• The PCV (positive crankcase ventilation) valve allows blowby from the engine head to be recirculated back into the intake manifold and burned along with fresh intake air. Since most engine/crank blowby is full of heavy oil and carbon smoke, it actually may slightly decrease the overall efficiency of combustion in the cylinders when compared with pure clean intake air. When you remove the PCV hose you are allowing fresh unfiltered and relatively cool air into the intake manifold, which is not wise - air filters are there for a reason. PCV is designed to decrease hydrocarbon emissions so you are supposed to keep the system hooked up at all times.
  • Your vehicle may be running rich (too much fuel). Disconnecting the pcv is allowing unmetered (vacuum leak) air into the engine, leaning out the air to fuel ratio. Also, on fuel injected vehicles, the computer will notice the extra air entering the engine by the oxygen sensor values. The computer will see a lean condition and add more fuel to compensate. When this process happens the engine speed will raise due to the extra air and fuel entering the engine.
  • If you want to run it that way, you need to cap both ends, but your emissions will rise and may not pass inspection. It could even throw a CEL P0174.

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