Why does oil cap have a hose attatched???

[VT Woodsman]

I have a AMC 304 v8 , and as i was adding oil i noticed the oil fill cap had what looked like a sponge tucked up in it with a hose coming off it going to the air cleaner. What is that all about? Do the crankcase fumes go back thru the airfilter???

 

[IOPort51]

actually, at least in theory, the air that the PCV draws from the crank case, above the amount of cylinder blow by, is drawn through the breather. You wouldn't want to draw dusty air into the valve cover , would you??

The crank case fumes should end up being drawn into the PCV and into the intake manifold. think of your line to the breather as "Make up air".

 

[derf]

I always figured it was there to give the fumes someplace to go when the PCV end didn't have enough vacuum to fully evacuate the crank case.

I doubt that there would ever be a case when the PCV was pulling too much vacuum. Maybe there is. I don't know. I suppose if that does happen, the filter in the cap would keep the big stuff out. After all, that breather hose would draw in pre-filtered air. Though later 360 engines (after the AMC 304 was phased out in the CJ I think) have a filter assembly that attaches inside the outer wall of the air cleaner housing, perhaps to provide more filtering.

But yea, basically the breather is there to work with the PCV.

 

[CJim7]

What they said ^
I have an aftermarket cap which has no hose attachment....rather a filtered baffle on the underside of the cap.

 

[VT Woodsman]

Wait, is the cap taking air in, or letting crankcase fumes out? I need a damn flow chart here.

 

[xena1]

I have found a small filter at Advance Auto Parts with goes over the 1/2" fitting that works and looks the best. I experienced too much vacuum in the intake valley with the PVC and a hose into the air cleaner.

 

[Old Dog]

Wait, is the cap taking air in, or letting crankcase fumes out? I need a damn flow chart here.

It's letting clean air in, unless the pcv is plugged than it will suck oil into the aircleaner.

 

[IOPort51]

I was thinking this afternoon I may try to remember to put a manometer on the line in question and see if it is in face positive or negative pressure.

I'l get back to you on this.

 

[VT Woodsman]

Thanks for doing that. If it help any, when i took the cap off and soaked it in gas, tons of mud and dirt came off and the spong disinagrated. I was finding dirt coming off from the saturday night fever days.

 

[IOPort51]

for the moment I suggest you assume that it is a negative and it sucked in the dirt and such.

Thanks for doing that. If it help any, when i took the cap off and soaked it in gas, tons of mud and dirt came off and the spong disinagrated. I was finding dirt coming off from the saturday night fever days.

 

[VT Woodsman]

Interesting how tucked up in the upper portion is foam like a baby air filter.

 

[IOPort51]

you can buy a replacement at the parts house, on the isle with the PCV valves.

Interesting how tucked up in the upper portion is foam like a baby air filter.

 

[IOPort51]

I just did this.

there is very definitely a negative pressure. I guess this should not come as a big surprise.

I do not know how much just yet but with as much as 25 inches at the PCV I guess I should not be surprised that is almost sucked water from the 8" water manometer I was using.

At this point I would say that running the engine with out a return filter on this intake makes about as much sense as running with out an oil filter. Maybe even less because the oil filter doesn't have to keep out dirt from outside the engine.

I was thinking this afternoon I may try to remember to put a manometer on the line in question and see if it is in face positive or negative pressure.

I'l get back to you on this.

 

[VT Woodsman]

thanks for testing it. So am i right in thinking air is sucked in thru the fill cap and air is pushed out thru the pcv . What was amc idea behind this, was it to move fumes out of the crankcase?

 

[IOPort51]

PCV is pretty much SOP on every engine newer than middle 60s I think.

Back in the old days there was what is called a "blow by tube that relieved crank case pressure and dripped oil on the ground up near the water pump. there was no PCV but there was still a breather on the valve cover.

then along came the EPA and decided this was not a good thing, I think they had a point. So a system was created that took the blow by, the gasses that get past the piston rings and the vaporized oil, and draw them into the intake manifold and down to the cylinder where they are burned as the engine runs. The PCV is sort of a restriction to keep control of the vacuum and as a check valve to make sure that fuel vapor can't end up in the oil pan. You may have noticed that with out the PCV the engine dies.

So yes, you have a grasp of things. I would suggest that you think of the flow starting with the vacuum in the intake manifold drawing fumes from the crank case rather than them being forced out, and the breather allows the pressure to remain just slightly negative rather than dropping to a real vacuum in the oil pan.

thanks for testing it. So am i right in thinking air is sucked in thru the fill cap and air is pushed out thru the pcv . What was amc idea behind this, was it to move fumes out of the crankcase?

 

[VT Woodsman]

Very, very well said, we all have learned something here. Thanks again.