Will a vacuum leak on the AOS affect the right bank more? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Dale_K · 03-01-2011, 04:33 AM

I got the 1126 CEL this week and I'm thinking about the repair strategy. Correct me if I'm wrong but the 1126 code is saying the right side cylinders are running lean. Seems to me there are two basic possibilites:

1 - The control system (MAF sensor & O2 sensor) is working correctly and I have a vacuum leak.

2 - There is no vacuum leak and I have a bad sensor.

The AOS seems to be a common vacuum leak failure point but I don't understand why I only get a code on one bank. Does a leaking AOS bellows tend to make the right side go lean or both banks equally?

I pulled the MAF sensor and sprayed it with some sensor cleaner but I didn't see anything obviously dirty so I doubt that's it, especially with the code on only one bank. The car runs OK so far, although it briefly ran weak at low speeds after I reset the codes. The CEL hasn't come back on yet but I doubt that will last.

Also critique this repair stategy...

Buy a propane torch and try to find a vacuum leak, especially at the AOS bellows. If I find a leak that's no doubt the problem.

If I can't find a leak replace the single pre-cat O2 sensor on the right side.

If that doesn't work replace the AOS on general principles because maybe it's leaking even though I couldn't find it. I'd do this first except for the fact I'm only getting the code for one bank and I would expect a vacuum leak to affect both sides.

chris97boxster · 03-01-2011, 07:48 AM

I'm in the same boat as u. Replaced MAF. ..now getting 1124 & 1126
been doing research on this site and now believe it's AOS
what r the chances of both O2 sensors going @ the same time?
reading more ....i also found out that O2 sensors last 100 000 miles
on average...which I'm well below.....so AOS is my next fix.
I've also read on here that is u get rough idle @ startups...
it's one of the symptoms of AOS starting to fail.
this was starting to happen more frequently on
cold mornings (around 0 degree C) & simply thought
its due to the cold weather....after reading more about it on here
Its definately my next DIY project
I'll subscribe to this thread so I can stay updated to ur progress
post anything relevant that u have found out.
I won't b able to tackle mine till april....
mother nature decided we need a constant replinishing of white
stuff @ the latitude I live @.
good luck. ..I'm. Sure ur on the right track
cheers
chris

blinkwatt · 03-01-2011, 08:14 AM

I've had P1126 on my 01' Boxster S......only one side showing off....shop said it was the MAF. They spent awhile trying to figure out why only one side showed as off...but at the end of the day the MAF voltage was way low and bad so it had to go and with it went the CEL.

chris97boxster · 03-01-2011, 09:54 AM

I did try and clean my maf. ....the codes came back
so I bought a new one from pelican
code went away. ..but then came back again after 200 miles
Sometimes would show only 1128...clear it
then other times both would show up 1126/28
this all happened within a month
then I got 0102 & 1126/28
but I just replaced the maf? ??
now looking @ other possibilities like AOS
I will change O2 sensors if I have to
but just checking other routes 1st

@dale k.....try AOS 1st and c what results u get
maybe what I've stated might help in ur diagnosis

Ok. ...other forum ppl mention that the AOS
when it starts to fail, has oil on top of intake
where the hose goes.

Another mentioned about taking the oil cap off at idle
still trying to find that thread on here
Something about if it runs rough with out cap means
one thing...have to find it again

Others have mentioned to take the AOS line off the intake
and vent it into a catch bottle as they do in race track application

Things I will try when I get a chance
then decide whether to replace AOS

Of course if this does't fix the 1126/28 cell
then I'll b looking for some more input from forum members

Chris

chris97boxster · 03-01-2011, 10:26 AM

This thread might help

P1128 p1130

Chris

blinkwatt · 03-01-2011, 10:34 AM

If the oil filler cap won't remove when your idle then it more then likely means your AOS sucks.

stateofidleness · 03-01-2011, 02:07 PM

i still have a nagging 1128 occasionally, but i replaced my AOS last year.

mine is caused (i believe) by a small exhaust leak. i'm not familiar with the 1126 code so I'm not 100% sure what that means.

BYprodriver · 03-02-2011, 05:41 PM

Dale, I don't know what year your car is but according to the Bentley manual: P1126 2000-2002 is drivers side bank running rich. 97-99 P1126 is pass. side bank running out of range, lean or rich.

Dale_K · 03-03-2011, 04:16 AM

Yeah, I should have provided more details about the car, which is a 2000 S Tip with 103k miles. I'm going to search for a vacuum leak today with a propane torch.

Dale_K · 03-03-2011, 08:24 AM

I jacked the car up and poked around with the propane torch turned on but unlit trying to provoke a response that I could hear such as a rising rpm. No dice on the propane but I'm pretty sure I found the leak just by probing around and moving things.

I could hear a little hissing sort of at the center of the top of the engine. If I move the rear of oil fill tube gently I can hear a very distinct change in the sound (louder) and also the motor runs different at the same time. I couldn't get any similar reaction when I pushed the AOS bellows with my fingers. I'm almost positive the vacuum leak is the oil fill tube.

BTW, and not to hijack my own thread, but my hat's off to you guys who have replaced the AOS yourselves. I had the pictures from the 101 projects and it looks like a piece of cake. In real life that job looks like a nightmare. The oil fill tube looks like a tough job too but I'm still pleased to find a real issue to focus on instead of just replacing a bunch of parts on hope & prayer.