"The function of a PCV valve is to control the flow of crankcase gases. Start your engine, then remove the oil filler cap (watch your clothes; you might get some oil splatters). Place your hand just above the opening. Feel the strong jet of air? That's "blowby", combustion gases that squirt past the piston rings into the crankcase below the pistons. Even new engines in good condition generate lots of blowby (but it's still a small
fraction of what passes through the throttle body).That jet of air has to be vented somewhere, otherwise it will quickly build up pressure inside the engine, and cause those aforementioned very bad things to happen. If the PCV valve gets plugged, blowby pressure has no proper place to go.
The primary reason for plugged PCV valves is neglected oil changes, which causes sludge. Secondarily, it also happens to engines that are primarily used for short trips, where moisture, fuel, and acids build up in the oil, emulsifying it into sludge, which is related to the neglect thing mentioned earlier. 3 months between changes might be too long depending on how you drive.
Also, an engine with worn compression rings will have massive amounts of blowby, which will overwhelm the PCV valve's ability to pass air and cause the excess vapors (and oil) to be pumped into the intake before the throttle plate.
Since the air inside the crankcase also contains contaminants like unburned fuel, water, acids and other things that your engine and its oil do not like, venting the excess pressure also helps to vent those contaminants out of the engine before they settle into the oil in the first place, so your oil stays a bit cleaner longer."
http://www.tegger.com/hondafaq/pcv-replace/