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Positive BACKPRESSURE EGR Valve

  1. Place transmission in Park or Neutral. Set parking brake and block drive wheels. Connect tachometer. With engine running at normal operating temperature and fast idle speed set to specification, run engine at 2000 RPM.
  2. On carbureted engines, place fast idle cam on high step. Disconnect vacuum hose from EGR valve and plug hose. EGR valve diaphragm should move down and engine RPM should increase.
    NOTE: On some engines with ECM-controlled EGR control solenoid, EGR vacuum is locked out in Park/Neutral and EGR control solenoid must be bypassed.
  3. Reconnect vacuum hose. Diaphragm should move up and engine RPM should decrease. A slight vibration of diaphragm plate may be noticed in backpressure EGR valves.
  4. If engine RPM changes and EGR diaphragm moves, EGR valve is okay. If engine RPM did not change and diaphragm did not move, remove EGR valve and apply 10 in. Hg to EGR vacuum signal tube. EGR valve should not open.
  5. If EGR valve opens, replace EGR valve. With vacuum still applied, direct a stream of air (15 psi maximum) into valve seat. EGR valve should open completely.
  6. If air is not available, connect a section of hose over EGR valve seat. Connect a vacuum pump to signal tube. With thumb plugging intake port of EGR valve, operate vacuum pump while alternately blowing and pausing.
  7. With vacuum present at signal tube, EGR valve should open while pressure is applied and should close when no vacuum is present. If further testing is required, see appropriate COMPONENT TESTING article in the EMISSION section.
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