OBJECTIVES

Advanced Engine Performance Diagnosis, 4/eBy James D. Halderman

OBJECTIVES

After studying Chapter 22, the reader will be able to:

1. Prepare for ASE Engine Performance (A8) certification test content area “D” (Emission Control Systems).

2. Describe the purpose and function of the exhaust gas recirculation system.

3. Discuss how the exhaust gas recirculation system is tested under OBD-II regulations.

4. Explain methods for diagnosing and testing for faults in the exhaust gas recirculation system.

NOX FORMATION

• Nitrogen (N2) and oxygen (O2) molecules are separated into individual atoms of nitrogen and oxygen during the combustion process.

• These then bond to form NOX (NO, NO2).

• When combustion flame front temperatures exceed 2,500°F (1,370°C), NOX formation increases dramatically.

CONTROLLING NOX

• The amounts of NOX formed at temperatures below 2,500°F (1,370°C) can be controlled in the exhaust by a catalyst.

• To handle the amounts generated above 2,500°F (1,370°C), the following are some methods that have been used to lower NOX formation:– Enrich the air-fuel mixture. – Lower the compression ratio. – Dilute the air-fuel mixture.

CONTROLLING NOX

FIGURE 22-1 Typical vacuum-operated EGR valve. The operation of the valve is controlled by the computer by pulsing the EGR control solenoid on and off.

EGR SYSTEM OPERATION

• Since small amounts of exhaust are all that is needed to lower peak combustion temperatures, the orifice that the exhaust passes through is small.

FIGURE 22-2 When the EGR valve opens, exhaust flows through the valve

and into passages in the intake manifold.

EGR SYSTEM OPERATION

• Because combustion temperatures are low, EGR is usually not required during the following conditions.– Idle speed– When the engine is cold

POSITIVE AND NEGATIVE BACK PRESSURE EGR VALVES• Many EGR valves are designed with a small valve inside that

bleeds off any applied vacuum and prevents the valve from opening. – Some EGR valves require a positive back pressure in the exhaust

system. – This is called a positive back pressure EGR valve.

• On each exhaust stroke, the engine emits an exhaust “pulse.” Each pulse represents a positive pressure.

• Behind each pulse is a small area of low pressure. – Some EGR valves react to this low pressure area by closing a small

internal valve, which allows the EGR valve to be opened by vacuum. – This type of EGR valve is called a negative back pressure EGR

POSITIVE AND NEGATIVE BACK PRESSURE EGR VALVES

FIGURE 22-3 Back pressure in the exhaust system is used to close the control valve, thereby allowing engine vacuum to open the EGR valve.

COMPUTER-CONTROLLED EGR

• Many computer-controlled EGR systems have one or more solenoids controlling the EGR vacuum.

• The computer controls a solenoid to shut off vacuum to the EGR valve at cold engine temperatures, idle speed, and wide-open throttle operation.

EGR VALVE POSITION SENSORS

• Late model computer-controlled EGR systems use a sensor to indicate EGR operation.

• On-Board Diagnostics Generation-II (OBD-II) EGR system monitors require an EGR to determine EGR gas flow.

• A linear potentiometer on the top of the EGR valve stem indicates valve position for the computer.

• This is called an EGR valve position (EVP) sensor.

FIGURE 22-4 An EGR valve position sensor on top of an EGR valve.

FIGURE 22-5 An integrated EGR valve system showing the pintle-position sensor and vacuum diaphragm.

EGR VALVE POSITION SENSORSDigital EGR Valves

• Unlike the previously mentioned vacuum-operated EGR valves, the digital EGR valve consists of three solenoids controlled by the PCM.

FIGURE 22-6 This 3800 V-6 uses three solenoids for EGR.A scan tool can be used

to turn on each solenoid to check if the valve is working and if the exhaust passages are

capable of flowing enough exhaust to the intake manifold to affect engine operation

when cycled.

EGR VALVE POSITION SENSORSLinear EGR

• Most General Motors and several others vehicles use a linear EGR that contains a pulse-width modulated solenoid to precisely regulate exhaust gas flow and a feedback potentiometer that signals to the computer the actual position of the valve.

FIGURE 22-7 A General Motors linear EGR valve.

EGR VALVE POSITION SENSORSLinear EGR

FIGURE 22-8 The EGR valve pintle is pulse-width modulated and a three-wire potentiometer provides pintle-position information back to the PCM.

OBD-II EGR MONITORING STRATEGIES

• In 1996, the U.S. EPA began requiring OBD-II systems in all passenger cars and most light-duty trucks.

• These systems include emissions system monitors that alert the driver and the technician if an emissions system is malfunctioning.

• To be certain the EGR system is operating, the PCM runs a functional test of the system, when specific operating conditions exist.

• The OBD-II system tests by opening and closing the EGR valve.

• The PCM monitors an EGR function sensor for a change in signal voltage.

• If the EGR system fails, a diagnostic trouble code (DTC) is set. If the system fails two consecutive times, the malfunction indicator light (MIL) is lit.

FIGURE 22-9 A DPFE sensor and related components.

FIGURE 22-10 An OBD-II active test. The PCM opens the EGR valve and then monitors the MAP sensor and/or engine speed (RPM) to meet acceptable values.

DIAGNOSING A DEFECTIVE EGR SYSTEM

• If the EGR valve is not opening or the flow of the exhaust gas is restricted, then the following symptoms are likely:– Ping (spark knock or detonation) during acceleration or during cruise

(steady-speed driving)

– Excessive oxides of nitrogen (NOX) exhaust emissions If the EGR valve is stuck open or partially open, then the following symptoms are likely:

– Rough idle or frequent stalling– Poor performance/low power, especially at low engine speed– To check for proper EGR flow, the engine vacuum should drop 6 to 8

in. Hg when the EGR is commanded on by a scan tool when the engine is at idle speed.

THE SNAKE TRICK

FIGURE 22-11 Removing the EGR passage plugs from the intake manifold on a Honda.

EGR-RELATED OBD-II DIAGNOSTIC TROUBLE CODES

SUMMARY

1. Oxides of nitrogen (NOX) are formed inside the combustion chamber due to heat exceeding 2,500°F.

2. Recirculating 6% to 10% inert exhaust gases back into the intake system reduces peak temperature inside the combustion chamber and reduces NOX exhaust emissions.

3. EGR is usually not needed at idle, and not wanted at wide-open throttle, for maximum engine performance, or when the engine is cold.

SUMMARY

4. Vacuum-operated EGR valves are usually exhaust back pressure controlled to help match EGR flow into the intake with the load on the engine.

5. Many EGR systems use a feedback potentiometer to signal the PCM about the position of the EGR valve pintle.

6. Some EGR valves are solenoids or pulse-width modulated pintles.

7. OBD II requires that the flow rate be tested and this can be achieved by opening the EGR valve and observing the reaction of the MAP sensor.

REVIEW QUESTIONS

1. What causes the formation of oxides of nitrogen?

2. How does the use of exhaust gas reduce NOX exhaust emission?

3. How does the DPFE sensor work?

4. How does the PCM determine that the exhaust flow through the EGR system meets OBD-II regulations?

CHAPTER QUIZ

1. What causes the nitrogen and the oxygen in the air to combine and form NOX?

a) Sunlight

b) Any spark will cause this to occur

c) Heat above 2,500°F (1,370°C)

d) Chemical reaction in the catalytic converter

CHAPTER QUIZ

1. What causes the nitrogen and the oxygen in the air to combine and form NOX?

a) Sunlight

b) Any spark will cause this to occur

c) Heat above 2,500°F (1,370°C)

d) Chemical reaction in the catalytic converter

CHAPTER QUIZ

2. Exhaust gas recirculation (EGR) is generally not needed under all the following conditions except ______.a) Idle speed

b) Cold engine

c) Cruise speed

d) Wide-open throttle (WOT)

CHAPTER QUIZ

2. Exhaust gas recirculation (EGR) is generally not needed under all the following conditions except ______.a) Idle speed

b) Cold engine

c) Cruise speed

d) Wide-open throttle (WOT)

CHAPTER QUIZ

3. Technician A says that a low-restriction exhaust system could prevent a back pressure-type vacuum-controlled EGR valve from opening correctly. Technician B says restricted exhaust can cause the EGR valve position sensor to fail. Which technician is correct?a) Technician A only

b) Technician B only

c) Both Technicians A and B

d) Neither Technician A nor B

CHAPTER QUIZ

3. Technician A says that a low-restriction exhaust system could prevent a back pressure-type vacuum-controlled EGR valve from opening correctly. Technician B says restricted exhaust can cause the EGR valve position sensor to fail. Which technician is correct?a) Technician A only

CHAPTER QUIZ

4. EGR is used to control which exhaust emission?a) Unburned hydrocarbons (HC)

b) Oxides of nitrogen (NOX)

c) Carbon monoxide (CO)

d) Both NOX and CO

CHAPTER QUIZ

4. EGR is used to control which exhaust emission?a) Unburned hydrocarbons (HC)

b) Oxides of nitrogen (NOX)

c) Carbon monoxide (CO)

d) Both NOX and CO

CHAPTER QUIZ

5. A typical EGR pintle-position sensor is what type of sensor?a) Rheostat

b) Piezoelectric

c) Wheatstone bridge

d) Potentiometer

CHAPTER QUIZ

5. A typical EGR pintle-position sensor is what type of sensor?a) Rheostat

b) Piezoelectric

c) Wheatstone bridge

d) Potentiometer

CHAPTER QUIZ

6. OBD-II regulations require that the EGR system be tested. Technician A says that the PCM can monitor the commanded position of the EGR valve to determine if it is functioning correctly. Technician B says that the PCM can open the EGR valve an monitors for a change in the MAP sensor or oxygen sensor reading to detect if the system is functioning correctly. Which technician is correct?

a) Technician A only

CHAPTER QUIZ

6. OBD-II regulations require that the EGR system be tested. Technician A says that the PCM can monitor the commanded position of the EGR valve to determine if it is functioning correctly. Technician B says that the PCM can open the EGR valve an monitors for a change in the MAP sensor or oxygen sensor reading to detect if the system is functioning correctly. Which technician is correct?

a) Technician A only

CHAPTER QUIZ

7. Two technicians are discussing clogged EGR passages. Technician A says clogged EGR passages can cause excessive NOX exhaust emission. Technician B says that clogged EGR passages can cause the engine to ping (spark knock or detonation). Which technician is correct?a) Technician A only

CHAPTER QUIZ

7. Two technicians are discussing clogged EGR passages. Technician A says clogged EGR passages can cause excessive NOX exhaust emission. Technician B says that clogged EGR passages can cause the engine to ping (spark knock or detonation). Which technician is correct?a) Technician A only

CHAPTER QUIZ

8. An EGR valve that is partially stuck open would most likely cause what condition?a) Rough idle/stalling

b) Excessive NOX exhaust emissions

c) Ping (spark knock or detonation)

d) Missing at highway speed

CHAPTER QUIZ

8. An EGR valve that is partially stuck open would most likely cause what condition?a) Rough idle/stalling

b) Excessive NOX exhaust emissions

c) Ping (spark knock or detonation)

d) Missing at highway speed

CHAPTER QUIZ

9. When testing an EGR system for proper operation using a vacuum gauge, how much should the vacuum drop when the EGR is commanded on by a scan tool?a) 1 to 2 in. Hg

b) 3 to 5 in. Hg

c) 6 to 8 in. Hg

d) 8 to 10 in. Hg

CHAPTER QUIZ

9. When testing an EGR system for proper operation using a vacuum gauge, how much should the vacuum drop when the EGR is commanded on by a scan tool?a) 1 to 2 in. Hg

b) 3 to 5 in. Hg

c) 6 to 8 in. Hg

d) 8 to 10 in. Hg

CHAPTER QUIZ

10.A P0401 DTC (exhaust gas recirculation flow insufficient) is being discussed. Technician A says that a defective EGR valve could be the cause. Technician B says that clogged EGR passages could be the cause. Which technician is correct?a) Technician A only

CHAPTER QUIZ

10.A P0401 DTC (exhaust gas recirculation flow insufficient) is being discussed. Technician A says that a defective EGR valve could be the cause. Technician B says that clogged EGR passages could be the cause. Which technician is correct?a) Technician A only

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