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1978 Models

  1. After inspecting vacuum hoses, start and run cold engine at idle. Ensure that secondary EGR valve does not operate when engine speed is increased to 2500 RPM.
  2. If secondary valve operates, replace thermo valve. When coolant temperature exceeds 131°F (55°C), secondary valve should operate with engine at 2500 RPM. If valve does not operate, replace thermo valve.
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