Knowing EGR codes helps regulate emissions

Here's why we need it, how the PCM checks it and why it helps reduce tailpipe gases.

Exhaust gas recirculation (EGR) is used to control emissions in every automotive engine built since the Clean Air Act of the 1970s. Those early EGR systems were only marginally successful at reducing tailpipe emissions, but they were very successful at reducing driveability. Even when they worked correctly, they often were disabled to satisfy customer complaints about surging and stalling.

After 30 years, EGR systems are more effective, completely transparent to the driver and rarely fail. But when there's a light on the dashboard and EGR codes appear on the scan tool, understanding the strategies behind them can help pinpoint the real problem.

When a mixture of gasoline and air is compressed and burned, some of the oxygen and nitrogen in the air charge combine chemically to form nitric oxide (NO) or nitrogen dioxide (NO2), which are abbreviated together as NOx. In the environment, NOx contributes to the formation of ground-level ozone, and when mixed with water in the atmosphere, it becomes nitric acid, which falls to the ground as acid rain. NOx is truly nasty stuff, and tailpipe emissions of NOx are regulated by law.

To lower those emissions, techs can reduce the amount of NOx generated by the engine or use a catalytic converter to break the NOx molecules apart into nitrogen and oxygen. Both techniques are needed to reach the regulated limit, but engine-out NOx emissions must be as low as possible.

In a spark-ignition engine, in-cylinder NOx formation increases with temperature, combustion pressure and excess air in the air/fuel mixture. NOx formation could be eliminated with an air/fuel mixture that is rich enough to ensure all of the oxygen is used in combustion, but a rich mixture increases fuel consumption and dramatically increases emissions of hydrocarbon (HC) and carbon monoxide (CO). Peak combustion pressure can be limited by retarding the ignition timing, especially when the engine is under a heavy load. But retarding the ignition timing over the whole speed/load range increases CO and HC emissions and reduces power output, too. Also, compression ratios have increased in recent years in the drive for increased engine efficiency and specific power output. This means combustion pressures are higher at all speeds and loads.

The last option for reducing in-cylinder NOx formation is to reduce combustion temperature, which is not hard to do in a spark-ignition gasoline engine. By recirculating exhaust gas back into the combustion chamber, the rate of combustion is slowed, reducing its peak temperature. Up to 15 percent of the cylinder volume can be filled with exhaust gas to achieve the desired effect. If done properly, exhaust gas recirculation can also reduce fuel consumption, especially under low-load cruise conditions that naturally produce less NOx. There are two ways to achieve EGR: internally and externally.

Internal EGR

Internal EGR occurs naturally when the intake and exhaust valves both are open at the same time, a condition known as valve overlap. As the piston rises on the exhaust stroke, a strong flow of exhaust gas is established through the exhaust port. If the intake valve opens just before the piston stops rising, even though intake manifold pressure is below atmospheric, the gas flow out of the cylinder is strong enough to pull fresh air from the intake manifold through the combustion chamber. This is known as scavenging, because it helps evacuate exhaust gases from the cylinder, but it's only effective at the higher exhaust gas flow rates that come with higher engine speeds.

When the intake valve opens early at lower speeds, there's less flow velocity, so the rising piston has time to push some of the exhaust gas into the intake port. That exhaust is sucked back into the cylinder during the intake stroke, providing internal EGR.

Valve overlap can be tuned to work at different gas flow rates, which correspond with different rpm ranges. Large valve overlap (early opening of the intake valve and/or overlap occurring over a large crankshaft angle) increases internal EGR, but idle and low-speed driveability suffer because intake airflow is unstable. This produces the loping idle that's common to some "big-cam" engines of the 1960s. But with variable valve timing and variable path intake manifolds, valve overlap can be tuned to provide internal EGR over a wide range of engine speeds and loads without hurting driveability. In fact, it can provide a significant improvement in gas mileage.

External EGR

External EGR uses a passageway to allow gases to flow from the exhaust manifold to the intake manifold or to each intake port. Sometimes the passageway is cast into the manifold or cylinder head, but usually it's an external tube, and there's always a valve to control the flow rate. The exhaust gases always are taken upstream of the catalytic converter, and at any rpm it's the difference between exhaust backpressure and manifold vacuum that causes the gases to flow in the right direction. That's why engines with forced induction often have internal EGR.

Since the 1996 model year, the on-board diagnostic program (OBDII) in the powertrain control module (PCM) has been monitoring the performance of the external EGR system. This requires a computer-controlled, closed-loop operating strategy with some kind of feedback sensor that allows the PCM to measure and adjust EGR flow in real time.

Operating strategies vary by manufacturer, but in general, the EGR valve is commanded closed at idle and at wide open throttle (WOT) and open to varying degrees during cruise. The amount of EGR flow required depends on engine speed and load, and the PCM uses a speed/load/flow rate look-up table to decide how much EGR is needed at any given time. Of course, the PCM also needs to know actual EGR flow rate, and each manufacturer has its own way of measuring this.

Vacuum-operated EGR

Ford uses a differential pressure feedback EGR sensor (DPFE) that monitors pressure differences to determine the flow of exhaust gas through the EGR tube. On earlier systems, the EGR tube contains an orifice, and as exhaust gas flows through it, the pressure in the tube is higher on the upstream side of the orifice. Hoses connect to the tube on each side of the orifice and to the DPFE sensor. The pressure difference on each side of the orifice indicates the EGR flow rate. On the later system, the DPFE sensor reads intake manifold pressure and exhaust pressure after the EGR valve seat.

The EGR valve itself is vacuum-actuated, and the vacuum is controlled by the PCM through a pulse-width modulated solenoid valve called the electronic vacuum regulator (EVR). The PCM monitors the DPFE and EVR circuits any time the engine is running, checking for short circuits or out-of-limit signals.

If the measured EGR flow is not what the PCM expects to see at the given speed/load, the PCM will manipulate the valve during acceleration, deceleration and cruise to check for plugged or leaking DPFE sensor hoses, a stuck-open EGR valve and the valve's response to the commanded duty cycle. If any problems are detected, the PCM will turn on the malfunction indicator lamp (MIL) and store fault codes. These are one-trip faults, and all the on-board test information is available in several places on a scan tool, including Mode $06.

EGR flow rate measurements are accurate only when the underhood temperature is above freezing and when altitude is below 8,000 feet.

Toyota also uses a vacuum-operated EGR valve, but vacuum is controlled mechanically by a vacuum modulator. Ported vacuum from just upstream of the throttle plate is supplied to the top of a diaphragm. As vacuum raises the diaphragm, an internal port opens and vacuum is supplied to the EGR valve. The vacuum modulator's diaphragm also is pushed off its seat by exhaust backpressure, so as engine load and exhaust backpressure increase, more vacuum is supplied to the EGR valve.

At wider throttle openings, another vacuum port in the throttle body is uncovered, providing still more vacuum for an even greater EGR valve opening. A solenoid valve between the EGR valve and vacuum modulator is used by the PCM to command the EGR valve closed.

To monitor EGR flow and for testing the system, there is a temperature sensor at the exhaust inlet side of the EGR valve. Increasing temperature means increasing EGR flow.

Honda uses vacuum-operated EGR valves, too, and vacuum is controlled with a pulse-width modulated solenoid valve. EGR valve operation is monitored with a position sensor built into the top of the valve.

Electronic EGR

General Motors takes a very different approach to EGR. Though the engine has a mass air flow sensor (MAF), it also has a manifold absolute pressure sensor (MAP). Any time the EGR valve is commanded open, the PCM compares the drop in manifold pressure to the specification in the speed/load/flow rate look-up table. The PCM uses this comparison to monitor EGR flow rate during normal driving and also for the OBDII system test.

Once per ignition cycle, the EGR valve will be commanded open when it normally should be closed and closed when it normally should be open, while the PCM looks for the specified change in manifold pressure. The PCM will do this several times to get an average value for each commanded action. A failed test turns on the MIL and sets codes in one trip.

Chrysler's EGR valve is similar to that used by GM, but it also has a position sensor on the pintle to tell the PCM how far the valve has opened in response to the command. Flow is monitored by looking for expected changes in the MAP sensor reading. The diagnostic test is unique. The EGR valve is operated during closed-throttle deceleration, and the PCM looks for a change in the oxygen sensor reading: rich when the valve is open and lean when closed.

Internal EGR Monitoring

On engines equipped with variable camshaft timing and internal EGR, that system must work properly for proper NOx control, so it's monitored by the OBDII system. On most vehicles, the monitor only checks the control valve circuits for shorts or opens, but on newer models, the camshaft position sensor (CMP) signals are also monitored. Problems in this system would also cause driveability issues, but as always, the first priority is emissions control.

As more engines come to market with variable camshaft timing, there will be fewer EGR valves. But external EGR systems will never go away and in some applications, might become even more complicated.