Major blow-by....??

[johnl]

I wonder what the pressure differential is between the two tubes is at WOT. Interesting stuff though!

I have no idea what the difference would be, but it would vary with a number of factors.

At smaller throttle openings plenum vaccum will be strong and will scavenge the crankcase quite effectively, and the tube that goes from the induction tube (in front of the throttle body) to the valve cover is needed to allow a sufficient air flow through the PCV (or else plenum vacuum might cause crankcase pressure to become negative, despite blow by gasses).

At WOT the blow by will be more substantial, and the plenum vacuum weak, and the PCV is only a small orifice. Not all that crankcase pressure is going to vent through the PCV at WOT, so air flow through the tube (that goes from the induction tube to the valve cover) reverses flow and blow by gas also passes 'backwards' through this tube into the induction tube. This is why you get gunk (oil residue) inside the induction tube and throttle body. This 'reverse' gas flow will be greater with older more worn engines.

It's pretty common for racing engines to delete the PCV valve and just run a vent tube into a catch can, though such a tube should be quite large in ID for free flow of gasses.

On older engines the blow by can be great enough at higher throttle / rpm that the stock crankcase breather system can't cope and too much pressure builds up in the crankcase and causes oil to start seeping all over the place from gaskets and seals. This is yucky, but the excess crankcase pressure can also affect engine performance, often quite significantly.

I had a Cressida that partially blocked up it's breather tubes (metal as well as rubber) and started building excessive crankcase pressure (Toyota engines are known for gunking up internally). This caused a multitude of small leaks and the performance started to get noticably poorer than it used to be. Cleaning the crap from all the breather tubes affected an instant cure for both the leaks and the performance.

[johnl]

After reading through the replies, I forgot properly address the original question.

I’ll bet you probably don’t have as much blow by as you think you do. I suspect that a lot of what you’re seeing / feeling when you take the oil filler cap off is just pressure fluctuations in the crankcase. I know this seems unlikely, but bear with me…

You get a momentary pressure rise that forces gas out of the filler hole (along with some oil spray), then a negative fluctuation that sucks air back in. This fluctuating pressure cycle happens very rapidly with each crank revolution, and if you hold your hand near the filler hole you can feel the gas coming out, but not the gas going in, so it feels like a fairly strong gas flow coming out.

Do try this at home:
With engine idling hold a sheet of paper above the filler hole where you can feel it being affected by the pressure exiting the filler hole. Now slowly lower the sheet of paper toward the hole. What I expect will happen (unless you really do have substantial excessive blow by), is that when the sheet of paper gets quite close to the hole it will suddenly get ‘sucked’ onto the hole and then be kept there by the difference between average crankcase pressure and atmospheric pressure.

Despite the fluctuating crankcase pressure the PVC will be ‘sucking’ hard enough to create a slight vacuum in the crankcase, despite what the pressure fluctuations as seen at the filler hole might suggest. Now rev the engine while pulling gently on the paper, I bet the suction decreases and the paper easily lifts off the hole, then may be blown off if you give the engine a hard rev.

Ages ago I did the same as the OP, i.e. took the filler cap off (on my f22A9 engine), and then thought I must have had excessive blow by judging from the apparent pressure escaping the filler hole. So after checking PVC function and all the breather hoses (all OK), I fitted an additional breather tube to the cam cover (feeding into a catch can) to assist in venting this apparent pressure.

When I had only the spigot for the new hose fitted to the cam cover I had the same sort of apparent substantial pressure escaping from the spigot, but when I fitted about one metre of hose to the spigot I could detect no pressure escaping from the end of the hose. In fact if I placed my finger over the end of the hose I could actually feel a suction. The hose was obviously damping the pressure fluctuations, so at the hose end the average (negative) crankcase pressure was being seen.

The PVC was obviously sucking hard enough at idle to draw air through not only the stock breather inlet tube but also through the new breather inlet tube (which I had expected to act as an outlet tube). Since this was obviously the case I then had to fit an air filter into the neck of the catch can (a small plastic oil bottle), for which purpose one of those small tubular lawn mower engine air filters works very well (jammed and taped into the bottle neck, with nearly all the filter inside the bottle).

Since fitting this additional breather hose I’ve never seen any signs of oil inside either the clear plastic hose or the catch can, but I still get oil inside the main induction pipe and the throttle body. This must be the result of the pressure in the induction pipe being less than atmospheric (as it must be at the catch can, i.e. atmospheric), so at higher rpm / throttle whatever blow by isn’t being drawn through the PCV must only be passing into the stock induction pipe breather tube.

[bcjammerx]

hmmm...I might wanna try that...sounds cool
I know I've (for specific reasons I can't remember right now) tried to pour oil in with the cap off and it just blew it everywhere...course that isn't proof of anything really

man I need to install that catch can though...

[alb_accord]

I think thats somewhat normal as long as it isnt blowing oil out at you...

that is normal on old engines

as Chrisisiced said. as long as it is not blowing oil you are fine.