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The Role Of PCV Valve

By Fred Gagnon

The blowby vapors that finish up in an engine's crankcase hold moisture and also combustion byproducts and unburned fuel vapors. The crankcase is covered to avoid the get away of these gases to the atmosphere, however the vapors need to be taken out so as to avoid oil contamination that will cause sludge formation. The positive crankcase ventilation (PCV) system siphons these vapors from the crankcase and tracks them into the intake manifold so that they can be reburned in the engine.

The primary component in the PCV system is the PCV valve, which is typically found in the valve cover. A hose attaches the PCV valve to the intake manifold. An additional hose between the air cleaner and crankcase or another valve cover (V6 or V8 applications) gives fresh air to help purge the vapors from the crankcase. Some engines have a split air filter for the PCV breather hose positioned inside the air cleaner.

The PCV valve is a spring-loaded valve with a certain orifice size intended to reduce the amount of air that's siphoned from the crankcase to the intake manifold. This is important because air drawn throughout the valve from the crankcase has a hovering effect on the fuel mixture the same as a vacuum leak. So air flow over the valve should be managed within certain limits. At idle, air flow is lessened because little blowby is made. When the engine is touring and vacuum is extremely high, airflow through the entire PCV valve is at an optimum to free the blowby vapors from the crankcase.

You have to keep in mind that PCV valves are assessed for certain engine applications. An inappropriate PCV valve for an application can pass way too much or too little air leading to drivability issues. Varnish remains can obstruct the valve, so replacement for protective maintenance is advisable (every 50,000 miles usually).

Its not all engines have PCV valves. Some ventilate the crankcase with a minimal breather hose and adjusted hole. There's no spring-loaded PCV valve. On these kinds of applications, no maintenance tasks are usually essential.

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