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YOUR RESEARCHTested my AOS today, what does this mean?
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Replace annually
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Extended periods of exposure to engine oils only changed annually, that become acid laden will create this over time.
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Tonight when I tested I had drive the car home from work, about half city, half highway, in 80 degree ambient temperatures and got the 5.5-5.8 inH2O.
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I use a manometer on a daily bases for work and they need to be calibrated.
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All this AOS talk got me nervous so...Got car hot, turned off A/C, put on test oil cap, used same tester as the OP and got 5.2 to 5.35!
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Suggestion
Based on Jake Raby's response above and the fact that you are having readings over the threshold that he recommends as well as the higher reading earlier, I would think about replacing the AOS.
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Unfortunately (for me), they mean replacing the AOS in the near future (it's not bad, but the results are above the norm), I will consider various options (Ultimate AOS as one of them).
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Although Pelican has a DIY article for replacing the AOS, I would search here on rennlist since I have found allot of additional helpful hints that the Pelican DIY article does not cover.
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I have seen some cars that had the bad AOS masked by a MAF that was dirty, or failing, and the negatives canceled each other out, and made for a perfectly running engine that did not smoke, yet ate engine oil like an intoxicated fish.
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I got a manometer and made a cap for testing. At idle, engine warm but not hot, the readings I got are below...showing -7.9 to -8.5. This seems to be at the higher end of OK?? Should I be concerned? Thanks in advance.
When I checked mine, the pressure cold was 5.68 in water column, warm but not driven was 5.37 and fully up to temp was 4.81 so try again after car is fully up to temp.
Mine is 5. It won't lower your reading but smear a little silicone grease on the seal of the cap to ensure you get a good seal. Calibrate the Manometer to prevent a false reading.
Check plenum for oil quickly. Once it spreads ,it gets everywhere!
Checked again today after getting the car good and hot. Reading is now 11.5-12.5 inH2O. Does this indicate my AOS is toast? I don't see any smoking at startup and the car seems to run well otherwise.
With engine running how hard is it to get the oil fill cap off? Just as a check for your manometer. If you have 11 in water column the cap should be very difficult to remove due to the negative crankcase pressure. Also hope Jake sees this thread and gives his expert advice.
you may also want to add a tube onto other nipple or move manometer away from motor, any positive pressure, wind, air movement from motor will add additional positive pressure to that nipple giving a compromised reading. I use a manometer on a daily bases for work and they need to be calibrated.
When I remove the cap there is vacuum I have to pull against to get it off. I would not say it's difficult to remove but there is some resistance. This manometer is the $30 digital one from Amazon, I see no calibration function in the owner's manual. It does show zero with nothing connected. There is a second nipple on it that I was just leaving open, is it a flow-thru design so one is the intake?
A reading of 11.5 in of water column is ea to approx 60 psf so it should be difficult to remove with that kind of pressure. My reading of 4.8 in is equal to approx 30 psf and I could feel the negative pressure when removing.
the two nipples are not a free flow but if you hook the hose to one side it will show measurement in negative (-) the other will show positive (+). The calibration is done in a lab and the cost is more than your instrument.
you could make your own fluid manometer with a piece of clear hose (just google it) and use light weight oil (not water) just in case some gets sucked into fill tube.
Yes, that is a fluid manometer but you can just buy a length of clear hose and make your own. Google it and you will see that u just measure both sides of fluid level from the zero mark and u get a very accurate reading in inches of water.
Ok so now I am really confused. Tonight after work I tested again using my homemade manometer as well as retested with the digital manometer.
Both of them show about 5.5-5.8 inH2O now.
Why would I be getting different readings now? When I tested and got the 12.5 on the digital manometer I had driven the car about 45 minutes in about 65 degree ambient temperatures. Tonight when I tested I had drive the car home from work, about half city, half highway, in 80 degree ambient temperatures and got the 5.5-5.8 inH2O.
Thoughts???? (Pics below).
In your first picture, you're measuring the manometer wrong. The measurement should not be from the open end to the top of the fluid, but the difference from one side's fluid level to the other. Doesn't look like a huge difference, but still worth mentioning. Kinda hard to judge perspective in your picture, but it looks like the measurement is around 7.5" of oil, or 6.4" of water. I would trust this measurement over what was done with the digital one since we don't know how that's calibrated or how good that instrument is.
The magic number is 5 inches of water, up to 6 is normal... When I see more than 6.25 I get suspicious.. When its over 7 the AOS is weak for sure.
I do the test with the AC off, idle speed of less than 1K, and the engine fully warmed up.
Yes I was trying to hold it and take a photo at the same time. I was intending to hold the left end of the tube even with the oil level on the right when measuring. While watching it I noted it hovered between 6.5 and 7.5" of oil depending on how high I held the apparatus relative to the engine level. Regardless whether it's 6.5" or 7.5" it's still well within spec for the AOS.
What is perplexing is I was seeing a measurement of 12.5 on my digital meter, but now it shows similar to the slack tube at 5-6 inches measuring the same way as I did yesterday getting 12.5. Is the AOS function temperature dependent? The car was hot when I was getting 12.5, but it is hotter today than yesterday when I got the 12.5 readings. I took all these readings more than one time so I don't think they were any sort of error. Thoughts?
Based on Jake Raby's response above and the fact that you are having readings over the threshold that he recommends as well as the higher reading earlier, I would think about replacing the AOS. If you have some mechanical skills, you should be able to do this yourself and save a ton. Although Pelican has a DIY article for replacing the AOS, I would search here on rennlist since I have found allot of additional helpful hints that the Pelican DIY article does not cover. Why take a chance on a $140.00 part that is known to go bad and in the worst case could cause catastrophic failure. Just my two cents.
After reading his post I am thinking the same thing. According to Pelican's tech article they feel that up to 7 is OK, maybe even up to 9. Clearly Jake is the authority on this so I'll have to go with his numbers.
I'll probably just drop the engine and transmission, it seems easier to do that than trying to change it with the engine in the car. I know my way around the garage and have done some engine, transmission, differential, etc building in the past and I have an EzCarLift in my garage which should make the drop relatively easy.
Thanks for everyone's help on this. I have various cooling system stuff to do this winter so I hope this AOS will last until then and I can tear everything apart at once.
Not to add to your confusion, but it looks like you have your home-made tubing filled with oil - good idea since it won't contaminate the engine. But, while the difference is minor, the density of water is 1000 kg/m3, and oil ca. 850. Your post #17 shows an instrument reading well within spec.. Without any symptoms (smoke, whale call noise, etc.) I'd be somewhat comfortable until you have reasons not to be.
So here is something interesting....I got to thinking about the two different testing results I got with the digital manometer. The first time I tested was with the A/C off and was getting readings of 10-12. Yesterday when I tested with both the digital and the homemade slack tube manometer I was getting similar readings of 6-7ish. So I realized that the first time I had the A/C off and yesterday I had it turned on.
So tonight after work I retested it with the digital manometer with the A/C off and On. Here's what I saw:
A/C on = 6.0-6.3 inH2O
A/C off = Vacuum steadily increasing from 6.0 up to about 10-12.5 after turning the A/C off.
Now why would turning the A/C off cause an increase in crankcase vacuum? Is there some component of the A/C that would affect crankcase vacuum?
No idea but in Jake's response he noted that he does his test with A/C off and rpm's under 1000. I can not remember if when I tested mine the A/C was on or off so I may play around with it tomorrow and see if I get those types of variations. Will keep you posted.
The difference comes from engine load that increases with the AC engaged. AOS vacuum depends on intake manifold vacuum, as the AOS bridges the crankcase to the throttle body.
The higher the load, the more the deviation you'll see from the AOS reading.
Dont over think this. Leave the AC off and test the AOS. All the empirical data we have for AOS health via manometer has been gathered and compared with the AC off. Leave it off.
Yes the engine can run perfectly and have a weak AOS that fails the manometer test.
Clearly based on that then my AOS is weak at the least as the readings w/ the A/C off are approaching 12.5 inH2O. Guess I'll keep the A/C on until I get a chance to replace it!
As Jake mentioned, we are over thinking this. Since his data is based on an up to temp engine at idle with air conditioning off, we should use his data as a bench mark. I do not know of anyone else that has evaluated the AOS in the detail that he has. With that said, the following if for curiosity only and should not be used as a reliable source in determining the condition of your AOS.
I performed some additional testing of the crank case pressure. I drove the car for approx 30 minutes on some back windy roads (wanted to have some fun) then performed the crankcase test with my Dryer Series 475 Mark II Digital Manometer. I have this Manometer calibrated annually as required in the standards I use it for.
The 1st photo shows the hook up of the manometer. The 2nd photo shows the reading of 4.55 inches of water with the air conditioner off and at idle. The 3rd photo shows the reading of 4.81 inches of water with the air conditioning on. There was a slight rise in pressure and I could hear the additional load on motor when turning on the Air.
Again, not sure this means anything but the OP had the same curiosity as I did although his readings were greatly different. Some additional info, my car has approx 60,500 miles on it and I had the 60K service performed approx 1,000 miles ago.
Thanks for posting that Tex. Clearly my readings are high indicating my AOS is failing. Strange that when the load of the A/C was removed from the engine (A/C off) my numbers actually went up, now down as you would expect. I can't explain that.
FWIW my car is a 2003 with 21K original miles. Age matters more than mileage at this point.
The only explanation I could come up with in my mind is that there is a leak in the diaphragm and the increase in load w/ the A/C on pulls the leak closed, such as if it was at the edge of the diaphragm or something. That is 100% speculation...I can take it apart when I get it out to see.
When the AOS gets tired, the diaphragm will often soften up. Extended periods of exposure to engine oils only changed annually, that become acid laden will create this over time. There's 3 ways that an AOS can fail, and 6 different forms of symptom that the engine can exhibit based on what part of the AOS failed.
I have seen some cars that had the bad AOS masked by a MAF that was dirty, or failing, and the negatives canceled each other out, and made for a perfectly running engine that did not smoke, yet ate engine oil like an intoxicated fish.
It took the Porsche world years to start using manometer readings to measure AOS health, and unlike BMW, Porsche has never released manometer values for these engines. Like everything else, we've had to do all of that ourselves.
Engine RPM will dip slightly when the AC compressor turns on - or at least it does in my car - if the car is warm and nothing else is on. Even a slight variation in speed at low RPM significantly affects vacuum, including crankcase ventilation.
If you turn the AC on/off at 7K RPM, the vacuum differential would be negligible, especially if the AOS membrane is damaged but not destroyed...
That would make sense given that my car is 12 years old with only 20K miles on it. I'm sure it sat a lot. The car is new to me and I don't have much history beyond the last 2 years. I do see suggestions that the car was tracked or autocrossed at some point, there are 5 point harness eyelets hidden under the carpet and I have found track rubber stuck under the wheel arch linings...so it could have been some hard miles in that 20K but given how nice the paint looks (and I believe it's all original paint) I doubt it saw more than occasional track use. Having owned a track-only car myself in the past I know how the track tears up the paint w/ rock chips.
The point is that the car probably sat around a lot, driven only occasionally over it's life. Most likely this was mostly street driving with a few trips to the track or autocross but probably not much. So not surprising to see the decay of what happens when cars sit around and are not driven.
All this AOS talk got me nervous so...Got car hot, turned off A/C, put on test oil cap, used same tester as the OP and got 5.2 to 5.35! I would be insanely happy if not for the ticking on the right side of engine!
Hello everyone in this great rennlist community!
I've added a Porsche 911 996 to my garage and started using the impressive knowledge base that this forum is.
Let me resurrect this thread, which inspired me to build my own AOS manometer from a silicone tube.
I compared the result with an electronic meter later and the results are practically identical as you might expect.
Unfortunately (for me), they mean replacing the AOS in the near future (it's not bad, but the results are above the norm), I will consider various options (Ultimate AOS as one of them).
I am resurrecting the thread, in case anyone is interested in how to perform this simple test - I recorded a video. It is in Polish, still has English subtitles, the actual test starts at 8:18.
In summary, You don't have to buy a manometer, the "garage test" is accurate.
Here it is:
Porsche 911 996 test AOS (DIY Vacuum Test and Electronic Verification)
