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gomi_otaku
gomi_otaku, Technician
Category: Audi
Satisfied Customers: 3379
Experience:  VW certified technician, Chicago VW/Audi Training Center, Seattle VW/Audi Academy
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Audi A4 has rough idle okay when rpm is above 1.2k
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Customer Question

Customer: Audi A4 has rough idle okay when rpm is above 1.2k
JA: Thanks. Can you give me any more details about your issue?
Customer: have 5 fault codes 16684 randon multi cyl misfire
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Submitted: 11 months ago.
Category: Audi
Customer: replied 11 months ago.
Ran diagnostic
VCDS-Lite Version: Release 1.2
Saturday, 09 January 2016, 15:03:07.
Control Module Part Number: 8E0 909 518 F
Component and/or Version: 1.8L R4/5VT G 0005
Software Coding:(###) ###-#### Work Shop Code: WSC 91450
VCID: 33199CCF122D
10 Faults Found:
17964 - Charge Pressure Control: Negative Deviation
P1556 - 002 - Lower Limit Exceeded - Intermittent
16804 - Shareware Version. To Decode all DTCs
P0420 - 002 - Please Register/Activate - Intermittent
16684 - Shareware Version. To Decode all DTCs
P0300 - 001 - Please Register/Activate
16685 - Cylinder 1: Misfire Detected
P0301 - 001 - Upper Limit Exceeded
16688 - Shareware Version. To Decode all DTCs
P0304 - 001 - Please Register/Activate
16686 - Shareware Version. To Decode all DTCs
P0302 - 001 - Please Register/Activate
16687 - Cylinder 3: Misfire Detected
P0303 - 001 - Upper Limit Exceeded
17536 - Shareware Version. To Decode all DTCs
P1128 - 001 - Please Register/Activate
17887 - Shareware Version. To Decode all DTCs
P1479 - 008 - Please Register/Activate - Intermittent
17887 - Brake Boost Vacuum System: Mechanical Failure
P1479 - 008 - Implausible Signal - Intermittentcleared codes and started engine again and reran diagnositcs
VCDS-Lite Version: Release 1.2
Saturday, 09 January 2016, 15:06:08.
Control Module Part Number: 8E0 909 518 F
Component and/or Version: 1.8L R4/5VT G 0005
Software Coding:(###) ###-#### Work Shop Code: WSC 91450
VCID: 33199CCF122D
5 Faults Found:
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 001 - Upper Limit Exceeded
16685 - Shareware Version. To Decode all DTCs
P0301 - 001 - Please Register/Activate
16688 - Shareware Version. To Decode all DTCs
P0304 - 001 - Please Register/Activate
16686 - Cylinder 2: Misfire Detected
P0302 - 001 - Upper Limit Exceeded
16687 - Shareware Version. To Decode all DTCs
P0303 - 001 - Please Register/Activate
Customer: replied 11 months ago.
2002 A4 1.8T
Expert:  gomi_otaku replied 11 months ago.

What engine do you have? it looks like you have a vacuum leak, not only the misfires at idle, but the fault for a "brake booster vacuum system mechanical failure". You should be able to hear the hissing if you listen in the engine bay with the engine running. Roll the vacuum lines and other vacuum hoses between your fingers to feel for cracking or flattened hoses.

Customer: replied 11 months ago.
I have 1.8T. Why does it seem to run fine at 1200rpm or more?
Expert:  gomi_otaku replied 11 months ago.

Because of a few different reasons- at idle, the intake manifold vacuum is at it's highest, so any air leak is going to be sucking strongest at that time. Also, at idle your throttle body is closed pretty far down, so any amount of "unmetered" air in the system is going to be pretty large compared to the amount coming through that tiny throttle body gap.

The 1.8T has some common vacuum leak points- if you take a look at the valve cover, there is a rubber "Y" pipe at the end of it, check that and then follow that to the plastic curved pipe below it which often cracks right below the rubber pipe. Also, on the underside of the intake manifold, there are a couple of vacuum fittings with half inch lines- they often crack there.

Customer: replied 11 months ago.
Okay, I will check tomorrow. Could it be the PCV?
Expert:  gomi_otaku replied 11 months ago.

That Y pipe at the end of the valve cover is part of the PCV system. It runs down to a one-way valve below the intake at the front of the block, and directs to a couple different hoses. The hoses break more commonly than the valve, but if I find anything broken in there I usually replace all the hoses and the valve (as trying to take any of it apart usually breaks it anyway)

Customer: replied 10 months ago.
Hi, bad news when I tried to pop my trunk the cable broke. I put it on a ramp and I can see the cable and the ball attached to a bracket dangling. Is there any way to open my trunk? I took it to a shop and they tell me they have to remove the fender or the bumper to open it. Is there an easier way?

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