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So a problem I've been putting off dealing with for a long time is the excess crankcase pressure in my 99 Nissan Almera 1.6L.

You can see here in the air box the results of this excess pressure:

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Blow by and crank case vapors are obviously being pushed out into the air box through the breather hose. When I remove the dipstick I can feel the puffs of air coming out. Measuring with my vacuum gauge, I get a reading of 0 inHg at idle, and about 1 inHg of positive pressure at around 2500 - 3000 rpm. I would expect to see about 0.5 - 1 inHg vacuum at idle in the crankcase...

Now here's the strange thing. When I remove the PCV valve at idle and place my finger over it I feel vacuum, and when I remove my finger the valve snaps back the way I would expect it to. Seemingly no problem there. So the next thing one would look at is intake vacuum to see if the rings are sealing OK. Intake vacuum is a steady at about 20 inHg.

Based on the few answers I got to my question three years ago about whether compression and oil control rings wear out at the same rate, my gut feeling is that my good intake manifold vacuum reading would imply that my oil control rings are probably OK also.

So, my PCV valve seems to be OK, the breather hose isn't restricted, I've got good intake vacuum which makes me assume both oil and compression rings are OK, so what could be going on here?

What am I missing?

PS

Something I've noticed is that the idle is intermittently slightly low, and that the ECU will usually think the engine is rich ( AF Ratio PID ) and peg the O2 lean, or think the engine is lean and peg the O2 rich. This only happens at idle, and the moment even the tiniest amount of throttle is given the O2 starts switching normally. Not sure if this is related, but maybe the PCV is getting stuck or is partially clogged, cutting off some air flow, lowering the RPM and confusing the ECU since it calculates in a certain flow from the PCV at idle?

  • Are you sure you don't have excessive blowby? Also, all oil control rings do is control the oil. They really shouldn't have anything to do with whether you have vacuum. The compression rings and valve sealing take care of that job. – Pᴀᴜʟsᴛᴇʀ2 Aug 19 '18 at 15:03
  • Your pressure readings ae lower than you expect so your analysis seems flawed... – Solar Mike Aug 19 '18 at 16:25
  • @Pᴀᴜʟsᴛᴇʀ2 Sure? No. I'm assuming I don't have excessive blow by because I've got good intake vacuum, and I would think excessive blow by would imply poor compression ring sealing and poor intake vacuum and visa versa. – Robert S. Barnes Aug 19 '18 at 17:58
  • @SolarMike I'm not sure I understand. I'm expecting between 0.5 to 1 inHg of vacuum and what I'm seeing is 0 to 1 inHg of positive pressure. – Robert S. Barnes Aug 19 '18 at 18:01
  • The thing is, blowby is caused from the pressure of the combustion process, which is a LOT more than what you get from pressures of a compression test. More pressure will bleed past the rings during combustion than it will during plain compression found during a compression test. – Pᴀᴜʟsᴛᴇʀ2 Aug 19 '18 at 18:27

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