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Home >> Mercedes Benz >> 1982 >> 300CD >> Repair and Diagnosis >> Engine Performance >> Engine Control Systems >> Exhaust Gas RECIRCULATION >> Testing >> 5-Cylinder Engines >> System Check
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System Check

  1. Connect vacuum gauge between thermal vacuum valve and EGR valve using "T". With engine idling and throttle against stop, no vacuum should be present. Open throttle until all slack in free travel rod is taken up. Vacuum level should be 5-7 in. Hg. If system operates correctly, test EGR valve. If not, proceed with system check, with engine at normal operating temperature.
  2. Check all vacuum lines for tight connections. Blow through lines to be sure none are plugged. Clean vacuum source fitting on pump.
  3. Disconnect vacuum line at EGR valve, then pull off White/Purple/Brown hose at thermal vacuum valve. Blow through hoses. If no air passes, replace thermal vacuum valve.
  4. Disconnect vacuum hose plug at switch-over valve plate. Connect a jumper hose from vacuum source point to point 1 at switch-over valve plug. Plug point 2 and connect a vacuum gauge to point 3.
  5. With engine idling, no vacuum should be present. Open throttle until slack is taken up in free travel rod. About 10-12 in. Hg should be present. Return engine to idle speed, then watch vacuum gauge. Vacuum should remain constant for at least 2 minutes.
  6. Pull plug from point 2. Vacuum should drop to zero. If valve does not operate properly, replace it. Connect vacuum source jumper hose to point 2 and plug point 1. Leave vacuum gauge connected to point 3.
  7. With engine idling, about 10-12 in. Hg vacuum should be present. Clamp vacuum source jumper hose tightly and stop engine. Vacuum should remain constant for at least 2 minutes. Move throttle to full throttle stop, pull off hose at point 1, and observe vacuum gauge.
  8. Vacuum should remain constant. Return throttle to idle and pull off jumper hose. Vacuum should drop to zero. If switch does not operate properly, replace it. If switch is okay, check EGR valve.
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