I see that my truck has a PCV valve and a breather tube on the oil cap going to the air cleaner. The breather setup seems cobbled together, and i was wondering if I'm supposed to have both. It seems like the breather tube would render the PCV system unnecessary and inoperative. Am I missing something, or is this the proper setup?

 

Oil cap should have a hose going to breather, and PCV valve has one going into the intake/base of carb.
Air goes in the oil cap, and out the PCV valve, to complete the system.

 

Ok, I was curious because I was working on the truck today (did plugs, wires, PCV and air filter) I noticed that while the truck is running, the air cleaner is obviously sucking air out the breather tube in the oil cap. I could see smoke being pulled out the oil cap, through the air filter, and into the carb.

 

First thing I would do, is check the hose and vacuum port that the PCV hooks to. Make sure it's not partly blocked. If that's working at peak flow, the engine may just be getting a little tired.

 

Checked the PCV, and it's getting plenty of vac, but there is no appreciable suction on the breather tube coming out of the oil cap. I assume there should be, but I also assume my engine isn't exactly in tip top shape after 250k miles.

 

As the engine wears, it gets more blowby past the piston rings. It will get to a point where the PCV valve cannot suck all of it out and starts to get overwhelmed. You will then start seeing the pressure go up the breather tube into the aircleaner. You can live with this as long as the sparkplugs don't start fouling, and it gets so bad it overwhelms the carb and makes it run rough.

 

Cool, thanks for the info. No idea the last time the plugs were changed before today, but the old ones I pulled out were pretty good looking actually. Probably could have just put them right back in. Not really planning an engine rebuild on a truck to drive to the dump a few times a month, so I won't sweat it.

 

Oh, and when I was testing for suction on the breather tube, there was no suction, but no pressure either, so I hope the blowby isn't too bad.

 

There won't be the same amount going into the engine at the breather tube, as there is going out the PCV valve. Even on a new engine.

Blowby gasses from the combustion process leak around the rings a tiny bit even on a brand new engine. The PCV valve sucks this caustic blowby gas out to be reburned.

Normally the amount of blowby gas is less than the PCV valves full "suction" capacity. If the blowby gas is at 20% of volume in the engine, it sucks that out, and whatever is left, in this case 80% of fresh air is sucked into the engine through the breather.

When the engine wears out, the amount of blowby increases. If the blowby gasses get above 100% of the PCV valves volume, then it overwhelms the PCV system. Lets say your blowby gasses are at 110% of volume the PCV can handle. The valve sucks the same amount of volume out the engine. So at 110% it can backflow out of the breather into the air cleaner and down the carb. In this case it's 10% coming back out the breather.

The more that the blowby gasses are above 100% of the PCV systems capacity, the worse this can get.

Sometimes replacing the PCV valve can fix this problem as it increases the PCV valves total volume of suction, due to a worn out or defective valve.

Some of the time it's an early warning that you may need a ring and valve job in the future, or have worn bearings or other internal problems.

Hope this helps.

 

On a new engine that has been broken in, you can pull the breather pipe off and plug it with your hand, and you will eventually feel vacuum.