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Hiddeous Intermittent Smoking But AOS is Good

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I am having the familiar intermittent smoke bombs out the exhaust but I have at least some reason to believe that it's something other than the air/oil separator. I say that because I have manometer I use and when things are normal I never see the vacuum get beyond about 6" of H20. But when it smokes the vacuum not only disappears but there's actually pressure in the crankcase up to about 25" of water (~1PSI). Yes, pressure as in the meter turns to a reading with a minus sign! I can almost cause this at will by alternating between full acceleration and full deceleration and the problem occurs during the acceleration phase. I don't understand enough about these engines to know why or how a positive crankcase pressure can be created as I gather the AOS regulates what is supposed to be a very small vacuum in the crankcase. Can someone help me out with this?




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Welcome to RennTech :welcomeani:


I would have to say that the AOS is bad.  We check every car that passes through the shop with a digital manometer, and if any one exceeds 5 inches, we recommend replacing the AOS.  You also should not be swinging from vacuum to pressure in the crankcase as the low tension piston rings will loose seal.  With the engine running, you should always have some level (3-5 inches of water) in the cases.  You also have a PCV-like valve at the cam cover end of the long AOS line, which may be momentarily sticking.  Not a lot of fun to get at, but not expensive to get a new one.

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Thanks for the advice as I didn't think 6" would indicate that the AOS could be suspect. It was replaced already but I've seen on the forums where new ones are sometimes defective. It seems pointless to replace it though until I track down the source of the pressure. I'll see what I can do with the PCV like valve that you mentioned. 

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A good AOS will pull between 3-6 inches of water on a first start of a cold engine, but will then settle down to 5" or less, so anything over 5" on a warm engine is suspect and should be replaced.  Secondly, with your crankcase jumping back and forth between vacuum and pressure, your AOS can also be "fluttering", working some of the time, but not all the time.


New AOS unit can be bad, we have seen more than a few.  Aftermarket (read non factory) AOS units have been a disaster, with many failing within days of being installed.  We do not use anything but factory AOS units, and we still check everyone after it is installed because we still see bad ones from time to time.

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I just replaced the AOS with a brand new one from the dealer. All works well but still having smoke bombs during sustained full throttle where vacuum turns to pressure up to about 25" of water. This vehicle is a 2000 S and I hooked my manometer up to a '99 to compare numbers. Generally the numbers are comparable but I never see a pressure condition on the '99. On the 2000 S when under full throttle you can see vacuum heading toward zero and a rapid pressure buildup and then a smoke bomb when it's sustained. 


I have not tackled the check valve yet as I'm trying to understand how that can cause a pressure condition. I am told on later model Boxsters that check valve is eliminated entirely. Any idea what the check valve "checks" and in which direction? 


In the end it seems that the smoke bomb is a symptom of the pressure condition occurring at wide open throttle but I don't understand what would be causing that pressure and only at wide open throttle. 

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It is a one way valve that is in the end of the cam cover that opens under vacuum.


I would also seriously consider running a leak down on the engine that develops pressure; if you have a bad ring in one or more cylinders, or the more common out of round cylinder liners, your problem is more serious.  You should not have pressure in the cases while the engine is running.

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  • 2 weeks later...

I tested compression (150-160) and leak down (<9%) on all cylinders so it checks out good. That's not a surprise because the car runs great aside from the smoke bombs. I initially thought the condition only occurs at WOT but it's much more interesting than that. Believe it or not it's somehow related to engine temp. When the car is cold there is no pressure build up and the crankcase maintains normal vacuum numbers. As the engine warms up the crankcase vacuum gets smaller until it turns into pressure. At this point I'm talking with respect to WOT. Once at normal operating temp the pressure numbers go crazy like in the 30s and 40s (inH2O) under WOT. But I also noticed that (when warm) the pressure buildup occurs even under moderate acceleration. With the manometer attached I'm able to cut the acceleration when I see the pressure building so as to prevent the smoke bombs. What I describe here is repeatable and not just a one off circumstance. I'm completely stumped.


Also, I have given more thought to the check valve you referred to that opens under vacuum. The only checkvalve I can find is 996 107 047 51 and that is a pressure relief valve that I gather opens somewhere about 65 - 80 inH2O. Is there something else that I'm missing?


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I think you will find that the valve opens much sooner than 65 inches of water.  And yes, that is the valve on the cam cover on the other end of the long hose from the AOS.


You still have something amiss in the sump evacuation system, as these are very low tension ring engines and not designed to work well unless there is slight vacuum in the cases.  When your engine goes to positive pressure, the rings are no longer able to control the oil and you get the smoke.  It should not be able to go from vacuum to pressure.

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  • 4 months later...


This is an odd problem and the numbers on the manometer are alarming. Just wondering if any work was done on the engine prior to this problem? If I had this problem ,I would be convinced I fitted something the wrong way round , left a hose off. broke something accidentally.

Thinking this through = the bomb as you call it is produced by combustion pressure blowing into the crankcase when hot but not when cold .But you have excellent compression on all cylinders when cold. Something changes when the engine reaches full operating temperature. What ? It would be difficult, but could you do either a blowby or compression test when hot ?

You might want to visit David Allen ?


Edited by Schnell Gelb
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JFP recently helped me with a similar problem. Like you I thought it was just the usual AOS issue so removed it and pondered replacing the crankcase vent valve that you were curious about. Here is what JFP said to help you :" The vent valve is rarely involved as when it fails, it tends to shut off or reduce vacuum flow to the crankcase.  As they are cheap, if you were to take it out to check it, I would just can in and put in a new one.  And by-the-by, it is not at all unusual to have short lived AOS units, particularly if they are non factory.  We have had exactly zero luck with aftermarket units and now only use factory. "

My(2001 Boxster S) AOS failed (or so I thought)while doing repeated Durametric tests on the Bank 1 Variocam Actuator+solenoid at 3000 rpm. Clouds of white oily smoke. It is a newly rebuilt engine(but diy so who knows !)

See Post "AOS + Crankcase vent valve -replace both ?" Not the ones below which I tried deleting.

I am beginning to think you and I  have the same problem . Here's why. I carefully dissected the 'defective' fairly new AOS. The diaphragm is perfect. I tested the diaphragm by filling one side of the unit with water to observe leakage.  Not even a pin hole. It was a fairly new AOS marked "MHO Germany" 996 107 023 04. It was purchased from Pelican and fitted by the P.O. Is a split diaphragm the only Mode of Failure for the AOS? MHO = Mann Hummel ? But no idea if they are defect prone.

Is it possible your 2nd AOS was also defective?

Until I read your Post , I never thought of the possibility it was anything but an AOS problem.

I also had black .sticky,oily liquid around the exhaust manifold of Bank 1. I removed the manifold and found it also on the metal gasket. Do you have this also? I had assumed this was caused by the AOS failing. Now I am questioning my diagnosis.It was way to smokey to contemplate doing a Manometer test.

Edited by Schnell Gelb
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