Friday, January 3, 2003


First off, we’re not talking about Poly Vinyl Chloride, the stuff you make pipes and plastic sorts of things from.

Positive Crankcase Ventilation is what we’re talking about today. PCV is probably the earliest form of pollution control to be fitted onto cars. This is one of the good things that the automotive industry did for everyone.

Inside the engine, a vacuum is developed that sucks fuel in and provides for such wonders as servo-assisted brakes. Vacuum is inversely proportionate to load. This means you normally get a high vacuum at idle and it drops way low when you step on it.

The PCV valve sits in line from the rocker valve cover to a vacuum port. At idle, and in a high vacuum situation, PCV sips blow-by gases that contain usable fuel vapors were simply vented to the air. Under a load, the special shaped plunger in the valve responds by allowing more gases to pass into the intake.

A special feature of the PVC is backfire protection. In the event of a backfire, pressure from the intake manifold forces the plunger to the closed or engine-off position. This prevents the backfire flame from reaching the crankcase and exploding the combustible vapor.

If the venting were too restrictive, a pressure would build up inside the engine and push oil out anywhere it could. You might remember the old breather caps that weren’t much more than a vented oil filler cap.

Using the right combination of breathers and a PCV create a negative pressure in the crankcase that make oil behave better and stay put while keeping positive oil pressure on the moving parts as it should. Blow-by gases are hosed from the PCV back to the intake for recycling and extracting more energy from the vapors.

What all this means is if you neglect your PVC system there’s big trouble. The fuel-air mixture will be messed up big-time if the system becomes clogged. The results of the mixture being wrong can be stalling, premature wear on spark plugs and all sorts of nasty thing that cost way more than a PVC.

(Comments? Questions? Slip them on over to

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