RPM Dropping When Slowing Down

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RPM Dropping When Slowing Down

Postby K1ng0011 » Tue Mar 26, 2019 11:19 am

I am running OSE12P on my 165 ECU here in the US. My car has a chevy 350 with a manual transmission. I am having a small tuning issue when I slow down my RPM fluctuates. My idle is set to 750RPM. As I am slowing down to come to a stop my RPM drops to 500-600RPM and then hunts until it finds the idle RPM I have it set too. It never actually stalls but it is a noticeable issue. As I am slowing down the MAP KPA goes between 30-40KPA. I have attached a short log file of just going down to the store near my home. If you look at the log file and look at about 4 minutes you will see the issue I am describing. Just curious if you might be causing this.
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Re: RPM Dropping When Slowing Down

Postby vlad01 » Tue Mar 26, 2019 4:31 pm

I found on my car the throttle stop was too much closed and it did that and stalling on top. I opened it some more and now my idle catches at the right rpm without stalling either. The problem is the IAC wasn't fast enough to catch it with no other air source, so I screwed the throttle stop open a little more now it has enough air to not entirely rely on on the IAC which are inherently slow to react.

I set the idle stop screw to the point the IAC settles on 20 steps in TP. If the number shows much higher it means the IAC is doing most of the work and you want it only doing idle adjustments and not supplying the primary air source for idle if you know what I mean.
I'm the director of VSH (Vlad's Spec Holden), because HSV were doing it ass about.

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Re: RPM Dropping When Slowing Down

Postby delcowizzid » Tue Mar 26, 2019 4:49 pm

Looking at the log the hunting is closed loop swinging the o2 back and fowards are you using simulated narrowband off your wideband
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Re: RPM Dropping When Slowing Down

Postby K1ng0011 » Wed Mar 27, 2019 12:36 am

My throttle body actually has the factory plug still on the back of the adjuster bolt for the throttle body blades. As far as I can tell it has never been removed and adjusted. Yes I am using a wideband o2 sensor with narrowband emulation. I am using the spartan 2 from the 14point7.com. Looking at the logs the IAC is staying about 15-16 steps with 0 throttle input as I slow down. The MAP KPA fluctuates from 30-40 and the mixture leans out when it gets around 40KPA to about 15-17AFR as seen by my wideband. Every peak at 40KPA is where my RPM dips below what I have set for the idle RPM (750).

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Re: RPM Dropping When Slowing Down

Postby delcowizzid » Wed Mar 27, 2019 6:14 am

The narrowband emulation isn't really working it never does with any controller that offers it its just a selling point .fit a real narrowband sensor and closed loop will start working.it only really worked at the very end of the log and caused all that surge
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Re: RPM Dropping When Slowing Down

Postby vlad01 » Wed Mar 27, 2019 8:56 am

looks like the throttle stop is fine then. 15 steps is good.
I'm the director of VSH (Vlad's Spec Holden), because HSV were doing it ass about.

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Re: RPM Dropping When Slowing Down

Postby K1ng0011 » Wed Mar 27, 2019 9:52 am

What you are saying is that the narrowband o2 sensor is sending improper readings to the ECU causing the the fluctuation in idle as I slow down and come to a stop? When I look at the logs I am always looking at the wideband o2 to make changes to the VE table if I need too. When I look at my log it looks like it is responding correctly as I am decelerating. As I get to 40KPA the engine leans out (15-17AFR) and the narrowband goes down to 0 milivolts where I would expect it to show when the narrowband shows lean. Since I am seeing lean spikes when the RPM is dropping I would guess this would be a fuel related issue and not related to air. DFCO and Decel Enleanment do not look like they would be active when this is occuring.

Tuner Pro Log RPM Dip On Decel.PNG

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Re: RPM Dropping When Slowing Down

Postby vlad01 » Wed Mar 27, 2019 10:14 am

What is your idle AFR table like? seems like the idle areas are too lean, either the VE table is way out or the idle AFR table is set to 14.7 . The narrow band should not go into closed loop at idle speeds unless the AFR table has been set like that. Though I am sure once in idle mode it should not use closed loop fueling anyway.

So I would check the idle AFR table and make sure the are in and around 13:1 and then work the VE table from there. For that make sure that "force open loop" option is checked so that the narrow band is not use for fuel trimming at this stage until the VE table is dialed in.

To me it looks like it's hunting due to the fueling being out.
I'm the director of VSH (Vlad's Spec Holden), because HSV were doing it ass about.

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Re: RPM Dropping When Slowing Down

Postby delcowizzid » Wed Mar 27, 2019 1:19 pm

Right click on the monitor window and select narrowband o2 volts you will see closed loop swinging the o2 back and foward as you till to a stop
If Its Got Gas Or Ass Count Me In.if it cant be fixed with a hammer you have an electrical problem

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Re: RPM Dropping When Slowing Down

Postby K1ng0011 » Wed Mar 27, 2019 2:17 pm

In the screenshot I posted it has narrowband o2 volts (0v to 1v) and wideband. The O2 milivolts is in the top graph. The narrowband o2 is swinging to 0mv when the mixture goes lean 16+AFR and like I would expect when it is very lean like the wideband is showing. From what I understand it does not go to closed loop idle unless its under a certain speed. My Idle Fuel Param - Max road speed for idle A/F ratio is set to 5KPH. In the example below I am going well over 5KPH so it should not be using the idle AFR table from what I understand. Also the idle fuel param - delay before using idle a/f ratio (manual) is set to the stock 20 seconds. This is when I am slowing down before the idle AFR should come into play. The open loop thing is a good idea. I will force it to open loop tomorrow and take it for a spin to see if the RPM dips on deceleration when I force it to open loop and I will let you guys know what I find.


MAP A: A/F Ratio - Idle A/F Ratio vs Coolant Temperature

DegC AFR
-28 13.00
-04 13.30
20 13.60
44 13.70
68 13.80
92 13.90
116 13.60

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