Honda Accord/Civic/Prelude 1973-1983 Repair Guide

Crankcase Emission Control System



See Figure 1

The Honda's engine is equipped with a Dual Return System to prevent crankcase vapor emissions. Blow-by gas is returned to the combustion chamber through the intake manifold and carburetor air cleaner. When the throttle is partially opened, blow-by gas is returned to the intake manifold through breather tubes leading into the tee orifice located on the outside of the intake manifold. When the throttle is opened wide and vacuum in the air cleaner rises, blow-by gas is returned to the intake manifold through an additional passage in the air cleaner case.

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Fig. Fig. 1: Crankcase ventilation system operation


  1. Squeeze the lower end of the drain tube and drain any oil or water which may have collected.
  3. Make sure that the intake manifold T-joint is clear by passing the shank end of a No. 65 (0.035 inch diameter) drill through both ends (orifices) of the joint.
  5. Check for any loose, disconnected or deteriorated tubes and replace (if necessary).