#1
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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 |
#2
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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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#3
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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? |
#4
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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 |
#5
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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 quattro 6-gang PF / 200 quattro 20V / 944 Turbo / 500 SEC Instagram @hptalli |
#6
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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. |
#7
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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 |
#8
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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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#9
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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 |
#10
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The idle valve is easy to get to yes. Pop off the plastic engine cover and it is right there at the back next to the throttle body. The larger pipes at the back are mostly the suction jet pump for the brake servo vacuum, I think fed from the throttle body. They are all pretty thick so I would doubt there's a problem with that system.
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