L20B crankcase vent hole - threaded? Normal for oil to come out?

[moocowman]

Is that hole supposed to be threaded or is something supposed to be friction fit in there? Mine was just left open. If I do some hard acceleration a a good amount of oil comes out the hole, enough to make smoke as it gets onto the exhaust and it leaves a stream of oil down the side of the block. Is this normal or is there only supposed to be vapor coming out?

[datzenmike]

The pipe is just press fit into the block. Always run a PCV valve and vent the block properly. 

 

[moocowman]

What are the upper holes in the head you circled? Are those coolant passages?

 

Is it normal for oil to spray out that hole on hard acceleration?

 

If I run that to a catch can, do I still need to install a PCV valve in the line that goes to the catch can?

[moocowman]

I'll try just a vent tube with little filter on top, but judging from the amount of oil that comes out on hard acceleration it might not do.

[datzenmike]

1 hour ago, moocowman said:

What are the upper holes in the head you circled? Are those coolant passages?

 

Is it normal for oil to spray out that hole on hard acceleration?

 

If I run that to a catch can, do I still need to install a PCV valve in the line that goes to the catch can?

 

 

Not my picture but those are fro coolant flow from the head into both arms of the intake manifold to keep them warm for better fuel evaporation. In the summer it also removes heat radiated from the close by exhaust manifold.

 

There should be a screen over the block vent to catch and allow oil droplets to combine and drip back into the oil pan. Excessive blow by from a worn engine will inevitably push some oil fog out the vent. The long tube from the vent to the PCV valve will also collect and help drain any oil back into the engine. Is there any chance the engine is over filled with oil? 

 

Forget catch cans. Catch cans are mandated on some race tracks to reduce oil spills from blown engines and have no place on the highway.  If your engine is in good shape, the screen and vent tube collect the oil spray before it gets to the PCV valve. Keep the PCV valve functional. There has to be excessuve blow by to push oil spray and fog out the vent. If still blowing oil excessively.... time to rebuild it.  

[moocowman]

That's good info! I really didn't want to have to do the catch can. I'm going to make a tube reaching upward and put a breather filter at the end. I didn't think about the screen, but that's a great idea to get it to condense and run back into the engine. Maybe I'll stick some coarse steel wool in the tube. So, is this vent tube for the engine block normally teed in with the valve cover crankcase vent? My valve cover has an elbow with a vent filter on the end. I'm not sure there's actually a PCV valve in there. It looks like a basic elbow someone epoxied to the valve cover.

 

Yes, it is possible it's overfilled. I just did my first oil change, filled it to the upper line. Before, it was middle, between the full and low lines. I'm thinking the previous owner intentionally didn't fill to the top line. I'll drain some and see if it stops spitting oil up.

[datzenmike]

There should be a screen mounted in the block to catch oil fog. The PCV valve has a very small orifice in it and is mounted in the intake manifold where vacuum draws damp air and fumes out of the block. Fresh air is drawn in through the valve cover hose from the air filter. Get this working properly. The fumes in the engine are water and gas vapors and some blow by past the rings from combustion. Without the PCV working, these vapors would condense over night and contaminate the oil. The flow of fresh air goes this way...

 

air filter > valve cover > engine block > block vent > PCV valve > intake. The engine intake vacuum sucks vapors out of the block and draws fresh air in. Everything should be there, makes me wonder what you have.

 

 

 

Find out exactly how much oil is used when changing it. Put that amount in and read the dipstick. That will be the new full up. Dipstick might be from a different vehicle and you are over filling. Spinning crankshaft will whip the oil up into a spray.

[moocowman]

Well, sir, i ain't got nothing you described. I just made that tube for the engine vent and stuck a breather filter on top. I had to remove the carbs and heat shield to get to the block vent hole. Elbow it to get it around the back of the carbs. I figured block fumes and valve cover fumes shouldn't be mixed if I'm going to have oil condensate drain back into the engine. Other than being ugly, should this setup be ok? I know it's nothing like you described, but I have no air filter. There are some holes in the intake manifold that have plugs installed in them. Maybe I could use one of those for the last part of your flow chart, "PCV Valve > Intake"?

 

I'm pretty sure the dipstick is isn't the correct one for this engine. Is there a spec as to how far the dipstick should reach under the engine block? I can remove the pan and mark it on the dipstick. 

[datzenmike]

Oh, right I remember this engine.

 

Block and valve cover 'fumes' are the same and come from the same place. You can run it the way it's pictured, but you don't really need the block vent pipe open, you can seal it off as there is nothing to draw the fumes out as it is. Running just the valve cover vent gives a longer distance for oil spray to travel and fall out of the air and return to the oil pan. The block is vented this way but there is no flow of air through the system as with a functioning PCV valve. Without the PCV working your oil change interval is the same as cars in the 50's, about 3,000 miles. That's how important that PCV system is. 

 

You don't need to remove the oil pan. Next oil change put the correct amount of oil into the engine. 4.3 liters or 4.5 US quarts. It's now correctly filled, put the dipstick in and what ever the reading is... this is the new fill line. Mark the stick with a scratch or center punch a line. 

[moocowman]

"Oh, right I remember this engine." Yeah, sorry. It's here again.

 

In my head I said, "aren't the block and valve cover fumes the same?" But I read a thread where someone said absolutely 100% do not plug off the block vent and the gases from the block are different that what comes out the VC. Great. I'm going to plug it and get rid of all that ugliness.

 

Can I install a PCV valve and route that to the intake manifold? There's a larger threaded plug at cyl #4 on the manifold. But that would introduce unmetered air to that cylinder. You mentioned something about an orifice. That would limit the unmetered air, but there would still be some. Wouldn't I need to compensate for than in the fuel mixture?

 

Good idea on the oil level. I'm going to drain the oil and measure out 4.5 quarts. I know for sure it's over-filled. I used whole 5qt jug. I don't remember if I had to add even more to get to the full line.

[datzenmike]

57 minutes ago, moocowman said:

 

 

In my head I said, "aren't the block and valve cover fumes the same?" But I read a thread where someone said absolutely 100% do not plug off the block vent and the gases from the block are different that what comes out the VC. Great. I'm going to plug it and get rid of all that ugliness.

 

Well whoever said that they are dead wrong or it was misinterpreted. The internal block is connected into the valve cover by the passageway the timing chain runs in. If you have no operating PCV then a second vent is redundant. 

 

 

57 minutes ago, moocowman said:

Can I install a PCV valve and route that to the intake manifold? There's a larger threaded plug at cyl #4 on the manifold. But that would introduce unmetered air to that cylinder. You mentioned something about an orifice. That would limit the unmetered air, but there would still be some. Wouldn't I need to compensate for than in the fuel mixture?

 

 

 

Better would be a hose to both intakes so the fumes are not in just one or two cylinders.

 

 

The PCV valve is pulled closed during high intake vacuum such as idle. As the throttle is opened the vacuum cannot hold the valve closed and crankcase air is sucked into the intake. The air removed from the crankcase is replaced by filtered air drawn into the valve cover.

 

 

 

 

Before the early 60s and the introduction of the Positive Crankcase Ventilation valve, engines just vented down beside the block. Here is the result. Those dark stripes are oil.

 

 

[moocowman]

I see! So, there isn't a need to compensate for the unmetered air because at idle the PCV is closed and idle is where unmetered air is most apparent. On acceleration when unmetered air is less of a problem, the PCV opens up. You're right, both carbs have the same plugged port. I could run it to both. Alright. I'm getting on it this weekend.

 

Here in Southern California, we still have those black streaks up the center of lanes from people failing to fix their oil leaks. I also ride motorcycles and I've learned to instinctively avoid the center of lanes.

 

Thanks for all the good info!

[Rustbin]

My wife used to ride a motorcycle and she could tell a newbie rider by what they wore, how they sat at a light or if the rode in the "oil slick" center of the lane. I'm thinking of how to make the PCV work when I have motorcycle carbs with no vacuum ports on my car.

[moocowman]

Absolutely. Noob riders are easy to spot. Those who actually wear gear have ill-fitting counterfeit Dainese or Alpine Stars jackets and pants off Temu that will explode off your body before they even touch the pavement. At red lights, they don't know what to do with their arms. So, they do nervous things like tap on the fuel tank as if they're listening to music or twist their bodies back and forth, elbows out, like they're doing stretches but really just looking around to see if anyone's watching. On the road, they're either in the center oil slick where they lose the front wheel with any braking or they ride as far away from adjacent traffic as possible, which invites people to switch into their lane.

[datzenmike]

On 8/28/2026 at 7:21 PM, Rustbin said:

My wife used to ride a motorcycle and she could tell a newbie rider by what they wore, how they sat at a light or if the rode in the "oil slick" center of the lane. I'm thinking of how to make the PCV work when I have motorcycle carbs with no vacuum ports on my car.

 

Well there are the individual holes threaded for fittings I was going to use for vacuum for the brakes. They are very small and probably would clog. I was looking at one way valves that fit the exhaust pipe and use the negative pressure after each exhaust pulse. It doesn't burn the emissions but it does ventilate the crankcase.