Hatchback with S6 ecu

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Philloy
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Joined: Sun Feb 15, 2015 6:29 pm

Hatchback with S6 ecu

Post by Philloy »

Hi I have a hatchback Impreza with a 2.0lt sti engine running a S6 ecu the problem I have is, If you rev the car stood still it revs and drops back to around 800 rpm,thats ok but when moving in slow traffic the revs will not drop below 1500 rpm untill you come to a stand still then it drops to 800 again but the second you move it goes back to 1500
do you think this is software or a engine sensor.
All so if you rev the car to 4750 rpm like when you get noise tested on a track day it starts missfire, it is all most as if the anti lag is switched on but is not
any help would be appreciated

Phil
pat
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Re: Hatchback with S6 ecu

Post by pat »

Phil,

both "issues" you describe are actually normal behaviour - let me explain :

With regard to the raised idle when moving, that is to do with the way that idle is controlled when stopped. When the car is stood still and idling there is a certain amount of torque that is needed to overcome the puping losses, and any additional load that may be present. Such loads are steering input, fans turning on and off etc. It takes time for the amount of air in the plenum to increase, so when an unexpected load appears it can take a while to react to it. To improve this, we run a less efficient idle where there is more air than is absolutely necessary and we adjust the ignition timing to reduce the torque at idle, so when a load change happens, it is possible to immediately react to that load by ramping the timing back in.... this works well but it does have the knock on effect that whilst the closed lop idle is not enabled the torque is higher than needed for normal idle and there is nothing to bring the idle down.... so it will sit at around 1500. It is possible to enter closed loop control at higher vehicle speeds but there is one gotcha with that - greater risk of stalling. Whenever the closed loop idle is on it will try to achieve the target idle speed and it does not take into account the inertia of the car. This can result in a situation where you approach a stop whilst still in gear and the inertia of the car will keep the revs up even though the idle air is being reduced, and reduced, and reduced some more.... when you then finally dip the clutch, there is then no longer enough air available to idle and the engine will stall. To mitigate this as much as possible, entry into the closed loop state is normally not until the car is pretty much stood still anyway (ie you must have already pressed the cluch). If you don't drive like that then the entry into idle could be raised on your car, which would keep the revs down for you even when moving.

In regard to the misfire when revving when stood still, that would be the launch control :) It is possible to disable this if you prefer it not to auto activate, but by default it is normally turned on when the car is stood still and you exceed a certain amount of throttle opening.

Hope this helps,

Pat.
Philloy
Posts: 2
Joined: Sun Feb 15, 2015 6:29 pm

Re: Hatchback with S6 ecu

Post by Philloy »

Hi Pat thanks for the quick reply I under stand what you say so will see what happens when I next drive the car,and now I know its only the launch control am happy

thanks again Phil
pavlo
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Re: Hatchback with S6 ecu

Post by pavlo »

Or the engine is still cold. There will be a lower limiter until the coolant hits around 50deg (it's variable but this is the default). SO if you're doing a track day and doing off to do your noise test early with a cold engine, this is possible.
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