Mazda Trucks 1987-1993 Repair Guide

Crankcase Ventilation System



See Figure 1

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Fig. Fig. 1: PCV system-3.0L engine

The function of the PCV valve is to divert blow-by gases from the crankcase to the intake manifold to be burned in the cylinders. The system consists of a PCV valve, an oil separator and the hoses necessary to connect the components.

Ventilating air is routed into the rocker cover from the air cleaner. The air is then moved to the oil separator and from the separator to the PCV valve. The PCV valve is operated by differences in air pressure between the intake manifold and the rocker cover.

The most critical component of the system is the PCV valve. This vacuum-controlled valve regulates the amount of gases which are recycled into combustion chamber. At low engine speeds the valve is partially closed, limiting the flow of gases into the intake manifold. As engine speed increases, the valve opens to admit greater quantities of the gasses into the intake manifold. If the valve should become blocked or plugged, the gases will bee prevented from escaping the crankcase by the normal route. Since these gases are under pressure, they will find their own way out of the crankcase. This alternate route is usually a weak oil seal or gasket in the engine. As the gas escapes by the gasket, it also creates an oil leak. Besides causing oil leaks, a clogged PCV valve also allows these gases to remain in the crankcase for an extended period of time, promoting the formation of sludge in the engine.


See Figure 2

  1. Remove the hose from the PCV valve.
  3. Start the engine and run it at approximately 700-1,000 rpm.
  5. Cover the end of the PCV valve with a finger. A distinct vacuum should be felt. If no vacuum is felt, replace the valve.

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


  1. Disconnect the hose from the PCV valve.
  3. Remove the valve from the mounting fitting.
  5. Install the valve in the fitting.
  7. Connect the hose to the valve.