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Old 02-19-2018, 06:59 AM   #1
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Rough Idle & P0171 + P0174. Vacuum leak?

Been having both P0171 and P0174 CEL's on the car, with a very rough idle, backfiring (quite fun), performance loss and a weird hiss when I turn the engine off. The hiss may be normal but I've never noticed it before.

Here's the rough idle (it shows the exhaust later on so you can hear it clearer - I have bypass pipes so it's supposed to sound throaty like that, I haven't got a blow in the exhaust!):

https://www.youtube.com/watch?v=3JDbdLx9e4U

And here's the hiss:

https://www.youtube.com/watch?v=o3yYn-_a2Sc

Is that hiss normal? If not, vacuum leak?

I can't see any obvious vacuum leaks in the engine bay. I've ordered two new O2 sensors anyway, but I was hoping I'd see an obvious pipe disconnected in the engine bay. The throttle body pipe was totally loose (I could move it with my hand), but after tightening it, it hasn't made any difference.

Help!

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Old 02-19-2018, 10:44 AM   #2
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Those 2 codes are referencing both banks 1,2 lean condition. Hissing does sound to be most likely vacuum leak after the maf. Did you check the hoses between the T and plenum? You are most likely introducing unmetered air thru a leak

Last edited by Lapister; 02-19-2018 at 10:47 AM.
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Old 02-20-2018, 01:30 AM   #3
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Those 2 codes are referencing both banks 1,2 lean condition. Hissing does sound to be most likely vacuum leak after the maf. Did you check the hoses between the T and plenum? You are most likely introducing unmetered air thru a leak
Already checked the codes and found that it meant too much air, so I've suspected a leak. I've checked the hoses, all seems okay. The hissing sound seems to be coming from here:



That's why I was thinking it may be that little valve on top, but I'll check. I'm going to take the engine cover off behind the seats to get a front-on look at the engine to see if I can see anything else.

The other thing is that I may be chasing a leak that doesn't exist. The sound could be normal, and it could be something else causing the codes and rough idle.

What could it be?
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Old 02-20-2018, 05:05 AM   #4
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Does the engine run differently if you remove the oil filler cap?
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Old 02-20-2018, 05:05 AM   #5
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Does the engine run differently if you remove the oil filler cap?
Haven't tried it yet, will give it a go later. When the engine is warm, or cold?
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Old 02-20-2018, 11:46 AM   #6
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I couldn't take the oil cap off while the engine was running, and after stopping it, removing it, then re-starting it, the engine ran like crap - so that looks okay.

The hissing is coming from around that area, and it seems like it's coming from underneath the resonator tube (underneath the tube in the photo). There's a bunch of vacuum lines under there, so hopefully it'll just be something that's popped out.

I'm going to remove the resonator tube when I get a free day, and will check all the rubbers and tubes. That, with the new O2 sensors (they're 15 years old anyway), should do the trick.
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Old 02-20-2018, 12:08 PM   #7
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*resonance tube, not resonator tube.
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Old 02-20-2018, 05:37 PM   #8
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If you couldn't get the oil cap off while the engine was running you have a failing AOS. Get it replaced before you destroy the engine.
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Old 02-20-2018, 08:42 PM   #9
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If you couldn't get the oil cap off while the engine was running you have a failing AOS. Get it replaced before you destroy the engine.
And would a failing AOS let in air, giving the lean CEL's? Could this be the problem for everything?
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Old 02-20-2018, 09:33 PM   #10
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Originally Posted by geraintthomas View Post
And would a failing AOS let in air, giving the lean CEL's? Could this be the problem for everything?
The oil fill pipe can collapse/ crack from the excessive vacuum in the crankcase when the AOS fails as yours appears to have done.

Last edited by 911monty; 02-20-2018 at 09:35 PM.
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Old 02-20-2018, 09:59 PM   #11
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Originally Posted by geraintthomas View Post
What could it be?
- Failing MAF
- Intake system air leak
- Low fuel pressure
- Low fuel volume
- Bad fuel injector(s)
- AOS failing
- Oil filler cap leaking or oil fill tube cracked and leaking
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Old 02-21-2018, 03:00 AM   #12
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Quote:
Originally Posted by thstone View Post
- Failing MAF
- Intake system air leak
- Low fuel pressure
- Low fuel volume
- Bad fuel injector(s)
- AOS failing
- Oil filler cap leaking or oil fill tube cracked and leaking
I'm aware of the symptoms - it's not fuel as it's fine under load, it's not the MAF as I've tested it, it's not the oil cap as the engine stalls when removing it.
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Old 02-21-2018, 03:01 AM   #13
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Originally Posted by 911monty View Post
The oil fill pipe can collapse/ crack from the excessive vacuum in the crankcase when the AOS fails as yours appears to have done.
Surely if the oil filler pipe is cracked, then the engine wouldn't sound much different if I ran it with it removed? It stalls when I remove it, showing there's a huge difference when it's in place.

Would a faulty AOS cause a vacuum leak?
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Old 02-21-2018, 05:54 AM   #14
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Would a faulty AOS cause a vacuum leak?
Yes! The reason why you can't get the oil cap off is because the inlet is sucking air out of the crank case through the split AOS diaphragm.
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Old 02-21-2018, 09:21 AM   #15
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Quote:
Originally Posted by geraintthomas View Post
Surely if the oil filler pipe is cracked, then the engine wouldn't sound much different if I ran it with it removed? It stalls when I remove it, showing there's a huge difference when it's in place.

Would a faulty AOS cause a vacuum leak?
I'll let the master answer those questions in the attached thread. The squeak or hiss you attached in your first post could be from your front main seal.

Did I toast the engine?
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Old 02-22-2018, 12:59 AM   #16
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Thanks for the tips guys, much appreciated. Glad that the AOS is most likely the issue - it makes total sense. The hiss on a second look does actually come from that area, and with the lean codes, the immovable oil cap, the oil around the AOS pipe and the rough idle, it all points to it.

Will let you know how I get on, will order it in the next week or two

Thanks again!

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