Positive Crankcase Ventilation Valve Problems

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Positive Crankcase ventilation valve problems

Positive Crankcase ventilation valve problems, Incorrect PCV valve, Rough Idle, Surging engine, Stalling engine, Sticking throttle plate, Excessive oil consumption, PCV System Functional Tests


The positive crankcase ventilation system

A positive crankcase ventilation system (PCV) is yesterday’s technology with a new set of challenges. While most consider the PCV system a simple and uneventful part of the emission control system, it can have a detrimental effect on the overall engine performance.A component failure can result in an array of problems, including rough idle, surging, stalling, a sticking throttle plate, or excessive oil consumption.

The internal combustion engine blow-by vapors

The blowby vapors that end up in the engine crankcase contain moisture as well as combustion concomitants and unburned fuel vapors. The crankcase is sealed to prevent the escape of these gases into the atmosphere, but the vapors must be removed to prevent oil contamination that leads to sludge formation. The positive crankcase ventilation (PCV) system siphons these vapors from the crankcase and routes them into the intake manifold for the gases to burn off in the engine for a second time.

The main component in the PCV

The main component in the PCV system is the PCV valve, which is usually located in the valve cover. A hose connects the PCV valve to the intake manifold. A second hose between the air cleaner and crankcase or another valve cover (V6 or V8 applications) provides fresh air to help flush the vapors out of the crankcase. Some engines have a separate air filter for the PCV breather hose located inside the air cleaner.

PCV valve is a spring-loaded valve

The PCV valve is a spring-loaded valve with a specific orifice size designed to restrict the amount of air that’s siphoned from the crankcase into the intake manifold. This is necessary because air drawn through the valve from the crankcase has a lean effect on the fuel mixture much the same as a vacuum leak. So the airflow is controlled through the valve within certain limits. At idle, airflow is reduced because little blowby is produced. When the engine is cruising, and vacuum is high, the airflow through the PCV valve is at a maximum to purge the blowby vapors from the crankcase.

PCV valve for an engine application

It is important to note that the PCV valves are sized for specific engine applications. The wrong PCV valve for an engine application can allow high-vacuum  flow or too little air causing drivability problems. Varnish deposits can clog the valve, so replacement for preventive maintenance is recommended (every 50,000 miles usually). Eventually, the PCV valve can be gummed up. Then it cannot move enough air through the engine to keep it working efficiently. If the PCV valve is sticking, you could have oil leaks, excess oil consumption, and a fouled intake system. If you experience hesitation, surging, or an oil leak, it may be a sign of PCV value problems.

PCV Valve System Operation

PCV valve  uses manifold vacuum to draw crankcase vapors back into the intake manifold. Typically, blow by production is the greatest during high load operations and very low blow by at idle on light load operations. Since the characteristics of manifold vacuum do not match the flow, requirements needed for proper crankcase Ventilation; a PCV valve is used to regulate the blow by flow back into the intake manifold.

• During idle and deceleration, blow by production is very low, but intake manifold vacuum is very high. This causes the pintle inside the PCV valve to retract fully against spring tension. The positioning of the pintle provides a small vacuum passage and allows for a low blow by flow to the combustion chamber.

  • During low engine load cruising, the pintle inside the PCV valve is positioned somewhatin the center of its travel. This positioning allows a moderate volume of the blowby flow into the combustion chamber.
  • During acceleration and heavy load operations, blowby production isvery high. The pintle extends out further from the restriction allowing the maximum flow of blowby into the combustion chamber. During extremely high engine loads, if the blowby volume exceeds the ability of the PCV valve to draw in the vapors, the excess blowby flows through the breather hose to the air cleaner housing where it can enter the combustion chamber.
  • When the engine is off or it backfires, spring tension closes the valve completely preventing the release of blowby into the intake manifold. The valve closes during a backfire to prevent the flame from traveling into the crankcase where it could ignite the enclosed fuel vapors.

PCV System Effects on Emissions and Driveability Because PCV Valve operation is factored into the proper operation of the feedback control system, problems with the PCV system may disrupt the normal air/fuel ratio balance. A clogged PCV valve will prevent the regular flow of crankcase vapors into the engine and can result in a richer than normal air/fuel mixture. A plugged crankcase breather hose may cause the engine to consume oil because of the increased level of crankcase vacuum.

In addition, depending on the location of the fresh air breather hose, a nonfunctional valve or restricted vacuum hose can cause oil contamination in the air cleaner housing or throttle bore coking. Always suspect and check the PCV system if you find traces of oil in the air intake system.

If the crankcase becomes diluted with fuel, carbon monoxide (CO) levels will likely increase because the PCV system will meter extra fuel vapor into the intake system. Always replace fuel diluted engine oil and identify and resolve the problem causing the fuel contaminated.

Although there are no mandatory maintenance intervals for the PCV system, periodically check the system for a plugged or gummed PCV valve and damaged hoses. Replace suspect components as necessary. Since PCV flow rates differ between vehicle models, it is important to use the correct replacement PCV valve to ensure proper operation. The installation of an incorrect The valve may cause the engine to stall, rough idle and other driveability complaints. Thus, never install universal type PCV valves.

PCV System Functional Tests

The following RPM Drop Test may be used as a basic quick check to confirm that the PCV system is functioning:

Start the engine and allow it to reach operating temperature.

  • Allow the engine to stabilize at idle
  • Pinch or block the hose between the PCV valve and vacuum source
  • Typically, engine RPM should drop around 50 RPM if engine rpm does not change, check the PCV valve and system hoses for blockage. Replace components as necessary and then retest the system.

Read More About PCV Valve Related Symptoms and Problems

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