OBD Idle Vacuum Data-Need a Intelligent Interpretation !

...a question for the technicians; using an obd can one confirm that the AOS and vacuum lines are good? My non technical brain is inclined to think that a strong idle vacuum reading is a sign of a healthy engine and AOS, and non leaky vacuum lines? Is my thinking correct? For example my 996 Gen 2 shows around 24 ish (idle intake vacuum) on a cold idle which is about the same as my newer BMW 630i....

That is an interesting question .. not one ive ever looked at before . For vacum lines ie splits then you have additional air entering the intake after the maf so it would show on a tester useing the addaption figures... Maf readings may also be low . If too much air then you WILL get a fault code as the car can only addapt so far. You would see that the car is haveing to richen the mixture to maintain Lambda . The car would probably still run ok as the car can compensate for the air leak ( it increases injector duration basically ) . For an AOS though .. hmm .. the main part is oil getting past the AOS and into the intake which in turns get burnt and causes smokeing . In theory you would get something on the addaption figures but i dont see how you could nail it down to an AOS .. it could be many things and without any smokeing its not something i would jump at. If i thought an AOS was in the early stages of failing then i would remove the throttle houseing and see how much oil was in the intake system . There is a problem with this method though .. there WILL be oil in the intake .. its normal .. so the decision is how much oil is normal or a problem. I have seen what was an AOS by this method before but ive seen many intake systems so know roughly what is normal .. even so i could get caught out so its not a method i would advise. Rule of thumb .. if its smokeing from both banks when warm then an AOS is the most obvious and you would then check as above if needed. Your reading of 24 .. is that maf kg/hr ? To test the maf .. Car running and warm .. a/c off and its about 16kg/hr .. a/c on and about 18kg/hr. Ign on but not running and look at the maf voltage on the tester 0.9-1.1 volts and its ok .. well .. it shows its not contaminated but imho it doesnt prove the maf is ok . On cold start it will be higher .. revs are higher Thats just my opinion though and im no expert .. if anyone has more info then i also would love to hear about it .

...thanks for this , I was hoping you might respond as I always find your input into questions raised really informative Demort. The reading from my Bluetooth OBD is for intake vacuum pressure measuring in - in hg. So my 3.6 shows around 24 in/ hg at idle. The same as my my much newer 6 series coupe, also a six cylinder. Having read some information on the interweb vacuum readings appear to reveal a significant amount about an engine overall health and condition? Thank you...

Well .. vacum used to be an important tool in the testing or checking of things . Ive still got a vacum/pressure gauge i used to connect with the readings of what it should be and what it would read if a cat was partially blocked. These days there are many sensors on a car which tell us an awfull lot . You could say the maf readings are similar .. the volume of air entering the car is indeed a usefull tool for diagnoseing some items. 996 /986 was the first Porsche with Can bus and various new and more accurate sensors .. The 991 has a lot of sensors , control units and if im diagnoseing one i tend to sit in the car stareing at pages of read outs on the tester for 45 mins easily before even bothering to look at the engine ! From the 997 onwards there is a seperate pump to supply vacum for the brakes as at several times there just isnt enough in a normal mainifold for them to work . Ive not seen those readings on a tester before but 24 is what i would expect on a cold engine if looking at the maf readings so perhaps its that :dont know: .. there are no pressure / vacum sensors on this car though . Mechanics may well still use this system .. i just use what works for me though .. and i love electronics and gadgets more EDIT .. One thing i would add is that im looking for a fault when i work on these .. for a general test then a compression check , leak down test and a scope check is what i would recomend .

...thanks for this Demort and no faults to report thankfully just interested in interpreting the reading as car and OBD are both new to me...it must be the maf reading ... Best wishes Giles

No worrys .. Feel free to pm me if needed or if you need any specific info .. i have " a few " interesting reads depending on how far into car details you want to go .