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Revs holding before return to idle


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Hi guys seems mainly on cruise the idle seems to  hold maybe about 5 seconds before returning to idle

so if you cruise upto the lights it'll  hold up while u sit there so I quite often have to ride the clutch till the idle drops back this sucks any idea's

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Hi efi265 ,

what ecu are you using, what engine is it and what type of idle control is it using or is using Electronic Throttle (DBW).

A copy of your pcl file will be very handy also.



Edited by Dave Kriedeman
added pcl request
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Am running an g4 extreme edelbrock air only throttle body with gm style idle air on a valiant hemi 265 ive actually got 2 issues as why I have 2 posts. sorry but away at the moment not back till next weekend

stepper motor idle control


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  • 2 weeks later...

hi guys as above if you could help please.my tuner had disconnected the idle controller as it was uncontrollable seems to think boost is moving it .my last motor i had setup to open loop control this seemed to work ok. but i have not tried this yet with my new engine ..i have just reconnected the idle controller and it has gone fine for 2 days but tonight it has started to hold up again and the other issue i seem to have is the revs returning to idle slower than it would or should 

tuned brent july 2015 rebuild.pcl

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Hi efi265,

I had a similar problem with idle hanging high, also using a GM style idle stepper motor that was part of a holley 1000cfm throttle body MPEFI chev 383 set-up using closed loop idle control.  It was a few years ago but from memory, here is how I licked it:

1.  Firstly you have to identify the stepper values that equate to the IAC valve being fully closed.  Do this by switching to open loop and physically looking at the IAC valve head while it is installed in the throttle body (car not running) and keep raising the stepper value in the table until the IAC no longer visibly moves any further closed.  Then back off 1 point, then set that as your max clamp value.  That means the IAC will never be instructed by the ECU to go beyond fully closed and will therefore never get out of kilter with the ECU.  To get a decent min clamp value, still in open loop, with a warm engine set the IAC far enough open to give you around 2500 rpm and note that value - set min clamp at that value.  Set TP Lockout to around 1% and RPM lockout to around 300-400.

2. Still in open loop, with the engine warm, set the stepper motor value to the max clamp level so the IAC is closed, and adjust the throttle stop so that the engine is idling about 100-150 rpm below your target idle speed.  Then do a TPS calibration and ensure that the TP position goes to 0% when the throttle is fully closed.

3. Still in open loop, with the engine completely cold, fire it up from cold and adjust the stepper motor values in the table for each temperature cell so that under 60C engine temp, you are idling around 150-200 RPM slower than your target closed loop idle speed for that temp, and above 60C you are idling around 100 rpm slower than your target RPM closed loop idle speed for that temp. Basically create a set of open loop stepper motor values that have the car idling quite a bit slower than desired at all temperatures.  The IAC should be very near the max clamp value with the engine at operating temp.  This set-up process is important and it's the key to success. 

4.  Switch back to closed loop and fine tune tune from there.  If it still hangs high, try increasing stepper motor value slightly, or increasing RPM lock-out slightly, or both.  If it drops under target and then recovers or starts to hunt, drop stepper motor value slightly.   

The reason the set-up approach in 3 is important is that when you set control to closed loop, when the throttle goes under 1% and RPM falls under the RPM lockout value (like when you coast to the lights), the ECU implements a ramping strategy that you have no control over.  To smoothly drop RPM down to target the ECU subtracts a fixed non-user-adjustable number of steps off the stepper motor table value for about a second, and then adopts the table value for about a second, and then adopts closed loop control.  If you have a stepper motor table value too low to start with, at the first stage of this ramping strategy the rpm will hold or rise outside the rpm lockout point again and the car will hang above this point or hunt around it.  The reason my car was prone to this is that the GM stepper motor flows shitloads of air and the non-user-adjustable number of steps that the ECU took off the table value at the first stage of this ramp back to closed loop actually increased my idle speed rather than gently guiding it down.  The bandaid approach was basically to have stepper motor values in the table that caused the IAC to be more closed during the non-controllable part of the ramping strategy than it otherwise would be. Idling when warm in closed loop my stepper values would generally be 5-8 steps higher than the base value in the table.  Let me know how you go.  My Chev was boosted too - I don't think your issue will be boost related.

Hope this helps.  Inserted a screen shot of my idle set-up, done using this approach.  Your stepper values won't be the same, but you get the idea.

Cheers, Andrew

Wellington NZ


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hi guys i cant see my idle air valve in the throttle body so i have tried and measured it seems to be OK no going crazy of late

haven't been on a good drive yet .the idle still hangs a little and i was thinking the steeper motor takes time to adjust is this perhaps why its a little slow at dropping to idle it seems to be better will try some more work on it  tomorrow

question so what Andrew is saying is set the idle low thru open loop. "IE stepper set to 300" idling at 750rpm-ish then switch to closed loop and the adjust to desired idle speed say 900rpm and perhaps 294 stepper setting with a max clamp of 305 ? should the operating temp stepper setting be max clamp ie closed is this the reason the throttle posistion set to 0%

with the RPM lock-out the manual says about 150 is good Andrew is 3-400 how does this effect idle as mine was on 2500rpm previous is now about 400 i think




idle change 1.pcl

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The throttle position is the lock out that stops the idle control operation above that %. This is to avoid the idle control operating while you are driving.

The RPM lock out is the figure at which the idle control is stopped from making corrections above the target RPM. So if you have a target RPM of 1000 and a lock out of 450 the RPM at which the idle control stops working is 1450rpm

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  • 4 weeks later...

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