I have a 79 f250 with the 300 six cylinder. I have a pcv valve at the rear of the valve cover by the firewall and at the front of the valve cover there is a oem twist on oil fill breather cap. I dont understand how the pcv valve can work if the oil fill cap is a breather type. I would think the pcv valve would just suck the vapors straight across the valve cover to the front breather. How would this setup scavenge the crankcase vapors. A v8 engine has a breather on one valve cover and pcv valve on the other side valve cover so it sucks the crankcase vapors out while fresh air comes in. I would think the oil fill cap on the 300 should be just a cap, not a breather so the pcv valve can pull vapors out of the crankcase since there is only one valve cover on these engines. I doubt anybody cares, "LOL" but any thoughts?

 

The PCV valve is connected to manifold vacuum. Manifold vacuum pulls crankcase fumes into the intake stream through the PCV valve (when it is open) to be re-burned. This is one end of the system. The other end of the system is the "input" and draws in fresh, clean air. This is done one of two ways: (1) a fitting with a hose connected to the air cleaner with a breather filter, or (2) an aftermarket breather cap. In your case (2), air is drawn in to the crankcase through the filler cap and is filtered by the breather filter in the cap itself.

It works on a straight six the same way it works on a V8. It's a cyclical system. If you're pulling stuff out, you've got to let stuff in.

 

Thanks for the reply. I might do an experiment by capping off the breather on the valve cover and use the dipstick tube where the fresh air will be pulled in by removing the dipstick and clamp a 3/8" rubber fuel hose to the dip stick tube with a gas filter on the end to filter out incoming air. This way i would think the pcv valve would pull more toxic fumes out of the engine since the air would be coming into the oil pan down low and up to the valve cover on top. I guess that sounds crazy but i will try it you never know, is the dipstick tube large enough for incoming air and as a breather?

 

That's not a very good idea.

The vacuum that is applied to the PCV valve is more than enough to evacuate the whole engine of harmful fumes.

If you hook a line to your dipstick tube in the way you describe, You may put too much pressure on the crankcase and the PCV valve will suck your oil up into the intake. It may also arriate and form bubbles in your oil.

 

I agree, there's been a lot of holiday cheer this way here of late that dulls the senses, merry christmas everybody and happy new year.