January '19

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50 THE SHOP JANUARY 2019 W hen discussing how to tune a vin- tage carburetor-equipped engine to run its best with today's modern unleaded gasoline, it pays not to over- look the crankcase ventilation system. The PCV (positive crankcase ventila- tion) valve is basically a controlled vacuum leak that can affect how the engine runs. In the early 1960s, it was determined that the vapors that came out of an engine's crankcase through the road- draft tube were a source of air pollu- tion. The PCV valve system was first introduced on some production vehicles in 1961, and by 1964 PCV systems were standard equipment on most vehicles sold in the United States. The PCV system removes the vapors from the crankcase via the PCV valve and uses the engine to burn these hydro- carbon vapors during the combustion process, thus reducing the air pollution emissions that were previously exiting the engine through the road draft tube. There was also the side benefit that a vehicle equipped with a PCV system doesn't emit the oil vapor smell that a road draft tube-equipped vehicle does. When a PCV system is working prop- erly you will not even notice it; but, when it is not working properly, it can cause the engine to have idle problems, oil consumption problems, oil contami- nation issues or oil leaks. HOW A PCV VALVE WORKS The PCV valve is connected to engine vacuum through a port on the intake manifold or carburetor so it can draw the gases that are in the crankcase out through the PCV valve to be burned by the engine. A PCV valve is a directional valve that will allow the engine's vacuum to pull the vapors out of the crankcase, but in the event of an engine backfire it will keep the backfire flame front from reaching the crankcase, which could cause a crankcase explosion. In addition to the PCV valve, the engine must have a source of clean air to replace the volume of vapors that the PCV valve is removing from the engine. On a V-8, the typical fresh air source is the opposite valve cover from the PCV valve. The airflow that is created by the PCV valve circulates through the engine, clearing away substantial amounts of moisture and harmful acidic blow-by vapors that the engine produces during normal operation. The PCV system also requires some type of separator that allows the harmful vapors to be removed from the crankcase without sucking out any engine oil. HOW A PCV VALVE VARIES FLOW RATE The engine rpm, vacuum and load all will cause the engine to create various amounts of blow-by gases in the crank- case. A PCV valve is designed to vary its flow based on the vacuum level of the engine. Typically, there are two flow rate modes for a PCV valve. The first mode flows a low amount of crankcase vapors at idle when vacuum is highest; thus, the PCV is in idle/low-load cruise mode. The second mode flows a larger amount when the engine is under a load (highway speed/lower-vacuum cruise mode) and the vacuum level is lower. When the PCV valve is functioning properly, it will help keep the oil clean, reduce oil leaks and allow the engine to Positive Reinforcement Proper PCV function is vital to carburetor-equipped engine performance. By Henry P. Olsen 50 THE SHOP JANUARY 2019 This illustration shows how engines use the PCV valve to remove crankcase vapors so they can burn them during the combustion process. Air Filter Air Input Carburetor or throttle body PCV Valve Crankcase vapors and gases Closed oil filler cap Intake manifold vaccuum

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