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#1
7th October 2020, 06:54 PM
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PF S8 variable idle
Have a look at the video. I am coasting on neutral, and as you can see, engine idle varies steadily from 600 to 900 rpm. It has never stalled and engine seems otherwise healthy. Strange thing is that it never does this when the car is stationary, only when it is moving. For example, if I coast and stop slowly to traffic lights, idle evens out as soon as I stop. Also, it does not do this all the time, but occasionally.
Any ideas?  I have a PF S8 AHC with manual transmission.
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S8 quattro 6-gang PF / 200 quattro 20V / 944 Turbo / 500 SEC Instagram @hptalli 
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#2
7th October 2020, 07:16 PM
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When you're coasting in neutral the ECU is getting a number of inputs that it's not expecting. With the engine idling and no gear selected the ECU expects the car to be stationary! However it will be getting inputs from the ABS sensors that say it's moving. The output shaft sensors won't match the input shaft sensors. The ECU is confused. When you stop it thinks all is as it should be and you get a steady idle.
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Conan (the Librarian) Watch it if you dare Currently, 2000 S8, Merlin Purple, Leather Pack 1, Winter Pack 1, SPAX adjustable suspension, AP Racing 6 pot front caliper, 385mm fully floating disks, EBC Yellow Stuff Pads, Black Diamond grooved rear disks and predetor pads, D2 Doctors Brake Block. 1997 4.2 A8SQ - the Projekt. 12 years and ongoing! 1997 4.2 A8SQ - Ming Blue. Restoration sat waiting; saved from the Scrapyard 2003 C5 Allroad 4.2Q - Cobolt Blue. Towing car. Laid up awaiting manual conversion. 2005 C5 Allroad 2.5TDi FE - Grey - Workhorse. 2015 Q3QS SLine - Daytona Grey. Lots of extras! Long Haired Admirals cutter. Previously 2003 C5 A6 1.9TDI FWD - Loaner from D2 Doctor Lifex 1997 S8 - Ming Blue. Loaned out and written off. I loved that car. My first engine swap after cambelt failure. 1996 4.2 A8SQ - Ming Blue. 178k on a cambelt! Trader's 8. Delivered to A8Parts at Lifex. 2003 2.8 A8SQ - powder blueish. Fill in for a while. Sold on. 1998 1.8T A4 Avant FWD. Ming Blue (there's a theme here), PEX for the 97 S8. And a list of non VAG going back 40 years before I saw the light. 
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#3
7th October 2020, 07:50 PM
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I agree ^ That behaviour looks very deliberate non-random behaviour from the ECU, rather than a fault. Its probably puzzled why its doing 70km/h in neutral.
Question: why are you doing 70km/h in neutral?
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#4
7th October 2020, 10:16 PM
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Should not be doing it, especially with the manual box.
Had similar behaviour on C4 A6 2.8 years ago. It gradually got worse until it was stalling when switching to neutral and going to a stop at junctions. It turned out to be a combination of sticking idle control valve and a small air leak in the hoses by throttle body
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Currently 8less 2011 Q7 S Line 3.0TDI, 2016 Tesla Model S 90D 8 history: 2006 A8 Sport 4.2TDI quattro SOLD, 1997 S8, reached end of life with gearbox failure
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#5
8th October 2020, 05:55 AM
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Quote:
Quote:
I am not so familiar with this engine, I would appreciate if there is some information about how idle control works on 32V engines, especially about the control valve. Thanks!
__________________
S8 quattro 6-gang PF / 200 quattro 20V / 944 Turbo / 500 SEC Instagram @hptalli
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#6
8th October 2020, 07:35 AM
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Duh. I missed the manual gearbox part sorry
 Thats such a rare thing!Idle control is done by a valve at the back of the inlet manifold - #11 here: This is a servo motor with essentially a gate valve on the end, which the ECU controls to allow more or less air to bypass the throttle body.
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#7
8th October 2020, 08:18 AM
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No problem!
 MJ, you must know if it's accessible without major dismantling? I think I should try to check and clean it, as well as those hoses displayed left of it in the diagram. I guess the hoses are behind the inlet manifod as well.
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S8 quattro 6-gang PF / 200 quattro 20V / 944 Turbo / 500 SEC Instagram @hptalli
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#8
8th October 2020, 08:46 AM
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The front breather pipe was always the go to failure item for erratic idling on the PF 4.2's, and it would stutter more under load from the air con, power steering etc, but would generally idle ok.
Never had a manual to contrast, but I guess if you were idling, but moving and other ancillaries were drawing load, it might cause the same effect ? Would be interesting to know if you're idle is affected when stationary, but turning the steering ?
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For parts enquiries and Forum User Administration please contact customer services directly, not me  - enquiry@a8parts.co.uk, www.a8parts.co.uk, or call 01642 783537The Daily Town Car - 2023 Tesla Model Y Performance. Black with Black. Giant ipad screen, huge sunroof, 4 wheel drive, Tow bar, lots of ooomph and made in China, so actually built properly unlike the Californian ones. The Family Wagon - 2018 Tesla Model X. Black, 7 Seats, Towbar, Homelink, Enhanced Auto pilot with Summon and self park, Heated front seats, CCS Charger upgrade, MCU2 Upgrade, Falconwing Doors, Self opening / closing doors, Netflix, Spotify, toys, Air suspension, alcantara roof + lots of other stuff...... The 17 year old Driver Wagon 2015 Skoda Citigo Monte Carlo. Alloys, Power steering, Aircon, Leccy windows, Sat Nav, bluetooth, central locking, cup holders, 59 whole bhp. Million times better than my first car. Well jealous..... The First 20 Audis: 2001 D2 4.2 QS Silver, 1997 D2 S8 Silver, 1999 D2 4.2 Silver, 2002 D2 S8 Dolphin Grey, 2003 C5 RS6 Blue, 1998 D2 S8 Black, 2000 D2 4.2 QS/S8 Ming Blue, 2003 D3 4.2 Ming Blue, 2005 B7 S4 Blue, 2006 D3 3.0 Tdi Black, 1999 D2 4.2 Silver, 2003 D3 4.2 LPG Crystal Blue, 2000 D2 S8 Silver, 2007 3.0 Tdi Q7 Black, 2009 B8 S4 Black, 1998 D2 S8 Agate Grey, 2006 D3 4.2 Tdi Blue, 2006 D3 S8, 2007 Q7 3.0 Tdi, 2006 C6 S6, The Second 20 Audis: 2011 A7 Sline, 2001 A2 1.4 Tdi Black, 2002 A2 1.4 Tdi Silver, 2006 A8 4.2 Tdi Blue, 2007 4.2 Tdi Q7, 2006 S4 Cab Silver, 2010 RS6 Saloon Black  , 2007 TT Roadster, 2010 A8 D4 4.2 Silver, 2006 C6 A6 2.7 Tdi Quattro, 2000 D2 4.2 QS/S8 Ming Blue (again), 2008 Q7 4.2 Black, 2009 C6 RS6 Saloon Daytona Grey, 2001 Fully Loaded A2 Tdi, 2006 Avant S6 Black, 2007 VW Caravelle Exec Black (almost an Audi) 2009 TT Roadster Daytona, 2015 Q7 4.2 tdi lava grey, 2002 GmbH D2 demonstrator, 2006 D3 A8 LWB 4.2 Tdi Black Most Recent Audis: 2001 A2 1.4 Tdi Black, 2007 4.2 FSi Q7 ABT Blue, 2016 S5 Coupe Launch model, 2004 S4 B6 Cabriolet, 2016 Q7 Sline White, 2006 Q7 4.2 FSi Black, 2012 Q7 Sline+ 4.2Tdi White, 2008 Blue Audi S8 Everything Else 1989 Subaru Justy, 1986 Vauxhall Cartlton 2.0 GL, 1985 Vauxhall Carlton 2.2 CDi, 1987 Ford Sierra XR4x4, 1983 Vauxhall Senator, 1985 Vauxhall Senator 3.0i, 1997 Polo Saloon, 1997 Peugeot 306 1.9 TDi, Ford Sierra Sapphire 2000E, 1990 Subaru Justy, 2000 Subaru Impreza, 2000 Subaru Impreza RB5, 1993 Range Rover LSE, 2005 VW Bora, 2003 Skoda Superb, 2003 VW Passat V6 4 Motion, 1996 Peugeot 106, 2000 Range Rover 4.0, 2019 Tesla Model 3 Performance
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#9
8th October 2020, 08:55 AM
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It idles otherwise smooth, no effect whether I am turning wheel stationary, AC or other consumables on or off.
Where can I locate the front breather pipe?
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S8 quattro 6-gang PF / 200 quattro 20V / 944 Turbo / 500 SEC Instagram @hptalli
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#10
8th October 2020, 09:03 AM
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Linear Mode
