GM Metro/Sprint 1985-1993 Repair Guide

Exhaust Gas Recirculation System

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OPERATION



See Figures 1 through 6



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Fig. Fig. 1: EGR system operation - carbureted engines



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Fig. Fig. 2: EGR system operation - fuel injected engines



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Fig. Fig. 3: EGR valve testing



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Fig. Fig. 4: EGR modulator testing



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Fig. Fig. 5: Bi-Metal vacuum switching valve (BVSV) testing



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Fig. Fig. 6: Vacuum switching valve (VSV) testing

The EGR system lowers combustion temperatures in the combustion chamber to reduce NOx (Oxides of Nitrogen) emissions. The exhaust gases are drawn from the cylinder head exhaust port into the intake manifold riser portion through the passages in the cylinder head, the intake manifold and the EGR valve.

The diaphragm mounted in the EGR modulator is operated by the back pressure of the exhaust gas on carbureted engines, or by the Engine Control Module (ECM) on fuel injected engines, to open and close the valve. By this opening and closing action the EGR modulator controls the vacuum transmitted to the EGR valve.

Under low load condition such as low speed driving, the exhaust pressure is low. In this state, the diaphragm in the EGR modulator is pushed down by the spring force and the modulator valve opens to allow the air into the vacuum passage from the outside.

As a result, the vacuum transmitted to the EGR valve becomes less and so does the opening of the EGR valve. Thus, less exhaust gas is recirculated to the intake manifold.

Under a high load condition such as high speed driving, the exhaust pressure is high. By the high exhaust pressure, the diaphragm in the modulator is pushed up and closes its valve. As the air does not enter the vacuum passage in this state, the vacuum transmitted to the EGR valve becomes larger and so does the opening of the EGR valve. Thus, a larger amount of exhaust gas is recirculated to the intake manifold.

When coolant temperature is low, the vacuum passage of the EGR valve is opened to the air through the bi-metal vacuum switching valve (BVSV) on carbureted vehicles, or the vacuum switching valve (VSV) on fuel injected vehicles. In this state, because vacuum is not transmitted to the EGR valve, it remains closed.

On the other hand, when the coolant temperature is normal, the BVSV or VSV is closed. So the EGR valve opens and closes in accordance with the EGR modulator operation.

TESTING



EGR Valve
  1. Run the engine until normal operating temperature.
  2.  
  3. Place a finger on the EGR valve diaphragm and accelerate the engine, the diaphragm should move.
  4.  
  5. Disconnect the vacuum hose from the EGR valve.
  6.  
  7. Apply 10 in. Hg of vacuum to the valve and place a finger on the diaphragm. The valve should move and the engine should stall.
  8.  

EGR Modulator
  1. Check the filter for contamination and damage. Clean using compressed air.
  2.  
  3. Remove the modulator and plug the nozzle with a finger. Blow air into another nozzle and check that air passes through to the air filter side freely.
  4.  
  5. Install a vacuum pump to nozzle P and plug nozzle Q with a finger. Blow air into nozzle A and operate the vacuum pump. Should not be able to obtain vacuum on the pump.
  6.  

Bi-metal Vacuum Switching Valve (BVSV)
  1. Disconnect the hoses from the valve.
  2.  
  3. With the BVSV cool (engine temperature below 113°F) blow into both nozzles individually. Air should come out of the filter.
  4.  
  5. With the BVSV warm (engine temperature above 140°F) blow into both nozzles individually. Air should not come out of the filter.
  6.  

Vacuum Switching Valve (VSV)
  1. Disconnect the hoses and electrical connectors from the solenoid.
  2.  
  3. Use a DVOM to check the resistance between the 2 terminals; should be 33-39 ohms resistance. If not, replace the solenoid.
  4.  
  5. Blow into tube A , air should exhaust through the filter, not tube B .
  6.  
  7. Reconnect the electrical connector. With the ignition switch ON and ALDL connector grounded, blow air into tube A . The air should exhaust from tube B .
  8.  

REMOVAL & INSTALLATION



EGR Valve
  1. Allow the engine to cool prior to removing the EGR valve
  2.  
  3. Disconnect the EGR valve vacuum hose.
  4.  
  5. Remove the EGR valve retaining bolts.
  6.  
  7. Remove the EGR valve.
  8.  
  9. When installing the valve use a new gasket.
  10.  
  11. Installation is the reverse of removal.
  12.  

EGR Modulator
  1. Remove the air cleaner assembly.
  2.  
  3. Label and disconnect the modulator vacuum hoses.
  4.  
  5. Remove the modulator.
  6.  
  7. Installation is the reverse of removal.
  8.  

Bi-Metal Vacuum Switching Valve (BVSV)
  1. Drain engine coolant below the level of the valve.
  2.  


CAUTION
When draining the coolant, keep in mind that cats and dogs are attracted by the ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a sealable container. Coolant should be reused unless it is contaminated or several years old.

  1. Label and disconnect the vacuum hoses from the valve.
  2.  
  3. Remove the valve from the intake manifold.
  4.  
  5. Installation is the reverse of removal.
  6.  

Vacuum Switching Valve (VSV)
  1. Disconnect the valve electrical connector.
  2.  
  3. Label and disconnect the vacuum hoses.
  4.  
  5. Remove the fasteners the hold the valve to the firewall.
  6.  
  7. Remove the vacuum switching valve.
  8.  
  9. Installation is the reverse of removal.
  10.  

 
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