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DBW Commanded Throttle Position Too High when Coasting Outside of Closed Loop Idle Control

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I have a Haltech Elite 2500 with DBW that I just recently got up and running and everything is working well with one exception. More often than not, when I'm coasting outside of closed loop idle control, an idle control output of 40% is commanded, whereas closed loop idle output is around 20%. So until I enter closed loop idle control (speed drops below 20KPH), it'll keep the engine spinning away at about 2000 rpm if I'm in neutral, or keep pulling the car along if I'm in gear. It's really irritating and I've been through every parameter I can think of, but can't figure out what's commanding the 40% open loop idle throttle position. The APP to throttle table is 0 at 0. APP is 0. Idle control base output is 21% at operating temps.

It seems like there must be a a base commanded idle control output when outside of closed loop mode, but I've looked at every setting in Idle Control a hundred times without any luck. Any ideas/suggestions?

Are you using the Idle Control Throttle Follower Target table?

The Idle Decel Offset will also influence the control percentage so if this is too high you might see what you're seeing.

Thanks for the reply!

I'm not using Throttle Follower, so presumably it's not that. I did have an issue with the Idle Control State being stuck in open loop due to Decel Detected almost always being active. I bumped up the RPM values in the Decel Min RPM table (big throttle body, so it idles on the high side) and that resolved that.

Now, when this ~40% commanded throttle position is occurring, Decel Detected is Inactive, and the Idle Control State says "Open loop due to Vehicle Speed". So it would appear that there would be nothing triggering the Decel Offset to be added. However, the numbers would add up to suggest it is. The base idle control output is 21%, and I have the Decel Offset set to 20%. So that would give the ~40%. Rereading the Decel Offset description it would appear that Decel Offset is not tied to Decel Detected, at least not entirely. It says that Decel Offset is activated when zero demand conditions are reached and remains on until idle control returns to closed loop, so not when Decel Detected changes to inactive.

It may be that I'm getting the Decel tuning and Idle Control Decel Offset functionality mixed up. I'll try reducing the Decel Offset back down to 10% or 5% and see if the open loop commanded throttle position drops proportionately and that would verify that the Decel Offset is causing this.

Nigel

Have you tried to disable dashpot function?

Hi Dominique

Thanks for the reply. The dashpot (throttle follower) function is disabled.

As per my previous post, I'm fairly confident I was misunderstanding the Decel Offset function. It's a matter of terminology. To me, it's more intuitive to call it an Open Loop Idle Offset. I should be able to test later today to see if I'm on the right track.

Nigel

Based on the help text the Decel Offset value is added to the base any time the pedal position is less than the zero demand threshold and the idle conditions are NOT met.

Once the idle conditions are met the Decel Offset is decayed and the idle control percentage returns to the normal amount.

Idle control systems in aftermarket ECUs often have to be written to suit solenoid valve, stepper and drive by wire so they can be a bit of a compromise.

Personally, I have had to tweak an Elite 750 with similar symptoms, although not DBW.

When set to not to take vehicle speed into account, the base duty cycle output could be lowered.

So, it was the Decel Offset. As I mentioned in a previous post, after rereading the description, it was becoming clearer that the Decel Offset is a throttle opening percentage for open loop idle control that's added to the base closed loop throttle map. So, for example, if you let off the throttle at 3000 rpm, the ECU will enter open loop throttle control and it will take your Base - Closed Loop idle throttle opening, say 30%, and it will add the Decel Offset, say 10%, to that for an open loop idle throttle opening output of 40%. It will hold that until closed loop control is entered, such as when the vehicle speed drops below 20kph, at which point only the Base - Closed Loop idle throttle opening will be applied (30% in this example). I tested it out and it works exactly as described.

Many thanks to all who replied!

Nigel

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