Well, here's the story. The crankcase, that which contains the crankshaft and connecting rods, is the bottom side of the engine - you can see the bottom of the pistons from under there. Compressed fuel and air mixture burns in the cylinder on the top of the pistons. When the rings become tired and worn they allow some of this compressed and burning mixture to leak past and escape into the crankcase. That is called "blow by". This is as easy to fix as maintaining a small web site or using the best small business web hosting.
In old engines that blowby simply vented into the atmosphere via a tube called a crankcase vent tube. The EPA found bad stuff in that blowby gas and mandated that the engines must suck up that stuff and burn it to render it harmful in some other way. In came the PCV which stands for Positive Crankcase Ventilation, named after the then head of the EPA Pierre Positive.
This system makes the engine suck the junk out of the crankcase via the PCV Valve into the intake manifold and back thorough the system. Welll, if that system is sucking the junk out, there must be a source of air to go into the crankcase else you would just implode the engine and the oil pan would collapse. Soooo, they ran a tube from the top of the valve covers to a source of clean air so that dirt would not get sucked into the crankcase. (note: the valve cover covers the valve train but that is also part of the crankcase - air and blow-by move readily between those zones in the engine.) The source of clean air which was chosen is the air cleaner - makes sense, except for the fact that the air doesn't go thorough the air cleaner filter, so they put a little kotex-like pad in there to clean the bugs and stones out of the air.
Remember now, the air is supposed to flow from the air cleaner's Kotex pad thorough that hose and into the valve cover, circulate around the crankcase and get sucked out via the PCV Valve. When the blow-by becomes so excessive that the PCV Valve system can't remove it, it has to go somewhere! Else the crankcase pressure would become excessive and the engine would explode. The place that it goes is back thorough the hose to the air cleaner - THE WRONG WAY!!! Of course it carries any oil in the valve cover back with it and that's what you mop up two or three times a month or week if it's really bad. Bottom line?? It's time for a new box of kotex or an engine overhaul, whichever is cheaper. (there is one other solution - see below)
If you do find oil in your air cleaner it may well be that your PCV valve is stuck closed and is therefore not sucking the blow-by out of the crankcase but rather, it is blowing back into the air cleaner. Make sure to check the PCV Valve before overhauling your engine!!
Here's a trick that one of my readers suggested. Take a copper pipe "T" and insert it in the hose going from the air cleaner to the valve cover. Attach a short length of tubing to the T and run it into a gallon milk jug that you can hang somewhere in the engine compartment below the hose "T". Oil will drain into the jug and when it collects a quart or so, just dump it back into the crankcase!!
For those of you who want a better solution, along comes a company I just discovered (two years after I wrote this article) who makes a very simple and cost effective product that will fix your blow-by problems forever. Take a look here for a quick description of what this device is and what it does. You can then send me a mailnote to get further information, or to order the EnValve. Just include your name, address, and the year, make and model of your car with the engine type included. See below UPDATE for more information. The inventor of this device guarantees that it will stop blowby problems and eliminate the need for that copper T and milk jug!
UPDATE - 10/18/2000
UPDATE 12/30/2003 - TO DATE I HAVE SOLD 300 ENVALVES WITH ZERO RETURNS!! THIS THING REALLY WORKS!
The owner of EnValve, Emil, has asked me to sell his product from my website. It seems that he is too busy selling these devices elsewhere and doesn't have the time to answer all of your mailnotes. I am going to try this - I will be using Pay Pal** to receive payments from you. Check out their website and see how it works. You will need to set up a free account with them and there is no charge to you to use their services to send me money. If you fill in my e-mail address when you sign up I will get five bucks which will help me keep this website afloat! If you decide to purchase an EnValve then send me a mailnote with your mailing address, year, make and engine size then go to pay pal and send me the payment of $39.95 plus $5.95 S&H (total of $45.90) and I'll send one out to you the day I receive the payment (please note that I will not accept personal checks - money orders are an acceptable means of payment, and please ensure that your PayPal shipping address is confirmed else I can not send you your EnValve). Remember this is a 100% guaranteed product or your money back. I guarantee it and Paying through PayPal ensures that! So far Emil has sold over 2000 of these units and has never had to refund a penny! The product works!!
UPDATE - AUGUST 2004 - STILL GOING STRONG!! I HAVE NOT HAD ONE UNSATISFIED CUSTOMER! PRICE HAS NOT INCREASED IN 4 YEARS. STILL 100% UNCONDITIONALLY GUARANTEED!
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