Have another question about my '86 F250 with a 351w. On the PCV valve there's 2 hoses, one goes to the oil cap but there's a smaller one on mine that doesn't connect to anything, where is it supposed to go? Just any vacuum source? I guess that would explain the oil leaking around it...

 

If the valve has been changed, it may go nowhere. Some trucks used that small line to purge the charcoal canister, some didn't. The aftermarket people put that nipple on all of them. If you need it, you snip the end to open it up and put the small line on there. If you don't need it, then you leave it plugged off with plastic. If someone mistakenly cut the plastic and you don't need it, then you would have to put a cap or a hose with a screw in it to plug it.

 

Where would I find this charcoal canister? I'm not sure if it was by mistake but somebody attached a vacuum type hose to the smaller nipple, it's sitting on top of the valve cover going the same direction as the oil cap but like I said, it's attached to nothing (not even plugged off, it's just an open hose end.) I bought a new PCV and grommet, should I just connect the oil cap line and leave it or how can I tell if I need the small hose?

 

The charcoal canister is a black plastic box mounted somewhere usually down low on either side of the radiator, and has vacuum lines going to it. There should be a small line leaving the canister going down the frame to the fuel tanks. And another line going to the carb.

You know how they only sell those sealed gas cans now for your lawnmower and weedeater? These trucks were built during the first crackdown, their fuel systems are totally sealed off so they do not emit gas fumes. Any fumes from the carb bowel or the gas tank run up those vacuum lines and the fumes are stored in the charcoal canister. When you start the truck, they use vacuum to suck these stored fumes into the engine to be burned.

They had many different ways they did this. Some had solenoids in the suction line to only let the fumes into the engine during certain conditions. Some had a simple vacuum line that sucked on the canister all the time the engine was running. And the lines went to the engine in different places. That's why even though all the aftermarket PCV valves have the small ports, they are not always used on every truck.

You will have to look on the vacuum diagram on your radiator support to determine what your truck originally had.

 

Appreciate the help, I'll look around for the canister and check out the diagram. One more question, I read on this forum somewhere that the PCV should "suck on your finger" like a vacuum. Is this not the case? If it is, how does it work i.e. where does it get vacuum from? You said my truck may only have the one line, does that line from the oil cap produce vacuum? Sorry for the dumb question.

 

The PCV system is a loop.
Starting at the valve in the valve cover or oil cap.
A hose goes from valve to a port on the cab base. this is where it get vacuum from.


Now if you are pulling a vacuum on the inside of the motor you need to let fresh air in.
You should have another hose in the other valve cover/cap or other end of valve cover on a six.
This hose goes up to the air filter housing to pull fresh air from. Inside the housing there should be a smaller filter type thing called a breather filter.


So air filter breather to valve cover> thru motor> out other valve cover thru PCV into carb.
Dave ----

 

The PCV valve should have a large line and large port on the bottom, and the smaller line/port on the top that you were questioning. The large line should go to somewhere on the engine or the carb, this is where the suction comes from. If you cut the small port open on the valve, then the large line will suck through the PCV valve AND the small port, which would most likely connect to the canister if your truck uses it that way.