EGR valves
EGR valves are critical to controlling and limiting exhaust emissions. Rob Marshall explains why EGR circuit faults are comparatively common and shows that they can be diagnosed and repaired correctly.
Everything you need to know about exhaust gas recirculation.
While you should view the exhaust gas recirculation (EGR) valve circuit as a system, modern EGR valves are very deceptive. Despite appearing to be simple, they are precision-made mechatronic components, which means that the majority of modern types combine a mechanical valve, the position of which is monitored – and controlled, in many cases – electronically. However, problems with them are not always highlighted by an engine management light, or fault code, so you might have to undertake manual checks to diagnose an issue. Even if you find that the valve needs replacing, this is not as simple as it appears. Most EGR valves are held to the engine by hard-to-reach bolts and a replacement valve could require diagnostic coding into the engine management system computer (ECU) in order to work properly.
Why is EGR necessary?
It might seem counterintuitive to recirculate dirty exhaust gases back into the cylinder, but this is the function of the EGR system. While this can create issues as a vehicle ages, it does have a number of benefits.
Primarily, EGR plays a vital role in reducing nitrogen oxides (NOX), a group of harmful pollutants produced by the reaction between oxygen and nitrogen, which have gained particular prominence in Europe since the ‘dieselgate’ emissions scandal in 2015. While EGRS have been a fixture on certain North American petrol engines since the early 1970s, in the UK, you are more likely to encounter EGR on diesel and gasoline direct injection (GDI) engines, both of which rely on the technology heavily to meet mandatory emissions legislation.
Oxygen and nitrogen reaction rates – and, therefore, NOX production – is related directly to in-cylinder temperature and pressure. Recirculating gases from the exhaust system back into the inlet manifold reduces combustion speed and lowers cylinder temperatures, because the ‘spent’ exhaust gas replaces the oxygen that would have been present had the cylinder been filled solely with fresh air. NOX levels reduce as a result. EGR has played a particularly important role in lean-burn petrol engines and subsequent GDI engines (especially turbocharged, low-capacity types), because these engines tend to experience particularly high in-cylinder temperatures, due to their relatively weak fuel mixtures under certain operating conditions.
EGR can also reduce fuel consumption and CO2 emissions. For example, when a vehicle is operated in overrun conditions (ie, coasting in gear with the throttle released), instead of the engine having to suck air into its cylinders via the air filter element, introducing recycled exhaust gases results in less drag on the road wheels due to reduced pumping losses. The quantity of fuel injected can be lowered under part-load conditions with the EGR valve open, with a corresponding reduction in emissions because less oxygen is available.
For those cars made within the past decade, there is no firm rule about EGR duration, because it depends greatly on software programming. In general, EGR tends to be most prevalent at lowto mid-load/rpm conditions, such as cruising. During heavy acceleration, EGR tends to be deactivated, to permit as much oxygen as possible to be available for burning at the temporary expense of higher emissions. The valve also tends to close during idling. Diesel and GDI engines rely more heavily on EGR than earlier port injection engines (many European models of which lack EGR altogether), but EGR valve operation depends greatly on the vehicle’s age, fuel type, engine management hardware and programming. For example, one of the official ‘dieselgate’ software modifications made to Volkswagen’s 2.0-litre engines is reputed to be more frequent operation of the EGR valve and some people have said that this additional exertion reduces the valve’s operational life.
What to expect from EGR
By engineering overlap between the engine exhaust and inlet valve timing, a maintenance-free ‘internal’ EGR system was employed on some older petrol engines, which permitted a very limited quantity of exhaust gas to remain within the cylinder. Variable valve timing (VVT) allowed petrol engine timing to tailor a greater proportion of exhaust gas recirculation rates more appropriately, depending on rpm and load.
As virtually no diesel engines feature VVT, external EGR systems were necessary. This consists of a separate EGR valve, with associated plumbing, to transfer the gases from the valve into the inlet, where necessary. These are not unique to compression ignition engines
Early EGR valves were controlled pneumatically (by vacuum) until around the early 2000s. The internal poppet valve that controls EGR flow is opened and closed by negative pressure, provided via a pressure pipe, supplied by the intake manifold. As a result, EGR rates were influenced by engine load alone, because inlet manifold vacuum pressure becomes closer to that of atmospheric pressure depending on engine load and speed. However, the system is fairly imprecise. Greater valve position accuracy came with greater complexity, where electronically-controlled valves were added to the system to control the vacuum supply to the main EGR valve. Additionally, most of these arrangements add electronics to the pneumatic EGR valve itself, although this tends to be in the form of a potentiometer that detects the EGR valve’s position and relays the information back to the ECU, which then adjusts the vacuum supply accordingly.
In order to simplify the EGR system, cut costs, enhance efficiency and integrate the EGR into the engines electrical system, vacuum activation has been superseded by electrics, especially on cars built within the last 15 years. The valves on these units are operated not by vacuum but by either a motor, or a solenoid. Valve design has also changed and rotary slide/flap valves may be encountered, as well as the original poppet type. A further advantage with fully-electronic control is that the rate of EGR flow can be tailored more accurately, based on other engine sensor outputs. You need to take this into account when trying to diagnose a problem. For example, the EGR valve can be opened to reduce engine warmup time, as coolant temperature sensor output is considered.
With increased in-cylinder temperatures on newer engines, especially turbocharged applications, the exhaust gas temperature became too high to control NOX. EGR coolers fulfil this purpose and consist of two separate chambers: one for engine coolant, the other for exhaust gases. The incoming exhaust fumes flow through a series of channels, the heat from which is shed into the antifreeze mixture. The cooled gases can then flow directly into the EGR valve.
Cooling the gases allows for a higher proportion of EGR to be used. Modern diesel and GDI engines can have as much as 50-60% of volume within the cylinder consisting of recirculated gas; beforehand, it may have been around half that figure. Extra complication comes with temperature sensors being fitted to some coolers and you may also find a bypass flap, which prevents cooled exhaust gases that are exiting the cooler from entering the EGR valve. This allows both the engine and catalytic converter to achieve faster warm-up times. The EGR valve, its cooler and bypass valve can be fitted together in a single compact unit that you have to unbolt from the engine as a single assembly in order to access, diagnose, or even separate and clean, the component parts.
The most recent development has been the introduction of low-pressure EGR. While the traditional high-pressure EGR circuit recirculates gases between the exhaust manifold and inlet manifold (and this system may be installed and used on newer cars only to aid engine warming), the low-pressure EGR circuit extracts exhaust gases just after they flow from the particulate filter/catalytic converter and introduces them first into a cooler, then into the turbocharger inlet. For the engine, the main advantage of this is that the gases entering the inlet are likely to be free of contaminants, because they are fed back into the engine after flowing through the exhaust system’s after-treatment hardware, such as a diesel particulate filter. From an engineering perspective, low-pressure EGR systems allow EGR to be activated at higher engine loads, which was not possible with older systems that employed high-pressure EGR alone.
Symptoms and faults
Unless alerted by an illuminated engine warning lamp, a faulty EGR circuit can create a variety of running issues, including difficult starting, misfiring at idle speeds, hesitation when driving, stalling, increased black smoke from the exhaust and, possibly, a surge in unburnt hydrocarbon (HC) emissions.
With constant exposure to high temperatures, plus oil mist and soot that may exist in the exhaust, coupled with inevitable mechanical wear, it is unsurprising that the valve itself is the most common Egr-related part to fail. However, air leaks at the intake can impede EGR flow, even if the main EGR circuit is sound, so check that all gaskets, hoses and pipes are fitted correctly.
Physically, a mixture of oil mist and soot/carbon deposits from the hot gases can build within the valve body, restricting valve movement and impeding exhaust gas flow. Even if not contaminated, the mechanical parts of the valve are unlubricated and have a finite life. Valve action can become sticky, or jam, over time and consideration should be given to replacing a suspect valve that has covered over 80,000 miles regardless of contamination level, because its best years are likely to be behind it. DIY reconditioning is not an option.
Should a pneumatic EGR system be fitted, check the circuit carefully for air leaks and crimped/kinked vacuum pipes, which will prevent the EGR valve from opening fully, or at all. Study the manufacturer’s data for the layout of ancillary parts, such as the extra vacuum control valves, noting that there is no single universal layout.
The mechanical EGR cooler bypass flaps can seize due to deposit build-up. Within the cooling jacket, internal corrosion is caused mainly by low antifreeze concentrations, or the coolant not being replaced on time. Last year, BMW issued a major EGR cooler recall on various diesel-engined models, made between 2010 and 2017, because of an internal breach that caused coolant to leak into exhaust gases. The antifreeze mixture, when mixed with soot particles and combined with the high temperatures within the EGR circuit, could result in a melted intake manifold or even a fire.
Pneumatic (vacuum) EGR diagnostics
To check a pneumatic single-diaphragm EGR valve for internal vacuum leaks on older petrol engines, you will need to connect its port to a vacuum pump, fitted with a gauge, and generate a negative pressure of approximately 300 millibar. Should the pressure hold for five minutes, the valve is serviceable. Repeat the test with the engine running at its normal operating temperature – obviously, the valve must be fitted when about 300 millibar is produced by your vacuum gauge – and the engine should either idle roughly, or stall.
Some of these valves are fitted with a temperature sensor as well. You can measure the resistance across its connections with a multimeter, while verifying the temperature with a digital thermometer. Approximate healthy values should be: 20°C above 1000 kω (kiloohms), 70°C from 60-280 kω and 100°C from 60-120 kω. Diesel pneumatic EGR valves can be checked using the same method but raising the vacuum to 500 millibar. If visible, check that the rod, located between the diaphragm and valve, moves smoothly as you apply the negative pressure.
More advanced pneumatic EGR valves possess a pair of internal diaphragms, one of which opens the valve slightly, while the other opens it fully. The vacuum connections on these designs tend to be positioned one above the other. The test is identical to that described for single-diaphragm valves, but performed at both vacuum ports. To verify the vacuum supply to the EGR valve, connect the pipe to the vacuum pump’s pressure gauge, start the engine and note if the reading is appropriate.
As mentioned earlier, pneumatic EGR circuits can include a number of separate systems that control the vacuum supply to the valve. Pressure transducers detect how much vacuum is present, mechanical versions of which can be tested by removing the pressure pipe that links the transducer to the EGR valve and connecting it to a vacuum gauge. Start the engine and note if the gauge reading alters as you move the pressure transducer’s linkage. Connect the gauge in the same way for electro-pneumatic transducers and, with the engine running, remove the electrical connector; the vacuum reading should not exceed 60 millibar. With the connector refitted, increase engine revs and note if the vacuum reading increases. With the engine off and using a multimeter, expect the pressure transducer winding’s resistance to be between 40 and 20 (ohms). You will need your car maker's data to locate the electrical pin locations.
Change-over valves possess either two, or three connections. Connect a vacuum pump to an output line – vacuum must be generated when a voltage is present. Thermovalves control vacuum according to engine temperature so, with the coolant cold and your vacuum pump connected to the outlet, no airflow should be present until the coolant has warmed to a specified temperature, such as around 70°C. For more details about connections and switching temperatures, consult your vehicle manufacturer’s data.
Electrical issues and diagnostics
Should a pneumatic EGR valve be fitted with an electric valve position sensor, known as a potentiometer, unusual valve position readings are more likely to lie with the vacuum system, so prioritise those first. To verify that the potentiometer is operating properly, apply vacuum to the appropriate EGR valve port to open the valve and connect your multimeter – the resistance values noticed at pins 2 and 3 should measure between 1500-2500 Ω. Afterwards, connect your multimeter to pins 1 and 2 and, using the vacuum pump, open the valve slowly – the resistance should start at approximately 700 Ω and increase to 2500 Ω. Again, note that this is only general advice and you should prioritise any vehicle-specific information.
With purely electrical EGR valves, view them as integral to the car’s entire engine management system. If you have managed to read any fault codes, check if any other faults have been registered with other sensors/systems that have a direct influence on EGR operation, such as turbocharger variable vane geometry controls or the MAP and MAF sensors. For example, should the ECU receive an incorrect value from the MAF sensor, the quantity of exhaust emissions to be recirculated will be calculated erroneously, causing running issues and excessive exhaust emissions. This might lead you to blame the EGR valve, when the real cause lies elsewhere.
Consider also that any mechanical restriction is likely to cause a higher current draw, the heat from which damages the electric side of the valve. While it is possible to dismantle the valve, the lack of replacement parts means that renewing the complete unit is the only alternative.
Electric EGR – basic diagnostic techniques
Fortunately, electrically-controlled EGR valves tend to be easier to diagnose even if an engine management light has not been illuminated. Start by using the most inexpensive tools – your own senses – and look for any obvious issues, such as loose pipework. Many electronically-actuated EGR valves are programmed to self-clean after a set mileage and the distinctive ‘click, click’ sound from the valve assembly on either engine start-up, or shutdown, might be familiar to a mechanic ally sympathetic owner; suspicions should be raised if the ‘click’ frequency either changes, or disappears. Alternatively, you can instruct the EGR to open and close manually by not even leaving the driver’s seat and using diagnostic test equipment plugged into the car’s onboard diagnostics (OBD) port. You will need more advanced kit than a simple code reader to do this and you need to follow any instructions carefully. Many EGR actuation tests require you to listen for a distinctive sound as the valve operates.