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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 V10 '08 / 200 quattro 20V '90 / 944 Turbo '88 / 500 SEC '85 / 740i '94 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!
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S8 V10 '08 / 200 quattro 20V '90 / 944 Turbo '88 / 500 SEC '85 / 740i '94 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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