application-specific equipment that few non-professional mechanics possess or have access to.

In addition, given that a suspect EGR valve may not actually be defective, the best course of action is to seek professional assistance with replacing a suspect or known bad EGR valve.