Intermittent crankcase pressure issues

-
Theres only one place pressure comes from: the rings. So you have an issue with the PCV, ring seating, ring seal, or all of them. Oil pumps and exhaust valves wont affect it.
 
Theres only one place pressure comes from: the rings. So you have an issue with the PCV, ring seating, ring seal, or all of them. Oil pumps and exhaust valves wont affect it.

Compression and leak down tests are both good so I doubt it's the rings. Exhaust valves staying open when they shouldn't will also cause pressure to leak past the valve seals.
 
Compression and leak down tests are both good so I doubt it's the rings. Exhaust valves staying open when they shouldn't will also cause pressure to leak past the valve seals.

Um... no it won't. Race engines don't even use exhaust valve stem seals and they don't have pressure leaking into the crankcase. Only way for pressure to get into your crankcase is through the rings.

If the engine was still in the car I would have said to run it with the PCV and breather removed and see how much blowby comes out when it's warmed up and idling.
 
Unless you have the crankcase breather with a hose routed to the air cleaner, you are only running half of the PCV system. You need to connect the crankcase breather to the air cleaner VIA a hose. That will probably eliminate your issue.
 
Think of the PCV system as an unbroken circle, beginning with the inlet at the air cleaner and ending at the outlet for the PCV valve. Without all the components, you WILL have issues on a street driven car. Here is a simple diagram.

PCV SYSTEM.gif
 
Um... no it won't. Race engines don't even use exhaust valve stem seals and they don't have pressure leaking into the crankcase. Only way for pressure to get into your crankcase is through the rings.

If the engine was still in the car I would have said to run it with the PCV and breather removed and see how much blowby comes out when it's warmed up and idling.

This has nothing to do with what I said. Yeah if the exhaust valves are shut when they need to be you are in fact correct. Please read where I had oiling issues and bent some pushrods last year. Logic would lead me to believe I probably messed up my valve guides as well to some degree.

Plenty of people run without a hose hooked up to the air cleaner. I have a big breather and pcv. Sure I could find a way to vent all this pressure but this pressure is telling me something is wrong.
 
Are you seeing any signs of oil on your spark plugs, either on the electrodes, insulator, or around the threads?
 
Are you seeing any signs of oil on your spark plugs, either on the electrodes, insulator, or around the threads?

GjT1ipj.jpg


Looks to be some around the threads but the area was kinda soaked on this one because it was sitting under the breather. The rest were pretty clean.

Should be pulling a head tomorrow and taking it apart tomorrow. I don't mean to offend anyone trying to help. I suppose NOTHING is really conclusive at the moment and we all just have educated guesses at this point. I'm not completely ruling out the rings which is why I pulled the whole motor and not just the heads.
 
Last edited:
OK, I just asked as oil in the chambers can fool you with 'optimisitic' compression and leak down readings... the oil can seal things up better that they really are when operating.
 
Plenty of people run without a hose hooked up to the air cleaner. I have a big breather and pcv. Sure I could find a way to vent all this pressure but this pressure is telling me something is wrong.

Yes, they do and they are running it WRONG if it's a street car. Without the system being complete and AS DESIGNED, any diagnosis is ineffective. Lots of people drank the kool aid Jim Jones served, but they were WRONG too.
 
Yes, they do and they are running it WRONG if it's a street car. Without the system being complete and AS DESIGNED, any diagnosis is ineffective. Lots of people drank the kool aid Jim Jones served, but they were WRONG too.


Fair point. I'll be honest I'm not even really sure what purpose that fulfills. I saw the diagram but what does that hose do that a breather and pcv doesn't? I know there's no significant vacuum at WOT so the PCV isn't as effective but wouldn't that be the same case at the air cleaner?
 
OK, I just asked as oil in the chambers can fool you with 'optimisitic' compression and leak down readings... the oil can seal things up better that they really are when operating.

Yeah I was reading about that. Some guy had an old jeep that exploded on him after giving a decent compression test. Apparently it was letting so much oil by that everything was "sealed" pretty good lol.
 
Fair point. I'll be honest I'm not even really sure what purpose that fulfills. I saw the diagram but what does that hose do that a breather and pcv doesn't? I know there's no significant vacuum at WOT so the PCV isn't as effective but wouldn't that be the same case at the air cleaner?

Well, actually there is TONS of vacuum at WOT, just not where YOU'RE thinking. Engine vacuum falls off on acceleration, yes, but vacuum increases severely at the air cleaner going into the engine. That's the other half of the PVC system. When one side falls off, the other side reverses flow into the engine intake pulling the crankcase gasses in through the air cleaner on the other side. This is why often times when you leave the breather disconnected from the air cleaner, you get oil all over the valve cover. That same oil mist can also over come the rings at high RPM and get blown out through the combustion chamber.

The PCV system works good only TWO ways. 100% complete, OR 100% eliminated. I like the system, because on a street car, it actually helps keep the rings sealed and keeps oil consumption down.

On the older cars (mid 60s and prior) they used "OCV" or open crankcase ventilation, which had what was called a road draft tube that stuck straight down into the air flow under the car. All it did was dump the crankcase vapors right into the atmosphere. At least with the PCV, not only are those vapors burned, but an active vacuum is put on the crankcase, helping to keep rings sealed. It's really a win/win.

Yes, people have run "only this" or "only that" for like ten thousand years and "never had a problem", but how would they know the difference if that system had never been complete?
 
This has nothing to do with what I said. Yeah if the exhaust valves are shut when they need to be you are in fact correct. Please read where I had oiling issues and bent some pushrods last year. Logic would lead me to believe I probably messed up my valve guides as well to some degree.

Plenty of people run without a hose hooked up to the air cleaner. I have a big breather and pcv. Sure I could find a way to vent all this pressure but this pressure is telling me something is wrong.

I read that part, if anything happened it could have bent some valves slightly which would have made it run not-so-great (bad valve sealing). However just by how small the "gap" is between the valve stem and guide and how briefly there's actually any pressure there (which is actually usually negative because the exhaust gases are moving at very high speed through the port) I don't see how it's possible to get pressure up past the guides into the crankcase that way.

When my Duster was still running with the 360 I had in it I initially had a PCV valve on one side and an open breather on the other. I found that under hard running the breather would push up like it was going to pop out the valve cover and would leak oil. The first issue is that most of those breathers you buy from the parts store are super restrictive; I pulled about half the "filter" material (some kind of foam like for 2-stroke engine air cleaners) until I could blow on it and actually have airflow. Then I bought a breather with the hose fitting on it (and again had to remove about half the filter media) but since I had an open-element air cleaner I just routed the hose to right in front of the air filter.

What some of us are trying to get at is: your engine may not actually have too much blowby it just doesn't have a good way to vent the pressure. Now that the engine is out of the car you can't monitor the actual amount of blowby being pumped out by the engine so now all you can really do is play Detective and tear down the engine looking for clues.
 
Well, actually there is TONS of vacuum at WOT, just not where YOU'RE thinking. Engine vacuum falls off on acceleration, yes, but vacuum increases severely at the air cleaner going into the engine. That's the other half of the PVC system. When one side falls off, the other side reverses flow into the engine intake pulling the crankcase gasses in through the air cleaner on the other side. This is why often times when you leave the breather disconnected from the air cleaner, you get oil all over the valve cover. That same oil mist can also over come the rings at high RPM and get blown out through the combustion chamber.

The PCV system works good only TWO ways. 100% complete, OR 100% eliminated. I like the system, because on a street car, it actually helps keep the rings sealed and keeps oil consumption down.

On the older cars (mid 60s and prior) they used "OCV" or open crankcase ventilation, which had what was called a road draft tube that stuck straight down into the air flow under the car. All it did was dump the crankcase vapors right into the atmosphere. At least with the PCV, not only are those vapors burned, but an active vacuum is put on the crankcase, helping to keep rings sealed. It's really a win/win.

Yes, people have run "only this" or "only that" for like ten thousand years and "never had a problem", but how would they know the difference if that system had never been complete?

I had intended to try just running double breathers for a bit but never got around to trying it. My dad built a nearly identical 318 8 years ago and never had any of these issues which is what I'm basing most of my reasoning on. Chances are if he got away with the setup he's running (the same as mine) then the issue is definitely elsewhere. There may be a more effective way to manage to blow-by for sure but right now my main concern is definitely tracking down the source. A fresh mild 318 shouldn't be having these problems.

What some of us are trying to get at is: your engine may not actually have too much blowby it just doesn't have a good way to vent the pressure. Now that the engine is out of the car you can't monitor the actual amount of blowby being pumped out by the engine so now all you can really do is play Detective and tear down the engine looking for clues.

Yeah the cat's out of the bag now. Like I said to RustyRatRod I have the luxury of comparing my motor to an almost exact replica since I based it on a build my dad completed a while ago and he helped me put it together. He had none of these issues but he also didn't bend a bunch of pushrods and run his engine with some upside down rocker shafts either lol. That's why I'm so certain the issue isn't just improper ventilation.
 
I had intended to try just running double breathers for a bit but never got around to trying it. My dad built a nearly identical 318 8 years ago and never had any of these issues which is what I'm basing most of my reasoning on. Chances are if he got away with the setup he's running (the same as mine) then the issue is definitely elsewhere. There may be a more effective way to manage to blow-by for sure but right now my main concern is definitely tracking down the source. A fresh mild 318 shouldn't be having these problems.

No offense but from my experience that kind of thinking leads to nowhere ("it worked with his setup so it should work with mine"). Your dad probably got lucky that both the breathers he happened to use could flow enough that the crankcase didn't over-pressurize. When the PCV valve senses pressure it shuts and forces the crankcase gases out the breather side; with 2 breathers the engine has 2 big holes to vent out through at all times.
 
No offense but from my experience that kind of thinking leads to nowhere ("it worked with his setup so it should work with mine"). Your dad probably got lucky that both the breathers he happened to use could flow enough that the crankcase didn't over-pressurize. When the PCV valve senses pressure it shuts and forces the crankcase gases out the breather side; with 2 breathers the engine has 2 big holes to vent out through at all times.

No he's running PCV and breather like I was.
 
What kind of rings did you use? Sometimes, moly rings take a while to fully seat. How many miles on it again?
 
I should have clarified too, when you check blowby you look at how fast/hard the blowby is flowing out the hole in the valve cover. If it's spewing a ton like an old steam locomotive then you have issues, but if it's a small whisp of vapor that's completely normal. BUT even if that small whisp of vapor has nowhere to go it will pressurize the crankcase and cause the issues you're having.
 
No he's running PCV and breather like I was.

Oh sorry not sure where I read about 2 breathers... in that case I'd be taking a hard look at the breather you have. If you can't blow on it easily with your mouth then you can imagine what's happening to those crankcase gases.
 
Think of the PCV system as an unbroken circle, beginning with the inlet at the air cleaner and ending at the outlet for the PCV valve. Without all the components, you WILL have issues on a street driven car. Here is a simple diagram.

View attachment 1715254715
I keep having an oil leakage problem. I have a pcv valve on the drivers side cover, and a breather on the pass side. No oil showing up anywhere on top, but can`t keep the stud girdle sealed, about ready to scrap the dam thing. I `ve tried about every sealer known to man on it, checked the shims multiple times on the studs etc.
I have wondered about running the hose to the air cleaner , like in rrr`s diagram , but have a 6 pack scoop /sealed to the throttle body.
Would the hose hooked up inside the filter area still work like its supposed too, or would the scoop add pressure to the crankcase ?------opinions please
 
With any outside scoop, I'd guess no more than 1/4" of pressure rise over atmospheric. I'd not worry about that.
 
Compression and leak down tests are both good so I doubt it's the rings. Exhaust valves staying open when they shouldn't will also cause pressure to leak past the valve seals.

So if the exhaust valves are open, how come the two tests are good? My point is, you're missing something. Pressure will leak out the rings, or out the valves, or both. You will have evidence of the leakage. The valves are not leaking. Pressure is pushing oil out. So it's rings. Just is what it is.
 
what steps did you go through to seat the rings? not sharpshooting, just curious
 
I think you are doing the right thing by pulling the motor . I know it's a PITA but you will definatly find the problem . Good luck !
 
-
Back
Top