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AndrewCS
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Re: Back to where I started... 2020/08/04 08:53:06 (permalink)
A bit `fancy` that …  
 
My cheapskate versions used plastic bottles from the recycling bin  ... allowed me to maintain the larger hose diameters of the original systems. Not sure if the small connections on ^ this type of tank create a `restriction` ?
 
Probably overthinking things though … again  

Aberdeenshire (R2) : Cayman GT4 981


911hillclimber
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Re: Back to where I started... 2020/08/04 18:07:49 (permalink)
We shall see, now on order.
 
Should add, tried to lift the oil filler cap on the running engine, only a light but noticeable resistance along with a note change.
If the AOS is done for the suction would be really high.

Club member since 1988
hotfrog1
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Re: Back to where I started... 2020/08/05 10:27:03 (permalink)
I have wondered whether the issue with your AOS is that it is overloaded with gas flow under certain conditions.   If there is gas leaking due to blow by for instance then it is possible that the gas flow is outside its capacity.  The outcome may then be that there would be oil carry over from the crankcase side to the clean air outlet.   Not sure whether you can work out any sort of correlation here, but the blow by would be highest under mid range open throttle periods.   Once the clean side was wetted with oil then this could be burnt under low load conditions with high visibility.    The effect of the age of AOS is not readily explainable.   However, the performance of devices tends to fall off with age and we are talking about a marginal indicator here.    Maybe a cylinder leakage test would confirm / remove any blow by issues.   

However you view the issue, oil is breaking through the AOS to the clean side under certain conditions.   How, when and why is the puzzle.
Sincerely hope that this thought is not a red herring.
 
Just to say that I am not a professional motor man but have meddled and rebuilt a number of varied engines over 50 years or so.   The first time I came across an AOS, described as a breather, was on my 1964 MGB.   It was primitive and simple but had a diaphragm controlled valve.   It had a Smiths logo on it and was mounted directly on the inlet manifold and connected to the crankcase by a side pipe into a baffled cam shaft cover.   It never caused any trouble but did create significant crankshaft suction.   I was never aware of it causing any oil burning.
911hillclimber
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Re: Back to where I started... 2020/08/05 16:23:58 (permalink)
Thank you for the thought, and I'm on the same page, hence me wanting to have Zuffenhaus do a leak down test to see if a bore is passing air from the chamber (valves at TDC), past the ring(s) and pressurising the case and hence the wet side of the AOS.
Now, as the suction on the oil cap is not that high, really quite easy to remove once unscrewed, it indicates that the AOS is sound and the diaphragm not split etc.
On the 987.2 engines the AOS chamber is clipped together, the diaphragm is pinched between each 1/2 of the chamber mouldings, the AOS could actually be stripped, washed and re used.
 
If a ring is passing combustion products the case would quickly 'pump=up' and unless something somewhere gave way (like an engine seal) the AOS would be torn open, so I'm quite sure the rings are ok, BUT, best to remove doubt and test them.
 

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hotfrog1
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Re: Back to where I started... 2020/08/05 17:17:14 (permalink)
Your conclusions stack up OK for me.   It seems that you are looking for something that only happens during a particular cycle that is yet to be identified and even then it is marginal.     The cylinder leak check will rule that worry one way or other.   Stripping and cleaning the AOS seems like a good idea.

I am looking forward to the next episode.
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