EGRs – what they do, how they work, how to troubleshoot
Part of a vehicle’s engine management system, the exhaust gas recirculation (EGR) valve recirculates finely metered quantities of exhaust gas to the engine intake system for increased engine efficiency, and reduced fuel consumption and NOx emissions.
With growing pressure to reduce emissions, the EGR valve will play an increasingly important role moving forward. It’s important to know what it does, why it fails and how to replace it when it does fail.
The EGR valve effectively changes the air entering the engine. With less oxygen, the air mixture burns slower, lowers temperature in the combustion chamber by almost 150°C, and reduces NOx production for a cleaner, more efficient exhaust.
The EGR valve has two primary settings: open and closed, although the position can vary between the EGR valve being closed and when the engine is starting up. During idle and at low speeds, only a small amount of power is required, and therefore only a small amount of oxygen, so the valve gradually opens – it can be up to 90% open at idle.
However, as more torque and power are required (eg. during full acceleration), the EGR valve closes to ensure as much oxygen enters the cylinder as possible. EGR valves can be used to improve combustion efficiency and knock tolerance. In diesels, it can also help to reduce diesel knock at idle.
Types of EGR valve
Although there are several types of EGR valves, earlier systems use a vacuum-operated valve, while newer vehicles are electronically-controlled. The main types of EGR’s are summarised below:
- Diesel high pressure EGR valves divert the high-flow, high-soot exhaust gas before it enters the diesel particulate filter – the soot combines with oil vapor to create sludge. The gas is then passed back to the inlet manifold, either via a pipe or internal drillings in the cylinder head. A secondary valve is also used to help create a vacuum in the inlet manifold, as this is not naturally present on diesel engines.
- Diesel Low pressure EGR valves divert the exhaust gas after it has passed through the diesel particulate filter. This gas has a lower flow, but is almost completely clean of soot. The gas is then passed back to the inlet manifold via a pipe.
- Gasoline EGR valves divert the exhaust gases, much like the high-pressure diesel equivalent. The vacuum created by cylinder depression, draws the exhaust gases in. The flow is regulated by the opening and closing of the EGR valve.
- Vacuum operated EGR valves use a vacuum solenoid to vary the vacuum to the diaphragm, and open and close the EGR. Some valves also include a feedback sensor to inform the ECU of the valves position.
- Digital EGR valves feature a solenoid or stepper motor and, in most cases, a feedback sensor. These valves receive a pulse width modulated signal from the ECU to regulate exhaust gas flow.
Why do EGR valves fail?
EGR valves operate in a complex environment and are prone to wear and tear. However, the single biggest cause of failure is the buildup of carbon particles from the exhaust gases along the EGR and intake system passages. Over time, this will clog tubes, exhaust gas channels and eventually the valve’s plunger mechanism, causing it to either stick in the open or closed position. Failures can also be caused by a rupture or leak in the valve diaphragm.
The symptoms associated with EGR valve failure are similar to those of many other engine management components, and because of this, EGR faults continue to be a source of headaches for many technicians. However, there are a few signs to look out for, such as:
- Check engine light: as with most engine management components, a problem with the EGR valve may trigger the check engine light.
- Engine performance issues: if the valve is stuck open, the vehicle’s air-fuel ratio will be disrupted causing engine performance issues such as reduced power, poor acceleration and rough idle. It may also produce turbo boost pressure leaks, causing the turbo to work harder.
- Increased NOx emissions: when the EGR valve remains closed, the resultant high temperatures in the combustion chamber will leave a lot of unburned fuel in the exhaust, leading to increased NOx emissions and reduced fuel efficiency.
- Engine knock: the higher temperatures and NOx may also result in increased detonation or knock, heard as knocking noises in the engine.
Troubleshooting an EGR valve
Given the different types of EGR valves, it is always best to follow the troubleshooting procedures detailed in the service manual. However, there are a few common steps that can help to perform diagnostics:
- Read fault codes using a Diagnostic Scan Tool* capable of reading fault codes in the EGR system.
- Check that all vacuum lines and electrical connections are connected and positioned correctly.
- 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.
- Check the vacuum solenoid while engine is running. On electronically-controlled EGR valves, activate the solenoid with a scan tool and check the vacuum at end of pipe. If the solenoid does not open when energised, is stuck in the open or closed position, or has a corroded electrical connection, loose wire or bad ground, EGR operation will be affected.
- 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.
- 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.
- Remove the EGR valve and check for carbon build up. Where possible, remove any carbon, being careful not to contaminate the diaphragm.
- Inspect the EGR passageway in the manifold for clogging and clean if required.
*The ease of replacing an EGR – as with the advancement in EGR technology – has become more complex and time-consuming. In the past, EGR replacement could be done in hours. Now, EGR replacement can take up to 8 hours on some passenger vehicles. That’s why it’s imperative to have a scan tool that has strong emissions testing and calibrating capabilities to help diagnose EGR related issues.
At Cornell’s we have put a lot of time and effort in understanding and diagnosing EGR systems and their failures. Our qualified technicians have built up a wealth of knowledge that helps them diagnose EGR system failures quicker, and helps to reduce the cost of EGR repairs. For further information please call us on 03 9267 8800