Diagnosing bad AOS (AirOilSeparator)?
03-06-2019, 03:46 PM
#1
8th Gear
Thread Starter
Diagnosing bad AOS (AirOilSeparator)?
My car (2005 turbo with 19 000 km on it ) has suddenly rough idle and generally uneven running. When checking with my Durametric there is no sign of bad ignition coils and other
error codes like MAFsensor or oxygen sensor. (I have changed one coil with very good result some years ago.)
I suspect a bad AOS Membrane. But it seems hard to remove the cap without destroying it.
Could this be a good way of diagnosing?: Disconnect the line between the air intake line and the AOS.
Put some tape over the air intake connection so it is air tight. Leave the AOS connection open. If the idle/ is OK then it is probably the AOS membrane. Or will I fail anyway due to open connection at the AOS?
My guess is that there is some minor over pressure from the engine.
(I found this here at Rennlist but don’t understand it. I will check it since it is so easy to do the check: The for sure way to tell if the AOS has a ruptured diaphram is to remove the oil filler cap. If the engine starts to run rough, that is the problem. If it idles NO differently, the AOS is not suspect.)
error codes like MAFsensor or oxygen sensor. (I have changed one coil with very good result some years ago.)
I suspect a bad AOS Membrane. But it seems hard to remove the cap without destroying it.
Could this be a good way of diagnosing?: Disconnect the line between the air intake line and the AOS.
Put some tape over the air intake connection so it is air tight. Leave the AOS connection open. If the idle/ is OK then it is probably the AOS membrane. Or will I fail anyway due to open connection at the AOS?
My guess is that there is some minor over pressure from the engine.
(I found this here at Rennlist but don’t understand it. I will check it since it is so easy to do the check: The for sure way to tell if the AOS has a ruptured diaphram is to remove the oil filler cap. If the engine starts to run rough, that is the problem. If it idles NO differently, the AOS is not suspect.)
03-06-2019, 11:17 PM
#2
RL Community Team
Rennlist Member
Rennlist Member
There are a couple of threads here about this. A search should turn up the specifics that you need. It is possible to remove the cap without damaging it and there are aftermarket diaphragms available on eBay. Save your old one to compare the size as you may have to trim it slightly. IIRC, Porsche only sells the complete valve cover and AOS together for $$$.
Removing the oil filler cap will help you diagnose if it is the AOS or not.
Removing the oil filler cap will help you diagnose if it is the AOS or not.
03-07-2019, 08:02 AM
#3
RL Community Team
Rennlist Member
Rennlist Member
The oil filler cap test is not a 100% type test. Cars where you could remove the cap can still have a bad AOS The only way to test it with 100% certainty is with a manometer.
Your car is really an 05 with 19k km on it? How long has it had the current tank of fuel in it and do you stabilize the fuel when you fill up the car since it must sit for long periods at a time? Gas can get stale within a few weeks. (3-4) where you can start having runnability issues. If the car isn't used very frequently, add a bottle of Stabil 360 each time you fill it up and deep it on a Battery Tender too. This is what I do with all my P-cars and something I started doing after two different episodes of fuel related runnability problems in my silver 911 in two different years. Problem completely went away on the next tank of gas, both times.
Your car is really an 05 with 19k km on it? How long has it had the current tank of fuel in it and do you stabilize the fuel when you fill up the car since it must sit for long periods at a time? Gas can get stale within a few weeks. (3-4) where you can start having runnability issues. If the car isn't used very frequently, add a bottle of Stabil 360 each time you fill it up and deep it on a Battery Tender too. This is what I do with all my P-cars and something I started doing after two different episodes of fuel related runnability problems in my silver 911 in two different years. Problem completely went away on the next tank of gas, both times.
