Positive Crankcase Ventilation: Description and Operation

Fig. 28 PCV system (typical):

Fig. 29 PCV valve cross sectional view:

POSITIVE CRANKCASE VENTILATION SYSTEM

Description
This system, Fig. 28, prevents crankcase vapors from entering the atmosphere by scavenging the blow-by gases in the crankcase and routing them into the intake manifold where they are burned along with the normal air fuel mixture. In addition to controlling the emission of crankcase vapors into the atmosphere, this system continuously ventilates the crankcase with fresh air which aids in the prevention of sludge formation.

Manifold vacuum controls the airflow through the PCV system. When manifold vacuum is relatively high, such as at idle or at cruising speed, fresh air is drawn through the air inlet filter into the crankcase. After circulating through the crankcase, the vapor-laden air is drawn through the PCV valve, Fig. 29, and into the intake manifold. The vapor mixes with the air fuel mixture and is burned in the combustion chambers. The PCV valve is calibrated to control airflow at a rate acceptable to the intake system.

If crankcase vapor pressure (blow-by) exceeds the flow capacity of the PCV valve, airflow in the system reverses. Crankcase vapor is then drawn through the air cleaner element and carburetor and burned along with the air fuel mixture.

Service
The PCV valve and filter should be replaced every 30,000 miles.

DIAGNOSIS

System Quick Check
A quick check of the system can be made by pulling end of valve out of valve cover and, then with engine idling, placing a finger over end of valve to block air flow. A strong vacuum should be felt and engine speed should drop if the system is performing satisfactorily. If there is no change in engine speed, a clogged system is indicated. To isolate problem, remove PCV valve from hose. If hoses and passages are clear, a strong vacuum will be felt with the engine idling. Engine idle will change drastically and the engine may stall when the hose is uncovered. If this occurs, the trouble is in the valve. If the engine continues to idle approximately as it did before the hose was uncovered, the hoses or passages are blocked.

PCV Valve Quick Check
1. Replace existing PCV valve with a known good one.
2. Start engine and compare existing engine idle quality with prior idle quality.
3. If loping or rough idle condition remains when the PCV valve is replaced, the PCV system is not at fault. Further engine component diagnosis must be made to determine cause of malfunction.
4. If idle quality improves, replace PCV valve and clean hoses and fittings.

Fig. 30 PCV valve test:

Fig. 31 PCV flow rate chart:

PCV Valve Test
PCV valve tester J-23111, Fig. 30, can be used to test valve and system for proper operation. To use tester, proceed as follows:

1. Remove PCV valve from its mounting grommet.
2. Connect vacuum gauge to read intake manifold vacuum. The PCV valve must be in a horizontal position and lightly tapped during testing.
3. Start engine and allow to idle. Compare vacuum and tester readings to flow chart, Fig. 31, and record readings.
4. Stop engine, then disconnect coil secondary wire from distributor cap and ground it to engine.
5. Crank engine while comparing vacuum and tester readings with chart, Fig. 31. Throttle must be at curb idle position during cranking speed test.
6. If valve flow is above or below the chart specifications, clean or replace valve.