1999 C2 AOS Manometer Readings

I�ve had some idle �instability� for awhile and have fixed many things as part of that process: vacuum leaks, misrouted vacuum lines, new plugs, new coils, new 996.2 injectors, new evap solenoid, and a new MAF. If idling with the AC off, she hovers right about 750rpm. But every minute or so while idling, she will dip down to about 600 rpm and it honestly sounds like she might die. The engine never does die, though. With the help and generosity of @De Jeeper , I was able to finally put a manometer on the car. The AOS is about 6.5 years old and here�s what the water column readings are. The first photo with higher numbers is with the AC Off. The second photo with the lower numbers is with the AC On. I guess the AOS is ok? I�m not sure what else could be causing the occasional idle instability other than possibly the Idle Control Valve? The age of the current one is unknown though it has been cleaned and tested for internal flap movement with a battery.

Isla, you can also remove the oil cap when the engine is running to see if its tough to get the cap off from high suction as well as when you do remove it, the engine begins to stall. If so pretty good chance you have a failing aos.

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I tried that trick a number of times before finally utilizing a manometer. It's not at all hard to remove the cap and I guess these manometer readings are inline with that.

I had my Indy check it last summer and my reading was the same as yours and he called it fine.

Make sure the -5inH2O reading is stable under starting conditions and when revving the engine. A little variation is OK, but I have seen them go way up when first started, then slowly come back to specs after a few seconds= NOT GOOD.

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This afternoon I went out and did a cold start with the manometer connected and the AC Off. The manometer reading immediately jumped up to 8.46� WC and then slowly drifted down to 7.xx, then to 6.xx and then settled at 5.9x by the time that 13 seconds had elapsed after start. I went and ran some errands and came home with the car nice hot (73� outside air temp in south ATL today). I turned it off and let it sit for about 15 minutes. I then reattached the manometer and started her up (again with the AC Off). The manometer reading jumped to 8.0x and then eventually drifted down to about 5.5x after about 10 seconds. @Porschetech3 You�re saying that the readings should never go any higher than 5.xx at any time?

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That's right, there is no reason for the readings to above the spec except that the AOS diaphragm is failing and unable to control the pressures. I have seen them go to -50 inH2O on start or after full throttle, then slowly come back to -5 in a matter of seconds.= FAILED The flow regulator uses 4 pressures to balance the spec. Atmospheric pressure and engine vacuum work to "close the flow valve" , Spring pressure and blow-by pressure work to "open the flow valve". Atmospheric pressure works/balances against blow-by pressure and engine vacuum works/balances against spring pressure, the result is a variation in flow needed to evacuate the blow-by gasses while keeping a constant slight negative pressure (-5inH20) or (1inH2O less with UAOS) on the entire crankcase.......that is until the diaphragm starts to fail and upsets the Atmospheric pressure in the equation.. The UAOS keeps a constant slight negative pressure on the crankcase during/after start-up or after full throttle, never goes above spec from the time the engine starts until it is shut off.

I am days away from having my AOS replaced due to oil consumption but smoke is almost zero other than start up, curious is you also have oil consumption issues? I�m about a quart every 600 miles but runs great. Thanks

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No smoke and no noticeable oil usage. My only issue thus far has been my mildly unstable idle.

Isla, get yourself prepared for a new aos. What you are describing is exactly how things unfolded for me over a 2 week period with my aos. In my case it was a motorsports aos not the oem aos. But here is what happened. No leaks. In my case no smoke. it tested fine at the beginning of the two week period with the manometer. Over the next week the manometer band started to widen but the car was running fine. Then one day I started up the car and I heard a muffled high pitch squeal coming from the aos. Removed the oil cap like i always do to test for vacuum, and it now had a lot of suction and then then the revs were falling and rising erratically with the cap off. My goal was to replace the motorsports aos with the uaos w/ the track drain anyways and then group a few other items together to replace as well such as imsb, oil cooler and a few other parts that just needed a refresh after 5-10 years so no biggie. But it was a fascinating process over 2 weeks as it started to fail. Everything fine then failure.

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You�re likely right. What�s interesting is that I�ve been dealing with this issue for about 6 months now. I�m a little gunshy on tearing out the AOS because I�ve spent so much time and money fixing other things (that, in hindsight, truly did need correcting) in hopes of getting to the bottom of this idle issue, but none thus far have done the trick. So dropping $800 on a UAOS or $500 on an ICV in hopes of the same gives me pause. But the manometer readings don�t lie and there�s no doubt that Skip�s UAOS is the way to go. I literally just got done doing a full suspension rebuild with new arms, hardware, and Koni Special Actives. I spent about $2500 and two weeks of my time to do it but the difference in handling is incredible. Hopefully the result from installing the UAOS will be the same.

Yep, I definitely hear you. If it's any consolation, I've had mine for 20 years and I am going thru my second major refresh of engine parts littered with many minor part replacements. I went thru 2 factory aos' in the first 4 years, then put on the motorsports aos way before the uaos was invented and now am I upgrading to the uaos. One less part to worry about over the next 20 years.