See Figures 1 and 2
Most of the gasoline engines covered in this guide use a PCV valve. The PCV valve meters the flow at a rate depending upon the manifold vacuum. If the manifold vacuum is high, the PCV restricts the flow to the intake manifold. If abnormal, operating conditions occur, excessive amounts of internal exhaust gases back flow through the crankcase vent tube into the air filter to be burned by normal combustion.
Some early carbureted gasoline engines do not use a PCV valve. In place of the valve is a regulating orifice in the intake manifold. Blow-by gases are drawn through the regulating orifice into the intake manifold for reburning. During wide open throttle, the engine vacuum is not sufficient to draw enough vapor through the manifold, allowing part to be drawn into the air cleaner via a hose at the rear end of the rocker arm cover.
The crankcase ventilation system must be operating correctly to provide complete scavenging of the crankcase vapors.Diesel Engines
See Figures 3 and 4
The crankcase ventilation system is a closed type, and is designed to force blow-by gas generated in the engine crankcase back into the intake manifold to return with the fresh air back into the combustion chamber.
When the engine is running at high speed, the high negative pressure from the intake manifold makes the diaphragm valve close, as a result, the blow-by gas passes through the regulating orifice. When the engine is running at low speed, the negative pressure from the intake manifold is so small that the cylinder head cover pressure makes the diaphragm valve open. As a result, the blow-by gas passes through both the regulating orifice and the diaphragm passage opened by the diaphragm valve.
See Figure 5
- While the engine is idling, pull the PCV valve from the valve cover.
- Place your thumb over the end of the PCV valve and check for vacuum.
- If no vacuum exists, check for a plugged PCV valve, manifold port, hoses or deteriorated hoses. Also, check for proper routing. Refer to the vacuum diagrams at the end of this section.
- With the engine off, remove the PCV valve and shake it. Listen for the rattle of the check needle inside the valve. If it does not rattle, replace the valve.
Clean the hoses and calibrating orifice internally. Blow away foreign matter with compressed air. Check the hoses for cracks, fatigue and swelling. Replace if necessary.
REMOVAL & INSTALLATION
- Label the PCV valve or regulating orifice hoses before removing.
- Disconnect the hoses and remove the valve/orifice.
- Connect the hoses to the new valve/orifice and install the valve/orifice.