GM Storm/Spectrum 1985-1993 Repair Guide

Exhaust Gas Recirculation System

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OPERATION



See Figures 1 and 2

The Exhaust Gas Recirculation (EGR) valve is used to allow a controlled amount of exhaust gas to be recirculated into the intake system. This limits peak flame temperature in the combustion chamber so the engine produces less NOx (oxides of nitrogen). On all vehicles, the amount of gas recirculated is controlled by a combination of exhaust gas backpressure and the vacuum applied to the EGR valve.

The same basic EGR valve is used on all systems, with minor housing modifications made for mounting on the various engines and for the various control systems. The valve contains a diaphragm which is normally held closed by spring pressure. When vacuum is applied to the valve port, the diaphragm will be raised and held against the spring pressure, allowing exhaust gas to recirculate. The greater amount of vacuum applied, the more the diaphragm will open allowing a greater amount of exhaust gas to be recirculated to the intake manifold.

For carbureted vehicles, vacuum is controlled by a Thermal Vacuum Valve (TVV) which opens once the engine coolant temperature has reached a certain point. Engine vacuum takes over from this point to open and close the EGR valve, as necessary for proper engine operation.

For fuel injected vehicles, an ECM controlled Vacuum Switching Valve (VSV) or Electronic Vacuum Regulating Valve (EVRV) solenoid is used to provide vacuum to the EGR valve. The ECM controlled valve allows for vacuum regulation based on additional factors other than vacuum and backpressure, such as throttle position. For 1990-91 vehicles, the vacuum is applied directly to the EGR valve, but is then throttled or modified by a exhaust backpressure modulator attached to the EGR valve through a second vacuum port. For 1992-93 vehicles, the backpressure modulator (or transducer) is located inline between the solenoid valve and the EGR valve. On this second group of vehicles, the backpressure modulator still works on the same principles to modify the amount of vacuum applied to the EGR valve, but it does so before vacuum is ever allowed to reach the EGR valve.

The modulator or transducer controls vacuum (thereby controlling the amount of exhaust gas which is recirculated) based on exhaust backpressure. Exhaust backpressure pushes against the valve and keeps the diaphragm pushed against a bleed hole. When the rpm is high but the throttle is closed, exhaust backpressure becomes negative and the diaphragm is pulled down just enough to uncover the bleed hole. The vacuum on top of the diaphragm leaks off and the EGR valve slowly closes.

For ECM controlled EGR systems (fuel injected, not carbureted) the solenoid valve will close, preventing vacuum and thereby preventing exhaust gas recirculation, under any one of the following conditions:

  1. Engine coolant temperature below a specified point (during engine warm up)
  2.  
  3. When the throttle valve is at the idle position.
  4.  
  5. When the engine is running under a low load.
  6.  
  7. When intake manifold pressure is low.
  8.  



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Fig. Fig. 1: EGR system schematic - Spectrum



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Fig. Fig. 2: EGR system control schematic - 1992-93 Storm

SERVICE



Carbureted Vehicles
CHECKING THE EGR SYSTEM

See Figures 3 and 4

  1. Locate and disconnect the 3-way vacuum hose connector to the charcoal canister, between the EGR valve and the TVV. Install a vacuum gauge between the EGR valve and the TVV.
  2.  

Before disconnecting ANY vacuum hose, always note and/or tag the position of each hose to be removed. Proper hoses routing is essential to proper system and engine operation. Although it may seem obvious where each hose is connected, before removal, vacuum hoses begin to look strangely similar come installation time.

  1. With the engine cold, turn the ignition ON and start the engine. Make sure the vacuum gauge indicates 0 kPa at any speed, while the engine is still cold.
  2.  
  3. When the engine reaches normal operating temperature, check that the vacuum gauge indicates 0 kPa at idle and 6.7 kPa or more at 3500 rpm.
  4.  
  5. With the engine running at 3500 rpm, disconnect and plug the vacuum hose to the EGR valve. Engine speed should increase slightly.
  6.  
  7. Stop the engine, then push the EGR diaphragm by hand and release it. Make sure the diaphragm plate returns smoothly to the original position.
  8.  
  9. If a problem is found, begin by inspecting the EGR valve. Connect a hand vacuum pump to the valve hose and apply 24 kPa of vacuum. The diaphragm should move fully upward and hold without leaking down.
  10.  
  11. If necessary, remove the EGR valve and check for excessive build-up of deposits which might prevent it from opening or operating properly. Clean any deposits.
  12.  

The EGR valve must be replaced if cracks in the diaphragm prevent it from holding vacuum or if deformation of the valve seat cause the valve to stick open.

  1. If the EGR valve is good, and a problem is still suspected, check the TVV valve. Remove the valve from the engine, install 2 short lengths of vacuum hose (through which to blow air and check for passage) and heat the sensing portion using hot water. Be very careful when working around the hot water, most oven mits will NOT protect you if they become soaked with boiling water.
  2.  
  3. When the sensor reaches 130°F (54°C) blow air into 1 of the hoses and check for air passage through the other hose. If the TVV does not open, it is defective and must be replaced.
  4.  
  5. Cool the sensor to 95°F (35°C) using cool water, then blow air into 1 of the hoses to make sure that air passage is now blocked. If the TVV does not close properly, it is defective and must be replaced.
  6.  
  7. Install the TVV and/or the EGR valve, then reconnect all of the vacuum hoses.
  8.  



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Fig. Fig. 3: Install a vacuum gauge between the TVV and EGR valves in order to check system operation - Carbureted vehicles



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Fig. Fig. 4: Heat or cool the TVV valve in order to check for proper operation

Fuel Injected Vehicles
CHECKING THE EGR VALVE
  1. Start and run the engine until normal operating temperature of approximately 120°F (60°C) is reached.
  2.  
  3. Observe the EGR valve diaphragm and operate the throttle lever to accelerate the engine. The diaphragm should move.
  4.  
  5. Disconnect the hose from the EGR valve and install a hand vacuum pump. Apply 10 in. of vacuum while watching the valve. The valve should move and the engine may stall.
  6.  
  7. If the valve passes the test, but a problem is still suspected. Check the other system components and refer to the diagnostic charts at the end of the section.
  8.  

CHECKING THE EGR MODULATOR/TRANSDUCER

See Figure 5

  1. Trace the vacuum hoses from the EGR valve and locate the modulator/transducer.
  2.  
  3. Tag and disconnect the 3 vacuum hoses from the modulator/transducer.
  4.  
  5. For 1990-91 vehicles:
    1. Place a finger over the tubes marked P and R and blow air into the tube marked Q. Air should pass through the filter portion of the modulator.
    2.  
    3. Connect a vacuum pump to the tube marked S and plug the tubes marked P and R using your finger.
    4.  
    5. You should NOT be able to obtain vacuum on the pump.
    6.  

  6.  
  7. For 1992-93 vehicles:
    1. Place a finger over the tube marked VSV and blow air into the tube marked EGRV. Air should pass through the filter portion of the transducer.
    2.  
    3. Connect a vacuum pump to the tube marked VSV and plug the tube marked EGRV using your finger.
    4.  
    5. You SHOULD be able to obtain vacuum on the pump.
    6.  

  8.  
  9. If the modulator/transducer does not perform as specified, it is defective and must be replaced.
  10.  



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Fig. Fig. 5: Checking the EGR modulator/transducer - Fuel injected vehicles

CHECKING THE EGR VSV/EVRV SOLENOID

See Figure 6

  1. Tag and disconnect the 2 vacuum hoses from the solenoid
  2.  
  3. Make sure the ignition is OFF , then unplug the electrical connector from the solenoid.
  4.  
  5. Use an ohmmeter, check resistance across the 2 terminals. Resistance must be between 33-39 ohms, or the solenoid must be replaced.
  6.  
  7. Blow air into tube A (closest to the electrical terminal); air should exit only through the filter and NOT through tube B.
  8.  
  9. Install the electrical connector.
  10.  
  11. Turn the ignition ON and ground the diagnostic terminal of the check connector. Refer to Self-Diagnostics, later in this section.
  12.  
  13. Blow air into tube A (closest to the electrical terminal); air should now exit through tube B.
  14.  
  15. The solenoid must be replaced if it fails to perform as specified.
  16.  



Click image to see an enlarged view

Fig. Fig. 6: Checking the VSV/EVRV solenoid - Fuel injected vehicles

REMOVAL & INSTALLATION



EGR Valve
  1. Remove the air cleaner assembly for access.
  2.  
  3. Tag and disconnect the vacuum hoses from the EGR valve.
  4.  
  5. Remove the valve retaining bolts.
  6.  
  7. Remove the EGR valve and gasket from the mounting surface. Discard the old gasket.
  8.  
  9. To install, reverse the removal procedure and always use a new gasket.
  10.  

EGR Modulator/Transducer
  1. Remove the air cleaner assembly for access.
  2.  
  3. Tag and disconnect the vacuum hoses from the modulator/transducer.
  4.  
  5. Remove the modulator/transducer from the vehicle.
  6.  
  7. Installation is the reverse of removal.
  8.  

EGR VSV/EVRV
  1. Disconnect the negative battery cable.
  2.  
  3. Unplug the electrical connector from the solenoid.
  4.  
  5. Tag and disconnect the solenoid vacuum hoses.
  6.  
  7. Remove the 2 fasteners securing the solenoid to the intake manifold.
  8.  
  9. Remove the solenoid from the engine.
  10.  
  11. Installation is the reverse of removal.
  12.  

 
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