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Why is there an idle setting for when vehicle is moving

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What about the vehicle being in motion would indicate a need for a different idle than when the vehicle is stationary? I see in the ecu the default is a slightly higher idle if the vehicle is moving. Why?

You need minium RPM for stable idle in both cases to reduce fuel consumption and produce minimum exhaust gases. When vehicle is in gear the engine is under load so idle RPM is slightly getting less than stationary because of the additional load. ECU needs to know how much less is "Ok" without stalling the engine and if RPM will tend to go below that figure ECU will increase ignition timing and open throttle to allow more air to the engine to increase torque and bring RPM back to stable value.

As Shota said, on a closed throttle there can be incomplete combustion and this is usually heard as back-fires and 'cackling' on the over-run - it's cool but emissions and cat's don't like it - and a little more throttle improved the burn in the cylinders.

However, many vehicles use a full shut-off on closed throttle above a specific rpm range, this saves fuel and is emissions friendly - this second 'idle' point may set the fuelling/fuel shut-off threshhold?

My ECU cuts the fuel on overrun so I figured this “moving idle” was for when the car is coasting in neutral or the cutch is held in until things settle down / decel has reached a plateu? What are the range of conditions inclusive of “moving idle”?

Seems like in my case no matter what I set the moving idle to it won’t go any lower than 1,150 anyway. Not sure what’s up with that? I can get my stationary idle down to whatever I want. Maybe from the cold air intake forcing a little extra air past the tiny air gap in the throttle plate when in motion? AFR is on point under this circumstance so dunno.

Have you tried to retard ignition timing in that area?

Yes, I’d have to retard it a lot to get there. It’s something about moving. I’ve got stationary idle at 900 and that’s no problem. I’m trying to set moving idle at 1k. The more I play with it, it seems that moving idle comes out 300 or so higher than whatever I request. When it was coming out 1,150 it was actually set to 840. If I set it to 700 it will probably come out closer to 1k. I’m sure no vac leaks, just went through the whole process with that and idle works as expected stationary. Throttle is closed just the same either way.

In some ECUs there is a special pin to get stationary or moving signal and switch between appropriate tables. Is it done on yours? Also idle step motor has to be working properly to control amout of air getting to the cylinder. Needless to say there are shouldn't be any leaks(including throttle body) in the intake to bypass air to intake manifold. I made leak test on Camaro LS engine a couple of weeks ago and was very surprised to find an air leak arond the head and front cover...

I’ve leak tested it and found all the leaks. One at the brake booster check valve and the other was because I was stupid and had cleaned the throttle body too good (removed all the factory sealant around the plate). That’s all sorted. I’m finding I can get the moving idle spot on if I reduce the IACV duty cycle however then the duty cycle is too low for the stationary idle. You’d think the same duty cycle would work the same for both. Getting closer though. ECU is Hondata s300, not sure what yoy are referring to with the pin? I’m not sure it’s relevant to mine. Maybe there is some other area of the ecu that is advancing the timing if the car is moving. I’ve noticed now when I have the stationary idle where I want it timing is at 18* on the Hondata display but when the car is moving but in neutral and idling slightly too high the timing is then at 20*. Getting there... How exactly does the IACV work? It’s a 2 wire edition. My understanding is that it’s kind of like an injector but injects/allows air rather than fuel. It seems like the duty cycle is fixed to a specific setting which I pick rather than being able to increase or decrease itself based on a target.

Well I’m pretty sure it’s impossible to get my moving idle to my target but it’s only off by 150 and frankly my target is a bit arbitrary anyway. I think I’m just bothered that I don’t get the result I ask for by the ecu. I’ve rediced timing on the main table a lot but oddly the live timing doesn’t change very much at rolling idle. I set that back and then played with the ignition controlled idle setting, giving it a lot more degrees it is allowed to pull if need be and the live timing is unchanged. I’m pretty sure the ignition controlled timing actually has no effect on my engine whatsoever, maybe it’s in the Hondata software but not actually available to my ECU.

The only factor that affects it is IACV duty cycle which I’ve come to understand just means how far open the IAC valve is and it’s a static setting. If I reduce that opening moving idle can get to my target but then stationary idle is below target.

I think the reason is one of two things, either because I’ve got a lightweight flywheel though I’d think the effect of that would be the same on stationary idle, or more likely because my air intake goes out to an opening in the bumper and when moving it’s just forcing some air down that piping and through the idle bypass. It does seem the faster I’m rolling the moving idle is increases slightly.

I mostly just want to understand this because I’m learning and find it interesting so if anyone has any bright ideas let me know.

Since your questions are very Hondata specific, perhaps you should check in with the Hondata forum. I found this related thread, but there are plenty of idle threads...


I haven’t had much luck over there unfortunately.

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