DESCRIPTION
The positive crankcase ventilation system prevents unburned gases (hydrocarbons) from entering the atmosphere by recycling them back into the combustion chamber. This is accomplished by drawing vapors from the crankcase through a Positive Crankcase Ventilation (PCV) valve and routing them into the intake manifold.
From the intake manifold they are drawn into the combustion chamber as the intake valve opens and the intake stroke of the piston occurs. The unburned gases are then burned during the combustion process. (Scheme 30)
Scheme 30
OPERATION
The crankcase ventilation system operates on manifold vacuum. Air is drawn from the crankcase, through the PCV valve and hose into the intake manifold becoming part of the calibrated air/fuel mixture. When the engine is not running, the PCV valve is held closed by spring pressure. This prevents hydrocarbon fumes from collecting in the intake manifold.
During engine operation, manifold vacuum pulls the valve open, against spring pressure, permitting crankcase fumes to enter the throttle body passage. Should the engine backfire, the PCV valve will close to prevent ignition of fumes in the crankcase.
Scheme 31
TESTING
- With engine running at curb idle, remove PCV valve from grommet. If valve is functioning properly, a hissing sound will be heard from the valve as air passes through it.
- With engine running, place finger over valve inlet. Strong vacuum should be felt over valve inlet. Stop engine. Remove and shake PCV valve to ensure a metallic clicking noise can be heard, indicating valve is free. Reinstall PCV valve.
- Remove the crankcase ventilation filter or oil filler cap from the valve cover. Hold a piece of stiff paper over the opening. After allowing about one minute for crankcase pressure stabilization, the paper should be drawn against the opening.
- If paper is held against opening, performance is okay. If not, replace PCV valve and retest. If performance does not improve, inspect system for restrictions and clean as necessary.
PCV VALVE
Engine idle and fuel economy may be adversely affected by a clogged PCV valve or ventilator system. Never adjust fuel system without first checking PCV valve and system performance. If PCV valve is clogged or replacement interval is near, replace PCV valve and inspect entire system for carbon buildup. See REPLACEMENT INTERVAL table in appropriate article in the TUNE-UP section.
CRANKCASE VENTILATION FILTER
This filter should be cleaned or replaced every 60,000 miles. Clean air cleaner assembly internally and check air inlet hoses for carbon buildup. Ensure filter fits tightly in grommet or air cleaner assembly.
THROTTLE BODY PASSAGE
Remove throttle body to service PCV vacuum passage. Turn a 1/4" drill bit by hand through the passage to dislodge any solid particles or carbon buildup. Complete the cleaning process by removing residues with an appropriate fuel system cleaner. It is not necessary to disassemble the throttle body for this service.
Note. It is recommended that the crankcase ventilation system be inspected more frequently if vehicle is operated under severe conditions such as extreme dust, prolonged idling, trailer hauling or short trips in cold weather.
