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Hello, I am really worried, I noticed a coolant leak on the driver rear side of the car. The leak is on the back. Starts leaking after the engine gets hot and it seems that stops after a while because I don’t lose that much coolant. I check the coolant and add a little bit before driving it. No overheating. 

at first I thought it was a hose coming out of what now I think is the AOS. But now I think is the AOS itself that might have a tiny crack. Is that possible? Coolant from the oil separator and not oil leak? How serious is this? Any help really appreciated. As much as I wish I could do mechanical work, I think this is out of my league.

Thanks!

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The AOS has two coolant lines connected to it, held by two small hose clamps. The leak could occur at the connection. One will need to pressure test the cooling system and visually check where the leak is coming from and go from there.

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Thank you. I saw the coolant spray out after around 20 minutes of the engine running. I thought it was the connection with the hose, however later I realized that it seems the coolant to be coming from the plastic itself, from the top close

to the hose connection, however the way it is spraying it seems like it is really coming from behind the hose which will be the AOS. I don’t have any idea about parts or how the flow of coolant works, however it seems the AOS is cracked and leaking coolant, so that is possible I guess? Also as long I keep watching coolant

level and temperature is it safe to drive it for few days until I get it fixed? It is not a mayor leak and the engine does not overheat, however I think it is also affecting my catalytic converters as I just got the P0430 and P0420 codes. Thank you for your help!!

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Agreed with everthing you said except that the AOS should not affect your cats. I'm curious why you get both p0420 and 0430 the same time. So your CEL just came on now?

 

Yeah, it's possible the AOS is cracked because it is made of plastic.

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So the AOS needs to be replaced. My 996 had the fvd Bronbracher exhaust, could that be the reason why? I bought the car with the exhaust already installed recently and it is possible the code was deleted by the previous owner and not disclosed to me. 

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If the aftermarket exhaust is after CAT mod, it should not affect the cats barring any exhaust leak. Yes, it's possible the codes were erased before because it takes time for the DME to run repeated CAT tests before it gives up at n then and trip the CEL. Could take hundreds of miles depending on how bad the cats are.

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My AOS casing cracked and leaked coolant.  It was not a major coolant leak, but enough that it leaked down the engine casing and left a small pool of coolant on the garage floor.  So yes, your AOS needs to be replaced.  Recommend only an original Porsche AOS replacement.

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Thank you. Do you think is a job that I can do myself? Or only an experienced Porsche mechanic or any mechanic could do it. I found instructions with pictures on how to do it but I am new in the mechanical world and I am trying to learn. Is it really time sensitive to be replaced? Or you think I could wait a couple of weeks?

thank you!

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Posted (edited)

Replacing the AOS is a major pain in the butt job.  It will test your overall mechanical skills, tool set and patience.  For that matter, it is a job I never want to do again. It took me 3 days.  Lowering the engine is only the first step, so you need to read up on how to do  it.  You also might get a couple of quotes from experienced mechanics to compare with how much time you will have to put into it.  The AOS  can be purchased for less than $200.  Only get an original Porsche AOS.  This is one job not to go cheap.

 

If it is not spraying coolant all over the engine bay, more like just weeping, you can wait.  Keep a very close eye on the coolant level.

Edited by DBJoe996

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+1

 

I so recommend anyone new to the mechanical world to tackle this job. You may want to try a reputable indie.

 

With the engine out, it's a 5 min job. With the engine in, 4hrs, if you are good. Can easily dragged to a day or two for the first time.

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Thank you for the reply. Do you know if the oil will need to be replaced because the AOS will be removed and replaced? Is coolant also lost while replacing the oil separator? I have an experienced mechanic that I think he could do it, however he is not specialized in German cars. I am learning a little bit more every time something happens so someday I may be capable of fixing it myself. Thank you for your help! 

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I would highly recommend finding someone who works on 996 (not just other German cars) on a daily basis for AOS replacement. You really don't want to pay someone to learn how to fix a 996.

 

Engine oil does not need to be changed at all. Some coolant will need to be drained and later reused and yes, you will likely need some coolant to top off.

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I am going to replace my AOS soon.  I noticed on Pelican that they show an AOS and an "Internal Oil Separator"?  Is this a completely different part and not related to the common AOS?  Or is it an internal part of the AOS?  I'm confused about this.  I've never seen mention of an internal oil separator.

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There are two versions of AOS (for mkI and mkII) and they both look similar to this

https://www.pelicanparts.com/More_Info/99610702355.htm?pn=996-107-023-55-M100&gclid=Cj0KCQjwnpXmBRDUARIsAEo71tREheU0y2P6haM4ucMF3bVezGJprKlDmefU8r39mEpL3M7oXVBgZc8aAkHJEALw_wcB

 

Make sure you get only a genuine AOS. Any aftermarket AOS is a waste of time.

 

The internal AOS you refer to are inside the oil sump, which normally do not need replacement.

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I’m going to tackle my water pump, coolant expansion tank and coolant hoses and so also decided to replace the AOS.  I know the AOS has several hoses attached to it. Not sure if these are coolant , oil and vacuum hoses. Just want to replace everything that is attached because I’m sure these parts are also old and brittle. 

 

Does anyone know which parts to replace and would replacing these be difficult?  Any parts names and or numbers along with any tips would be great. I’m feeling a bit intimidated by the AOS procedure but while I have everything apart want to replace everything that could fail later because I don’t want to do this again for a long time. 

 

Car ar is a 1999 C4 manual. 

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You will find all the parts you need here.  There are two coolant hoses (supply and return) and two air hoses, one to the throttle body and one to the bank2 head.

 

The coolant hoses and the air hose to the throttle body are easy as well. The air hose to the bank2 head will be more involved because you have to snake it through a lot of stuff over the top of the engine.

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Update. I took my car to the Porsche dealer in Fort Worth and after getting a quote from another place the dealer

was cheaper. AOS replacement parts and labor $885 plus tax. Seems reasonable considering the other shop even though they say they specialize

in Porsche, they quoted me $1,000 plus tax. I think I will want to flush the coolant and replace the coolant in the near future. Does anyone know if that is a job I could do or should I ask the dealer to do it? 

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Yes, check out this thread

 

 

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I just realized that I am not as handy as I thought I was so the dealer will flash the coolant and replaced it. Porsche Autobahn in Fort Worth. So far happy with them. I will wait until I pick up the car Monday to see if they really did their job.  I mean it is a long story but.... 

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