HCM911

Hi! I have a 996.1 Carrera Tiptronic (3.4, M96.01, E-Gas, ROW, without secondary air pump), which recently began to produce the following symptoms:

The first thing I noticed was that the fuel consumption increased. On a trip where it used to be 11 l/100 km, now it’s around 13-14. (17-18 mpg instead of 21.) Once warmed up, the idle is slightly uneven. The first time it was much worse, sometimes close to stall Then I thought there might be a problem with the fuel. I refueled with a better octane (ROZ 100 instead of ROZ 95), it has been much better since then, but it always occurs.

The left exhaust tipe is sooty, the right one is completely clean. You can smell too, that there is too much fuel from the left exhaust. In cold weather you can see it also, that more vapor coming from left than from right.

So I think the problem is that only the right bank (due to the cross design exhaust) running a little too rich.



No CEL, perfect idle when cold, no oil consumption, no smoke from exhaust after the cold vapor.

Unfortunately, I can't do a diagnostic during the holidays. A very basic OBD scanner sees no error codes.

But I noticed that the AOS to crankcase breather (996 107 047 51) has a slight oil weeping, I attached the picture.



What do you think the problem might be?

A failing AOS? But that would affect both exhaust same, right? Only this breather valve?

Or is this oil not even related to it? Maybe a fuel injector, O2 sensor?



Thank you very much in advance for any help or advice,

Martin

yelcab

Bore scope the cylinders 4/5/6. You might not like what you find, but you need to be sure.

DBJoe996

Just a wild guess...your AOS has failed. You could test the AOS/crankcase vacuum with a manometer. There are detailed instructions on how to do this, search this forum. Also, for replacement get the UAOS and solve the problem for life.

jumpy chunky

My WAG will be that this sounds like a cam position sensor is not sensing any longer.

The one on that bank is located just below that crankcase relief.
I’d think a scanner would show a fault.

Have to get greasy to replace that and put new o-rings on the relief.

HCM911

Thank you very much everyone for the answers!

Because of the oil weeping, my first assumption was also something related to AOS. The cam position sensor wouldn’t came to my mind, but it is right that even that could cause these symptoms.

However a better OBD scanner with live data clearly showed an O2 sensor issue (ROW car, so only have pre-cat sensors, failing one on the right bank). After replacing it, the rich mixture immediately disappeared. The warm idle is also smooth as before, no more sooty exhaust, fuel consumption is back to normal.

EVOMMM

Had a recent similar issue put in a set of 02 spacers and did a hard reset and the issues have vanished

sublm8

The oil on the AOS breather may simply be due to the o-ring failing. The replacement o-ring is available, and there is a single 10mm bolt holding it to the head.

Porschetech3

That breather/connector is almost certain to leak at higher blow-by/age........

I would suggest testing the blow-by using this thread https://rennlist.com/forums/996-foru...=engine+health

Alternatively you can just cap it off, I know why Porsche put this on there, but I'm not sure why they thought it was a good idea. Porsche eliminated this on the 2003 up models...

jandackson

@Porschetech3 Did Porsche only eliminate the vent or the entire port to the AOS? I too wondered the logic behind porting the crankcase to atmosphere.