GM Prizm/Nova 1985-1993 Repair Guide

Exhaust Gas Recirculation (EGR) System

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OPERATION



The EGR system reduces oxides of nitrogen. This is accomplished by recirculating some of the exhaust gases through the EGR valve to the intake manifold, lowering peak combustion temperatures.

Whenever the engine coolant is below 122°F (50°C), the thermostatic vacuum switching valve (TVSV) connects manifold vacuum to the EGR vacuum modulator and at the same time to the EGR valve.

The EGR vacuum modulator controls the EGR valve by modulating the vacuum signal with an atmospheric bleed. This bleed is controlled by the amount of exhaust pressure acting on the bottom of the EGR vacuum modulator (diaphragm).

Since recirculation of exhaust gas is undesirable at low rpm or idle, the system limits itself by sensing the exhaust flow. Under low load conditions, such as low speed driving, the exhaust pressure is low. In this state, the diaphragm in the modulator is pushed down by spring force and the modulator valve opens to allow outside air into the vacuum passage. The vacuum in the line is reduced, the EGR valve does not open as far, and the amount of recirculation is reduced.

Under high load conditions or high rpm driving, the exhaust pressure is increased. This pushes the modulator diaphragm upwards and closes the bleed valve. A full vacuum signal is transmitted to the EGR valve; it opens completely and allows full recirculation. The slight reduction in combustion temperature (and therefore power) is not noticed at highway speeds or under hard acceleration.

Prizm vehicles also control the EGR with a vacuum solenoid valve (VSV), 1989-92 or exhaust gas recirculation solenoid valve (EGR SV), 1993. These devices allow the ECM to further control the EGR under certain conditions. The ECM will electrically close the VSV or EGR SV if the engine is not warmed up, the throttle valve is in the idle position or if the engine is under very hard acceleration. Aside from these conditions, the Prizm EGR operates in accordance with the normal vacuum modulator function.

TESTING AND SERVICING VIN CODE 4 (CARB.) ENGINE



EGR System Operation

See Figures 1 and 2



Click image to see an enlarged view

Fig. Fig. 1: EGR system components - 1985-88 VIN code 4 engine

  1. Check and clean the filter in the EGR vacuum modulator. Use compressed air (if possible) to blow the dirt out of the filters and check the filters for contamination or damage.
  2.  
  3. Using a tee (3-way connector), connect a vacuum gauge to the hose between the EGR valve and the vacuum pipe.
  4.  
  5. Check the seating of the EGR valve by starting the engine and seeing that it runs at a smooth idle. If the valve is not completely closed, the idle will be rough.
  6.  
  7. With the engine coolant temperature below 122°F (50°C), the vacuum gauge should read zero at 2000 rpm. This indicates that the thermostatic vacuum control valve (TVCV) is functioning correctly at this temperature range.
  8.  
  9. Warm the engine to normal operating temperature. Check the vacuum gauge and confirm low vacuum at 2000 rpm. This indicates the TVSV and the EGR vacuum modulator are working correctly in this temperature range.
  10.  



Click image to see an enlarged view

Fig. Fig. 2: EGR vacuum modulator filter

  1. Disconnect the vacuum hose from the R port on the EGR vacuum modulator and, using another piece of hose, connect the R port directly to the intake manifold. Check that the vacuum gauge indicates high vacuum at 2,000 rpm.
  2.  

As a large amount of exhaust gas enters, the engine will misfire slightly at this time.

  1. Disconnect the vacuum gauge and reconnect the vacuum hoses to their proper locations.
  2.  
  3. Check the EGR valve by applying vacuum directly to the valve with the engine at idle. (This may be accomplished by bridging vacuum directly from the intake manifold or by using a hand-held vacuum pump.) The engine should falter and/or stall as the full load of recirculated gasses enters the engine.
  4.  
  5. If no problem is found with this inspection, the system is OK; otherwise inspect each part.
  6.  

Thermostatic Vacuum Switching Valve

See Figure 3

  1. Drain the cooling system.
  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. Remove the thermostatic vacuum switching valve.
  2.  



Click image to see an enlarged view

Fig. Fig. 3: Port identification for testing the TVSV. Make sure you label each hose during disassembly

  1. Cool the thermostatic vacuum switching valve to below 45°F (7°C).
  2.  
  3. Check that air flows from pipe J to pipes M and L , and flows from pipe K to pipe N .
  4.  
  5. Heat the thermostatic vacuum switching valve to 63-122°F (17-50°C) generally room temperature).
  6.  
  7. Check that air flows from pipe K to pipes N and L and flows from pipe J to pipe M .
  8.  
  9. Heat the TVSV to above 154°F (68°C).
  10.  
  11. Check that air flows from the pipe K to pipes M and L , and does NOT flow from pipe J to any other pipes.
  12.  
  13. Apply liquid sealer to the threads of the TVSV and reinstall.
  14.  
  15. Refill the cooling system.
  16.  
  17. If a problem is found with any of the above procedures, replace the valve
  18.  

EGR Valve
  1. Remove the EGR valve.
  2.  
  3. Check the valve for sticking and heavy carbon deposits. If a problem is found, replace the valve.
  4.  
  5. Reinstall the EGR valve with a new gasket.
  6.  

EGR Vacuum Modulator

See Figure 1

  1. Label and disconnect the vacuum hoses from ports P , Q , and R of the EGR vacuum modulator.
  2.  
  3. Plug the Q and R ports with your fingers.
  4.  
  5. Blow air into port P . Check that the air passes freely through the sides of the air filter side.
  6.  
  7. Start the engine and maintain 2,000 rpm.
  8.  
  9. Repeat the test above. Check that there is a strong resistance to air flow.
  10.  
  11. Reconnect the vacuum hoses to the proper locations.
  12.  

Check Valve

See Figure 1

Inspect the check valve (1-way valve) by gently blowing air into each end of the valve or hose. Air should flow from the orange pipe to the black pipe but SHOULD NOT flow from the black pipe to the orange pipe.

TESTING AND SERVICING 1988 & 1990 VIN CODE 5 ENGINES 1989-93 ALL ENGINES



EGR System Check
1988 VIN CODE 5 ENGINE

See Figures 2, 4 and 5

  1. Check and clean the filter in the EGR vacuum modulator. Use compressed air (if possible) to blow the dirt out of the filters and check the filters for contamination or damage.
  2.  
  3. Using a tee (3-way connector), connect a vacuum gauge to the hose between the EGR valve and the vacuum pipe.
  4.  
  5. Check the seating of the EGR valve by starting the engine and seeing that it runs at a smooth idle. If the valve is not completely closed, the idle will be rough.
  6.  
  7. With the engine coolant temperature below 95°F (35°C), the vacuum gauge should read zero at 3500 rpm. This indicates that the bimetal vacuum switching valve (BVSV) is functioning correctly at this temperature range.
  8.  
  9. Warm the engine to normal operating temperature. Check the vacuum gauge and confirm low vacuum at 3500 rpm. This indicates the BVSV and the EGR vacuum modulator are working correctly in this temperature range.
  10.  
  11. Disconnect the vacuum hose from the R port on the EGR vacuum modulator and, using another piece of hose, connect the R port directly to the intake manifold. Check that the vacuum gauge indicates high vacuum at 3500 rpm.
  12.  

As a large amount of exhaust gas enters, the engine will misfire slightly at this time.

  1. Disconnect the vacuum gauge and reconnect the vacuum hoses to their proper locations.
  2.  
  3. Check the EGR valve by applying vacuum directly to the valve with the engine at idle. (This may be accomplished by bridging vacuum directly from the intake manifold or by using a hand-held vacuum pump.) The engine should falter and/or stall as the full load of recirculated gasses enters the engine.
  4.  
  5. If no problem is found with this inspection, the system is OK; otherwise inspect each part.
  6.  



Click image to see an enlarged view

Fig. Fig. 4: EGR system inspection, preparation - 1988 VIN code 5 engine



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Fig. Fig. 5: R port connected to the intake manifold - 1988 VIN code 5 engine

EGR Valve
1988 VIN CODE 5 ENGINE
  1. Remove the EGR valve.
  2.  
  3. Check the valve for sticking and heavy carbon deposits. If a problem is found, replace the valve.
  4.  
  5. Reinstall the EGR valve with a new gasket.
  6.  

1989-93 ALL ENGINES
  1. Start the engine and allow it to warm up completely. The coolant temperature must be above 120°F (49°C). The following tests are performed with the engine running.
  2.  
  3. Place a finger on the EGR valve diaphragm. Accelerate the engine slightly; the diaphragm should be felt to move.
  4.  
  5. Disconnect a vacuum hose from the EGR valve and connect a hand held vacuum pump.
  6.  
  7. Apply 10 inches of vacuum to the valve. The diaphragm should move (check again with your finger) and the engine may momentarily run rough or stall.
  8.  
  9. An EGR valve failing either of these quick tests should be replaced. The valves cannot be cleaned or adjusted.
  10.  

EGR VACUUM MODULATOR
1988 VIN CODE 5 ENGINE

See Figure 6

  1. Label and disconnect the vacuum hoses from ports P , Q , and R of the EGR vacuum modulator.
  2.  
  3. Plug the P and R ports with your fingers.
  4.  
  5. Blow air into port Q . Check that the air passes freely through the sides of the air filter.
  6.  
  7. Start the engine and maintain 3500 rpm.
  8.  
  9. Repeat the test above. Check that there is a strong resistance to air flow.
  10.  
  11. Reconnect the vacuum hoses to the proper locations.
  12.  



Click image to see an enlarged view

Fig. Fig. 6: EGR vacuum modulator test - 1988 VIN code 5 engine

1989-93 ALL ENGINES

See Figures 7 and 8

  1. Label and remove the 3 hoses from the modulator.
  2.  
  3. Place your fingers over ports P and R ; blow into port Q . Air should flow freely from the sides of the air filter on the modulator.
  4.  



Click image to see an enlarged view

Fig. Fig. 7: EGR vacuum modulator test - 1989-93



Click image to see an enlarged view

Fig. Fig. 8: EGR vacuum modulator and EGR solenoid valve location - 1993

  1. Connect a vacuum pump to port S (on the bottom of the unit) and plug tubes P and R with your fingers. Blow air into tube Q and attempt to draw a vacuum with the pump. You SHOULD NOT be able to develop a vacuum within the system.
  2.  
  3. If the modulator fails any of these tests, it must be replaced.
  4.  

Vacuum Switching Valve (VSV)
1988 VIN CODE 5 ENGINE

See Figures 9 and 10

  1. The vacuum switching valve is located on the left strut tower. The vacuum switching circuit is checked by blowing air into the pipe under the following conditions:
    1. Connect the vacuum switching valve terminals to the battery.
    2.  
    3. Blow into the tube and check that the VSV switch is open.
    4.  
    5. Disconnect the positive battery terminal.
    6.  
    7. Blow into the tube and check that the VSV switch is closed (no flow).
    8.  

  2.  



Click image to see an enlarged view

Fig. Fig. 9: Vacuum switching valve battery connected - 1988 VIN code 5 engine

  1. Check for a short circuit within the valve. Using an ohmmeter, check that there is no continuity between the positive terminal and the VSV body. If there is continuity, replace the VSV.
  2.  
  3. Check for an open circuit. Using an ohmmeter, measure the resistance (-) between the 2 terminals of the valve. The resistance should be 33-39- at 68°F (20°C). If the resistance is not within specifications, replace the VSV.
  4.  

The resistance - ohmmage - will vary slightly with temperature. It will decrease in cooler temperatures and increase with heat. Use common sense; slight variations due to temperature range are not necessarily a sign of a failed valve.



Click image to see an enlarged view

Fig. Fig. 10: Vacuum switching valve short circuit check - 1988 VIN code 5 engine

1989-92 ALL ENGINES

See Figure 11

  1. Label and disconnect the 2 hoses from the VSV.
  2.  
  3. With the ignition OFF, disconnect the connector at the VSV.
  4.  
  5. Check the resistance between the 2 terminals on the VSV. Look for 33-39- resistance. If the resistance is incorrect, replace the unit. If the resistance is proper, proceed with the next step.
  6.  



Click image to see an enlarged view

Fig. Fig. 11: Vacuum switching valve - 1989-93 VIN code 5 engine

  1. Gently blow air into port A . Air should come out through the filter but SHOULD NOT come out through port B .
  2.  
  3. Reconnect the electrical connector.
  4.  
  5. Turn the ignition switch ON (but don't start the motor) and ground the Diagnosis Switch Terminal. This is found in the diagnostic connector near the air cleaner assembly and is labeled as terminal T . Use a jumper wire with clips to ground the terminal.
  6.  


WARNING
Be careful not to ground other terminals in the connector (even accidentally) - severe electrical damage may result!

  1. Blow air into port A ; the air should exit through port B .
  2.  
  3. If the VSV fails either of the air flow tests, it should be replaced.
  4.  

EGR Solenoid Vacuum Valve (EGR SV)
1993 ALL ENGINES

See Figure 8

  1. Label and disconnect the 2 hoses from the EGR SV.
  2.  
  3. With the ignition OFF, disconnect the connector at the EGR SV.
  4.  
  5. Check the resistance between the 2 terminals on the EGR SV. Look for 33-39- resistance. If the resistance is incorrect, replace the unit. If the resistance is proper, proceed with the next step.
  6.  
  7. Gently blow air into port A . Air should come out through the filter but SHOULD NOT come out through port B .
  8.  
  9. Reconnect the electrical connector.
  10.  
  11. Turn the ignition switch ON (but don't start the motor).
  12.  
  13. Blow air into port A ; the air should exit through port B .
  14.  
  15. If the EGR SV fails either of the air flow tests, it should be replaced.
  16.  

Bi-metal Vacuum Switching Valve (BVSV)
1988 VIN CODE 5 ENGINE 1989-90 VIN CODE 6 ENGINES

See Figure 12

This valve does nothing more than allow vacuum to flow through the system depending on engine coolant temperature. The bimetallic element within the switch reacts to temperature changes, opening or closing the valve at a pre-determined level. To test the valve:

  1. Drain the coolant from the radiator into a suitable container.
  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 hoses from the BVSV.
  2.  
  3. Remove the valve from the intake manifold.
  4.  
  5. Using cool water, cool the threaded part of the valve to below 95°F (35°C). Blow air into the upper (center) port; there should be NO air passage through the valve. It does not allow vacuum to pass until the engine warms up.
  6.  



Click image to see an enlarged view

Fig. Fig. 12: Testing the BVSV for correct function

  1. Using warm water, heat the threaded part of the valve to above 122°F (50°C) and blow into the port again. The valve should allow the air to pass through.
  2.  
  3. If a problem is found with either the ON or OFF functions of the valve, replace it with a new one.
  4.  
  5. Using an ohmmeter, measure the resistance between the terminal on the VSV. Resistance should be 33-39- at 68°F (20°C).
  6.  
  7. Apply liquid sealer to the threads of the BVSV and reinstall it. Connect the vacuum lines.
  8.  
  9. Refill the radiator with coolant.
  10.  

COMPONENT REMOVAL & INSTALLATION



Exhaust emission control equipment is generally simple to work on and easy to get to on the motor. The air cleaner assembly will need to be removed. Always label each vacuum hose before removing it - they must be replaced in the correct position.

Most of the valves and solenoids are made of plastic, particularly at the vacuum ports. Be very careful during removal not to break or crack the ports; you have almost no chance of regluing a broken fitting. Remember that the plastic has been in a hostile environment (heat and vibration); the fittings become brittle and less resistant to abuse or accidental impact.

EGR valves are generally held in place by 2 bolts. The bolts can be difficult to remove due to corrosion. Once the EGR is off the engine, clean the bolts and the bolt holes of any rust or debris. Always replace the gasket any time the EGR valve is removed.

 
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