JFP in PA

I'm not fan of fixing cracked parts unless there are no replacements. Usually, they just crack again.

Sorry, but there are no real "magic Band-Aids" for this, if you have a cracked component, it needs to be repaired or replaced.

More than the data this could yield, I'd still like to know what the crankcase vacuum level is on your car; one min. of diagnostic time would immediately confirm or deny the AOS is going bad. I'd be willing to bet that we would see a vacuum signal somewhere between 5 and 10 inches of water; enough to be annoying and causing running issues, but not yet shot enough to disable the car and turn it into a mosquito fogger. Oh the joys of long distance diagnostics...........

John, this is interesting. Why will AOS leak make MAF read higher? Even I have a setup like that at home :) You have hit on the $64 dollar question. We know from direct experience that MAF's on their way to "aging out" often read higher than expected air volume flows; we have seen this on Porsche's as well as other brands, so it is not an unexpected event. So when we saw higher than normal MAF air volume numbers on cars with obviously dying AOS units, we kind of wrote it off to an aging sensor. But then someone brought it to my attention that they had seen similar high air flow volumes on cars with bad AOS units, that subsequently (but not always) dropped back into range when the AOS was replaced, so it was not always a dying sensor. Always intrigued by a new mystery on these cars, and not wanting to needlessly be replacing expensive MAF units, we started looking at cars we got in that failed the AOS vacuum test (vacuum signal at the oil fill caps greater than 5 inches of water). And what we saw mimicked what the other observer noted, the air flow volume often changed, which set us scratching out heads. On the 986 in question for example, the AOS line to the intake system enters behind the throttle body and ahead of the intake plenum, and the MAF is located a couple of feet "upstream" of the throttle body, near the air filter box on the driver's side rear fender; so intuitively it would seem rather too remote to see a change in the vacuum signal level where the AOS line enters the intake. But it apparently does under some conditions. Why this happens remains an item of speculation, and the amount of air volume change is not always large, but always seems to be in the same direction when it happens. An interesting experiment would be to set up a car to mimic a failing AOS (which would require being able to "throttle" the AOS crankcase vacuum level up or down, while monitoring both the intake plenum vacuum levels and the MAF air flow volumes) and collect some additional data points in an attempt to see what is going on. Unfortunately, in the shop we are nearly always in a time crunch to figure out what is wrong and get the car back into the owner's hands, so we have not attempted this. It would be an interesting R&D experiment however. Thanks John for the detailed reply. My thinking is as follows: Suppose everything is working correctly on a warmed up idling engine. Now all in a sudden the AOS diaphragm cracks. That allows MORE crankcase gas (oxygen deprived and even fuel-enriched) into the throttle and LESS fresh air from intake, Both effect will create an instantaneous rich condition. Engine will start to stumble due to wrong a/f mixture. To maintain a minimum rpm, the DME compensates by opening up the throttle a bit more to let more fresh air (oxygen) into the engine to achieve the correct a/f ratio. At the end the MAF will see more flow (in mass per second) and the rpm may be raised a little too (not sure about this though). When you observed the MAF read a bit higher than normal, I assume the rpm was at a normal value and not raised? I mean if the rpm was also raised, it's just natural the MAF read higher. I think it will be quite difficult to simulate the crack inside the AOS precisely. One will need to build an adjustable bypass valve between the AOS vent tube that's connected to the crankcase and the AOS vent tube that's connected to the throttle body. Will be an interesting experiment nonetheless :) I would expect that if the your scenario were correct, the idle should go up and down, and with the repeated A/F issues, the car would also code because the mixture would eventually have to swing out of range one way or the other more than once. We observed the higher MAF volume numbers at a normal (and steady) idle speeds. The basic problem with the AOS design is that it is way too complicated, too fragile, and has too many ways of going south on you. We thought about how you would have to modify one to test our observations and it quickly turned into a plumbing and electrical nightmare. Sometimes the view just ain't worth the climb.............

I will repeat what I have said many times before: I have never seen one of these engines blow a head gasket, nor have I ever heard of one from a credible source. These engine's use a multi layer steel head gasket, which is way stronger than either the cylinder head or engine case castings. You have a crack somewhere.

John, this is interesting. Why will AOS leak make MAF read higher? Even I have a setup like that at home :) You have hit on the $64 dollar question. We know from direct experience that MAF's on their way to "aging out" often read higher than expected air volume flows; we have seen this on Porsche's as well as other brands, so it is not an unexpected event. So when we saw higher than normal MAF air volume numbers on cars with obviously dying AOS units, we kind of wrote it off to an aging sensor. But then someone brought it to my attention that they had seen similar high air flow volumes on cars with bad AOS units, that subsequently (but not always) dropped back into range when the AOS was replaced, so it was not always a dying sensor. Always intrigued by a new mystery on these cars, and not wanting to needlessly be replacing expensive MAF units, we started looking at cars we got in that failed the AOS vacuum test (vacuum signal at the oil fill caps greater than 5 inches of water). And what we saw mimicked what the other observer noted, the air flow volume often changed, which set us scratching out heads. On the 986 in question for example, the AOS line to the intake system enters behind the throttle body and ahead of the intake plenum, and the MAF is located a couple of feet "upstream" of the throttle body, near the air filter box on the driver's side rear fender; so intuitively it would seem rather too remote to see a change in the vacuum signal level where the AOS line enters the intake. But it apparently does under some conditions. Why this happens remains an item of speculation, and the amount of air volume change is not always large, but always seems to be in the same direction when it happens. An interesting experiment would be to set up a car to mimic a failing AOS (which would require being able to "throttle" the AOS crankcase vacuum level up or down, while monitoring both the intake plenum vacuum levels and the MAF air flow volumes) and collect some additional data points in an attempt to see what is going on. Unfortunately, in the shop we are nearly always in a time crunch to figure out what is wrong and get the car back into the owner's hands, so we have not attempted this. It would be an interesting R&D experiment however.

I went to the largest Porsche dealership in the area, and they seemed familiar with the AOS when I asked them to specifically check it as well. Just remember one thing about dealerships: Good techs typically leave dealerships and go to work for independent shops, both because they can make more money, and don't have to toe the corporate line. A lot of dealerships out there will still tell you that it is impossible to replace the IMS in one of these cars without tearing the engine apart, even though it has been done everyday of the week for over eight years now in the field.

In general, no, because an air leak into the engine cases would lower the vacuum signal, not raise it. This is specific to measuring the amount of total vacuum the AOS is putting on the system. When the AOS goes bad, the vacuum level gets too high (greater than 5 inches of water), not lower, which would be a sign of a leak elsewhere in the system.

Unfortunately, it is that simple. Everyone in the Porsche repair trade knows of the fragility and problem prone nature of the AOS, and should be ready to properly test them. This is what you should find at any Porsche shop worth its salt: It consists of a digital manometer (accurate +/- 0.1 inches of water), some rubber tubing and a modified oil fill cap. Screw the cap on the car, hook up the manometer, and start the car; you immediately know the health of the AOS. I also hope you ordered an OEM AOS replacement, we have had very poor luck with aftermarket units.

Which is why I suggested running a manometer test on you AOS. Going all the way back to some of your earliest posts on this, I noted what appeared to be an overly high flow volume in your MAF data, which is exactly what a low level AOS leak would give you. You need to get that AOS either tested (just a few min.), or simply replace it (several hours work). I know what I would do if it was in my shop; it would have the manometer on it in about 1 min. to confirm or deny the AOS as problematic.

Or you could simply go to board sponsor Sunset Porsche, and get the real thing at a great price.

They need to connect a digital manometer in place of your oil cap and measure the vacuum in the engine case; on cold start it should vary between 4-7 inches of water, but when warmed up should read 5 inches. Any higher and it is a sign of a vacuum leak in the AOS.

How difficult was it to remove the oil fill cap? If it required some effort, you have a bad AOS (a good AOS would show a vacuum signal of 5 inches of water, which is relatively weak).

Many.

Welcome to RennTech :welcome: Other than by long term corrosion, I have never seen anyone shear those bolts off. Something must be misaligned or otherwise incorrect.

Your "PIWIS" sounds like one of the many Durametric system clones sold on the internet. A real Durametric system would give you the option to activate the ABS system for bleeding/flushing purposes. You can always get one of the real Durametric systems and do it yourself sooner.

If you do that, you will also lose any data that could have helped you permanently correct the problem. Bad idea.

Without a scan for what is causing the issue, fixing it quickly becomes a potentially expensive guessing game. Do you have access to an OBD II scan tool of any kind?

When we get one in the shop with a bad bleeder assembly, the car gets an entirely new tank; you can sometimes replace just the valve itself, but repeat failures are far too common. Get a new tank, you will save yourself grief in the long run, and do not buy an aftermarket tank, get a Porsche unit. We have seen way too many aftermarket tanks fail after short periods to ever consider using one.

Welcome to RennTech :welcome: Yes, you need to be careful about what year roller you buy as there are multiple system changes (CAN bus, VarioCam, fuel system, etc.) between the model years. You might want to look for someone with an IMS or engine failure, as the those cars should otherwise be in good condition. Breaking up the car is always a crap shoot as some pieces would move quickly, others not so much, and the process takes a lot of time and effort. Very few people ever benefit from going this route.

The Durametric system will activate your ABS system during a bleed/flush to remove any trapped air, which is always an issue after draining the system, eliminating the need for two people. I am also not completely convinced that using two people would work on a car that had been fully or even partially drained, as activating the ABS system with a diagnostic tool activates components that pumping the pedal simply will not do.

A quick scan of the car with a Porsche diagnostic tool would give you better information, but it may be the sensor.

Hello Loren; Thank you for your reply; EEPROM is still working; reading and writing; but it has been deleted, so it is empty. Do you think a dealer can reprogram the key? The car is programmed to accept the key, which is what Loren is referring to.

Not really, some of these "stealth" units would be so difficult to remove, it simply isn't worth while, and their removal would actually diminish the car's value.