Mazda 323/MX-3/626/MX-6/Millenia/Protégé 1990-1998 and Ford Probe 1993-1997

Crankcase Ventilation System



See Figure 1

All Mazda engines are equipped with the Positive Crankcase Ventilation (PCV) system. The PCV system vents crankcase gases into the engine air intake where they are burned with the fuel and air mixture. The PCV system keeps pollutants from being released into the atmosphere, and also helps to keep the engine oil clean, by ridding the crankcase of moisture and corrosive fumes. The PCV system consists of the rocker arm cover mounted PCV valve, the nipple in the air intake and the connecting hoses.

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Fig. Fig. 1: Typical PCV system schematic

The PCV valve regulates the amount of ventilating air and blow-by gas to the intake manifold. It also prevents backfire from traveling into the crankcase, avoiding the explosion of crankcase gases.


See Figure 2

  1. Visually inspect the PCV valve hose and the fresh air supply hose and their attaching nipples or grommets for splits, cuts, damage, clogging, or restrictions. Repair or replace, as necessary.
  3. If the hoses pass inspection, remove the PCV valve from the rocker arm cover. Shake the PCV valve and listen or feel for the rattle of the valve plunger within the valve body. If the valve plunger does not rattle, the PCV valve should be cleaned or replaced.

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Fig. Fig. 2: Check the PCV valve for vacuum at idle

  1. Start the engine and bring it to normal operating temperature.
  3. Unplug the PCV valve and check for vacuum at the valve. If no vacuum is felt check the hose for clogging.
  5. Plug the valve back in. Remove the fresh air supply hose from the throttle body air hose nipple, and plug or cap the nipple immediately to keep the engine from stalling. Check for vacuum at the end of the supply hose using a stiff piece of paper. If the paper is retained by vacuum at the end of the hose, the PCV system is okay.
  7. If the paper is not held by vacuum, check the fresh air and PCV hoses for leaks or loose connections. Also check for a loose fitting oil fill cap or loose dipstick. Correct as required until vacuum can be felt at the end of the supply hose.

If air pressure and oil or sludge is present at the end of the fresh air supply hose, the engine has excessive blow-by and cylinder bore or piston ring wear.


See Figures 3 and 4

  1. Remove the PCV valve from the mounting grommet in the valve cover.
  3. Disconnect the valve from the PCV hose and remove the valve from the vehicle.
  5. Installation is the reverse of the removal procedure.