The Exhaust Gas Recirculation (EGR) system's purpose is to reduce NOx emissions that contribute to air pollution. The first EGR systems were added to engines in 1973, and today most engines have an EGR system.
As long as the EGR system is functioning properly, it should have no noticeable effect on engine performance. But if the EGR system is leaking or inoperative, it can cause driveability problems, including detonation (knocking or pinging when accelerating or under load), a rough idle, stalling, hard starting, elevated NOx emissions and even elevated hydrocarbon (HC) emissions in the exhaust.
Exhaust gas recirculation reduces the formation of NOX by allowing a small amount of exhaust gas to "leak" into the intake manifold. The amount of gas leaked into the intake manifold is only about 6 to 10% of the total, but it's enough to dilute the air/fuel mixture just enough to have a "cooling effect" on combustion temperatures. This keeps combustion temperatures below 1500 degrees C (2800 degrees F) to reduce the reaction between nitrogen and oxygen that forms NOx.
HOW EGR WORKS
To recirculate exhaust back into the intake manifold, a small calibrated "leak" or passageway is created between the intake and exhaust manifolds. Intake vacuum in the intake manifold sucks exhaust back into the engine. But the amount of recirculation has to be closely controlled otherwise it can have the same effect on idle quality, engine performance and driveability as a huge vacuum leak.
Most older EGR systems use a vacuum regulated EGR valve while newer vehicles tend to have an electronic EGR valve to control exhaust gas recirculation. When the engine is idling, the EGR valve is closed and there is no EGR flow into the manifold. The EGR valve remains closed until the engine is warm and is operating under load. As the load increases and combustion temperatures start to rise, the EGR valve opens and starts to leak exhaust back into the intake manifold. This has a quenching effect that lowers combustion temperatures and reduces the formation of NOx.
In addition to EGR, other methods may also be used to minimize NOx. These include increasing camshaft valve overlap, redesigning the combustion chamber and modifying ignition advance curves. Three-way catalytic converters also reduce NOx in the exhaust. Some engines run so clean that they do not need an EGR system to meet NOx emission standards.
If the EGR system is rendered inoperative because it was disconnected or tampered with, the cooling effect that was formerly provided by the EGR system will be lost. Without EGR, the engine will often knock and ping (detonate) when accelerating or lugging the engine. This can cause engine damage over time.
TYPES OF EGR VALVES
There are six different types of EGR valves:
Positive backpressure EGR valves (1973 & up). Backpressure EGR valves use exhaust backpressure to vary the point at which they open and their flow rates. On GM cars, they are identified by the last letter on the part number on top of the valve. A letter "P" indicates a positive backpressure valve, and a letter "N" indicates a negative backpressure valve. Inside a backpressure EGR valve is a second diaphragm that reacts to backpressure in the exhaust system. The backpressure diaphragm opens and closes a small bleed hole in the main EGR vacuum circuit or diaphragm chamber. Opening the bleed hole reduces vacuum to the main diaphragm and prevents the valve from opening fully. Closing the bleed hole allows full vacuum to reach the main diaphragm so the valve can open wide and allow maximum EGR flow. With positive backpressure EGR valves, any increase in exhaust backpressure causes the EGR valve to open. This reduces backpressure somewhat, allowing the backpressure diaphragm to bleed off some control vacuum. The EGR valve begins to close and exhaust pressure rises again. The EGR valve oscillates open and closed with changing exhaust pressure to maintain a sort of balanced flow.
Negative backpressure EGR valves (1973 & up). The negative backpressure type of EGR valve reacts in the same way, except that it reacts to negative or decreasing pressure changes in the exhaust system to regulate EGR action. A drop in backpressure occurs when there is less load on the engine. This causes the backpressure diaphragm to open a bleed hole and reduce EGR flow. It's the same principle as with the positive type except that the control function occurs when backpressure goes down instead of up.
NOTE: Most precomputer EGR systems have a temperature vacuum switch(TVS) or ported vacuum switch between the EGR valve and vacuum source to prevent EGR operation until the engine has had a chance to warm up. The engine must be relatively warm before it can handle EGR. If an engine runs rough or stumbles when cold, it may indicate a defective TVS that is allowing EGR too soon after starting. A TVS stuck in the closed position would block vacuum to the EGR and prevent any EGR operation. The symptom here would be excessive NOx emissions and possible pinging or detonation.
Pulse-width modulated electronic EGR valves (early 1980s & up). First used in 1984 by General Motors, this type of EGR system uses a pulse width-modulated EGR control solenoid. With this technique, the powertrain control module (PCM) cycles the EGR vacuum control solenoid rapidly on and off. This creates a variable vacuum signal that can regulate EGR operation very closely. The amount of "on" time versus "off" time for the EGR solenoid ranges from 0 to 100 percent, and the average amount of "on" time versus "off" time at any given instant determines how much EGR flow occurs.
Digital electronic EGR valves (late 1980s to 1990s). On some applications, a "digital" EGR valve is used. This type of valve also uses vacuum to open the valve but regulates EGR flow according to computer control. The digital EGR valve has three metering orifices that are opened and closed by solenoids. By opening various combinations of these three solenoids, different flow rates can be achieved to match EGR to the engine's requirements. The solenoids are normally closed, and open only when the computer completes the ground to each.
Linear electronic EGR valves (early 1990s & up). Another type of electronic EGR valve is the "linear" EGR valve. This type uses a small computer-controlled stepper motor to open and close the EGR valve instead of vacuum. The advantage of this approach is that the EGR valve operates totally independent of engine vacuum. It is electrically operated and can be opened in various increments depending on what the engine control module determines the engine needs at any given moment in time. GM started using this type of valve on many of its engines in 1992. Linear EGR valves may also be equipped with an EGR valve position sensor (EVP) to keep the computer informed about what the EGR valve is doing. The EVP sensor also helps with self-diagnostics because the computer looks for an indication of movement from the sensor when the it commands the EGR valve to open or close. The sensor works like a throttle position sensor and changes resistance. The voltage signal typically varies from 0.3 (closed) to 5 volts (open).
APPLICATIONS WITH NO EGR VALVE
On many late model engines with Variable Valve Timing(VVT), there is no EGR valve because the VVT system varies the timing of the exhaust valves to provide the same effect as EGR. By changing the point at which the exhaust valves close when the engine is working hard under load, a small amount of exhaust gas can be retained in the cylinders for the next combustion cycle. This has the same effect on reducing combustion temperatures and NOx as recirculating exhaust gas from an exhaust port back into the intake manifold through an EGR valve. The big difference is that the VVT system can react to changing engine loads much more quickly and precisely than a traditional EGR valve. Using VVT for EGR also eliminates many of the problems associated with EGR valves such as carbon buildup and valve sticking or failure.
COMMON EGR PROBLEMS
Pinging (spark knock or detonation) because the EGR system is not working, the exhaust port is plugged up with carbon, or the EGR valve has been disabled.
Rough idle or misfiring because the EGR valve is not closing and is leaking exhaust into the intake manifold. You may also find a P0300 random misfire code on OBD II vehicles.
Hard starting because the EGR valve is not closing and is creating a vacuum leak into the intake manifold.
Find out what kind of EGR valve is on the vehicle so you can use the appropriate test procedure. Examine the valve or refer to a service manual. On some vehicles, you may find this information on the underhood emissions decal. Also, find out what kind of vacuum controls are used in the vacuum plumbing. Does it have a ported vacuum switch or a solenoid? Follow the vacuum connections from the valve, refer to a service manual or the underhood emissions decal for vacuum hose routing information.
There are several ways to troubleshoot an EGR system. You can follow the EGR troubleshooting procedure that's listed in a service manual for the engine. On late model computer controlled engines, there may be trouble codes that relate to the EGR system. On such an application, the first step would be to read out the code or codes using a scan tool or code reader. You would then refer to the specific diagnostic charts in a service manual that tell you what to do next.
EGR Trouble Codes:
P0400....Exhaust Gas Recirculation Flow
P0401....Exhaust Gas Recirculation Flow Insufficient Detected
P0402....Exhaust Gas Recirculation Flow Excessive Detected
P0403....Exhaust Gas Recirculation Control Circuit
P0404....Exhaust Gas Recirculation Control Circuit Range/Performance
P0405....Exhaust Gas Recirculation Sensor 'A' Circuit Low
P0406....Exhaust Gas Recirculation Sensor 'A' Circuit High
P0407....Exhaust Gas Recirculation Sensor 'B' Circuit Low
P0408....Exhaust Gas Recirculation Sensor 'B' Circuit High
P0409....Exhaust Gas Recirculation Sensor 'A' Circuit
On pre-OBD II GM applications, a code 32 indicates an EGR problem. The logic by which the onboard diagnostics detects trouble follows one of two routes. On some applications, a code 32 is set when the computer detects a richer fuel mixture off idle (indicating no EGR). On others, a code is set if the computer energizes the EGR vacuum solenoid but does not detect a corresponding drop in intake vacuum.
On pre-OBD II Fords, a code 31 indicates a problem with the EGR valve position sensor (EVP). It works like a throttle position sensor, going from high resistance (5500 ohms) when the EGR valve is closed to low resistance (100 ohms) when it is open. You'll find these EVP sensors mostly on Ford EEC-IV V6 and V8 engines. Other codes include a code 32 which indicates the EGR circuit is not controlling. A code 33 is triggered when the EVP sensor is not closing, and a code 34 indicates no EGR flow. Any of these codes could indicate a faulty EGR valve as well as a problem in the EGRC or EGRV vacuum solenoids. Other codes include a code 83 (EGRC circuit fault) and code 84 (EGRV circuit fault). Both indicate an electrical problem in one of the solenoid circuits. The solenoids should have between 30 and 70 ohms resistance.
See Emission Guide for emissions testing and diagnosis information. Emission Guide is a quick reference program that covers basic emission controls and emissions testing.
FORD EGR PROBLEMS
On 1995 and newer vehicles with OBD II, P0400 to P0409 codes indicate various faults in the EGR system.
Click to see larger image of Ford DPFE sensor
A common EGR problem with many Fords is a bad DPFE (differential pressure) sensor. The DPFE sensor is part of the EGR system and senses EGR flow when the EGR valve is open. It provides a feedback signal to the engine computer so it can vary EGR flow to meet changing engine loads. The DPFE sensor is usually mounted on the engine and is connected to the pipe that runs from the exhaust manifold to the EGR valve with two rubber hoses. When the sensor goes bad, the Check Engine light comes on and typically sets any or all of teh following fault codes: P0171 & P0174 (lean codes), and/or P0401 (insufficient EGR flow). Nine out of ten times, the fault is not a bad EGR valve or a vacuum leak, but a bad DPFE sensor. A replacement costs about $112 at Ford, or about $48 at an aftermarket auto parts store.
EGR TROUBLESHOOTING PROCEDURE
The following "generic" procedure can help you troubleshoot EGR problems.
1. Does the engine have a detonation (spark knock) problem when accelerating under load? Refer to the timing specs for the engine and check ignition timing. The timing may be overadvanced. If the timing is within specs, check the engine's operating temperature. A cooling problem may be causing the engine to detonate. If the temperature is within its normal range and there are no apparent cooling problems, other possibilities to investigate include a spark plugs that are too hot for the engine application, a lean air/fuel mixture, low octane fuel or too much compression (due to a buildup of carbon in the combustion chambers or because of pistons or heads that have too much compression for the fuel you're using). Be sure you've ruled out all the other possibilities before focusing on the EGR system.
2. Use a vacuum gauge to check the EGR valve vacuum supply hose for vacuum at 2000-2500 rpm. There should be vacuum if the engine is at normal operating temperature. No vacuum would indicate a problem such as a loose or misrouted hose, a blocked or inoperative ported vacuum switch or solenoid, or a faulty vacuum amplifier (or vacuum pump in the case of a diesel engine).
Sometimes loss of EGR can be caused by a failed vacuum solenoid in the EGR's vacuum supply line. Refer to a vacuum hose routing diagram in a service manual or the hose routing information on the vehicle's emission decal for the location of the solenoid. If the solenoid fails to open when energized, jams shut or open, or fails to function because of a corroded electrical connection, loose wire, bad ground, or other electrical problem, it will obviously affect the operation of the EGR valve. Depending on the nature of the problem, the engine may have no EGR, EGR all the time, or insufficient EGR. If bypassing the suspicious solenoid with a section of vacuum tubing causes the EGR valve to operate, find out why the solenoid isn't responding before you replace it. The problem may be nothing more than a loose or corroded wiring connector.
3. Inspect the EGR valve itself. Because of the valve's location, it may be difficult to see whether or not the valve stem moves when the engine is revved to 1500 to 2000 rpm by slowing opening and closing the throttle. The EGR valve stem should move if the valve is functioning correctly. A hand mirror may make it easier to watch the valve stem. Be careful not to touch the valve because it will be hot! If the valve stem doesn't move when the engine is revved (and the valve is receiving vacuum), there's probably something wrong with the EGR valve.
Another way to "test" the EGR valve on some engines is to apply vacuum directly to the EGR valve. Note; This only works on ported vacuum EGR valves, not backpressure EGR valves or electronic EGR valves. Vacuum should pull the valve open creating the equivalent of a large vacuum leak. This should cause a momentary drop in idle speed and a noticeable increase in idle roughness.
Backpressure type EGR valves are more difficult to check because there must be sufficient backpressure in the exhaust before the valve will open when vacuum is applied. One trick that's sometimes used is to create an artificial restriction by inserting a large socket into the tailpipe, then applying vacuum to the valve to see if it opens. Don't forget to remove the restriction afterwards.
4. Remove and inspect the EGR valve if you suspect a problem. Most failures are caused by a rupture or leak in the valve diaphragm. If the valve is not a backpressure type, it should hold vacuum when vacuum is applied with a hand-help pump. If it can't hold vacuum, it needs to be replaced. Note: This test does not work on backpressure EGR valves.
Backpressure EGR valves sometimes fail if the hollow valve stem becomes clogged with carbon or debris. This you can see for yourself. It's almost impossible to remove such a clog, so replace the EGR valve.
Carbon accumulation around the base of the EGR valve can sometimes interfere with the opening or closing of the valve. These can be removed by careful brushing or by soaking the tip of the valve in solvent. Do not soak the entire valve in solvent or allow solvent to get anywhere near the diaphragm. The solvent will attack and ruin the diaphragm.
5. Inspect the EGR passageway in the manifold for clogging. Use a pipe cleaner or small piece of wire to explore the opening for a blockage. Sometimes you can dislodge material that's clogging the opening by carefully poking at it. Other times, it may be necessary to remove the manifold and have it professionally cleaned. Also recommended is to clean the throttle body and intake manifold at the same time to remove varnish and carbon deposits.
HOW TO REPLACE EGR VALVE
With so many variations from one vehicle application to the next in emission control systems and calibration, it is extremely important that you get the correct replacement EGR valve for the application. Two EGR valves may look identical but be calibrated differently in terms of flow and the amount of vacuum and/or backpressure it takes to open the valve. Therefore, you may have to refer to the vehicle's VIN number as well as year, make, model and engine size when ordering a replacement EGR valve. It may also be necessary to refer to the OEM part number on the old EGR valve (if possible) when ordering a replacement, so don't throw the old EGR valve away until you have the new one, have installed it and made sure it's working correctly.
Many aftermarket EGR valves are "consolidated" so fewer part numbers are necessary to cover a wider range of vehicle applications. Some of these valves use interchangeable restricters to alter their flow characteristics. Follow the suppliers instructions as to which restricter to use for the correct calibration.
More Emissions Articles:Positive Crankcase Ventilation (PCV)
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Understanding OBD II Driveability & Emissions Problems
Basic Emission Control Systems Overview
Exhaust Emissions Diagnosis
Troubleshooting a P0420 Catalyst Code
Spark Knock (Detonation)
Finding & Fixing Vacuum Leaks
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