Idle BLM

[BLG355]

I have a question about BLM at idle. I've ran several logs and corrected the fuel table numerous times, but it seems at the end of every log when I let the truck idle, it goes crazy rich and the BLM drops down to 110-112. Here is a example of a log I just ran. The other day I got everything down to with +/- 4 blm, then today I ran another log and adjusted some more, most were from 127-131. So I ran a second log today and the idle is just horrible with the BLM. Also, is there a way to smooth out the idle? My cam should idle silk smooth but it feels like its a 230@.050 duration cam. It literally shakes the whole truck at an idle, open and closed loop, the idle is crap. One thing I didn't do is lower the timing at an idle, its close to 30, but when it was at 25 and even 20, it wasn't a whole lot better, if at all.

From what I've understood, perhaps this is incorrect, the idle VE table needs to be blended in the with off idle VE table...right? If anyone has any free time and gets a chance to look and could offer some advice, I greatly appreciate it.

BLG

[steveo]

you have to experiment with idle timing and idle speed. try to get high vacuum without any 'roughness'. too little timing is rough, and so is too much.

your cam is very small, i would expect no more than 20 degrees timing would be required for a smooth idle. 30 degrees is for a big cam. imo, of course.

have you forced open loop to see if the blm trims are why it's rough? is it rough while warming up? if it's rough before it even hits closed loop, it's probably not an AFR problem. it's timing or something else.

your trims will be screwy if your timing is out to lunch, or if there's some other problem...

are you sure it's running evenly on all cylinders? perhaps there's some other reason it's idling rough?

[Roadknee]

Hot idle is still around 46 IAC counts. Open up the throttle blades. The increased airflow around the butterflies mixing with fuel may improve fuel distribution and idle quality. It might not, but at least you can rule it out.

[BLG355]

Hot idle is still around 46 IAC counts. Open up the throttle blades. The increased airflow around the butterflies mixing with fuel may improve fuel distribution and idle quality. It might not, but at least you can rule it out.

I did want to lower it some more. I remember Mark telling me, ideally you want it close to zero on the hottest day of the year with the engine warmed up. I'll get it closer.

BLG

[BLG355]

you have to experiment with idle timing and idle speed. try to get high vacuum without any 'roughness'. too little timing is rough, and so is too much.

your cam is very small, i would expect no more than 20 degrees timing would be required for a smooth idle. 30 degrees is for a big cam. imo, of course.

have you forced open loop to see if the blm trims are why it's rough? is it rough while warming up? if it's rough before it even hits closed loop, it's probably not an AFR problem. it's timing or something else.

your trims will be screwy if your timing is out to lunch, or if there's some other problem...

are you sure it's running evenly on all cylinders? perhaps there's some other reason it's idling rough?

I seen in another post your mentioned setting idle timing from vacuum and I am going to try that next. I was messing with it from 16 to 22, i did have some good results but i was messing with a bunch of other things and the idle never seemed to bad until recently. I figured i'm overlooking something simple. I want to get a bung welded in to move my O2 front and i'm going to use the stock O2 bunch for WBo2.

BLG

[Roadknee]

Take a look at this. I've come to respect Danny a lot over the years.

http://www.chevytalk.org/fusionbb/sh...hp?tid/253395/

This information was written for Holley C950 and HP aftermarket fuel injection. The principles work for GM injection too. Pay particular attention to his instructions on the base fuel table.

[brian617]

Ive noticed with my truck that idling for 1 minute at idle vs idling for 5 mins or better that the BLMs will come back around. The way I understand it, the reason to keep the idle VE and Off Idle VE close is for transition.

[brian617]

Take a look at this. I've come to respect Danny a lot over the years.

http://www.chevytalk.org/fusionbb/sh...hp?tid/253395/

This information was written for Holley C950 and HP aftermarket fuel injection. The principles work for GM injection too. Pay particular attention to his instructions on the base fuel table.

As he mentions Cell Boundaries, I often wondered if increasing idle speed too much would have a bad affect on idle. Getting too close to the cell boundary given the stock TBI engines will idle hot at 525rpm. I recently lowered mine and it seemed like it made a difference. Of course I have no idea where to find the cell boundary table for reference.

[Roadknee]

Barry - what valve springs you running and what are your open and closed pressures?

[Fast355]

you have to experiment with idle timing and idle speed. try to get high vacuum without any 'roughness'. too little timing is rough, and so is too much.

your cam is very small, i would expect no more than 20 degrees timing would be required for a smooth idle. 30 degrees is for a big cam. imo, of course.

have you forced open loop to see if the blm trims are why it's rough? is it rough while warming up? if it's rough before it even hits closed loop, it's probably not an AFR problem. it's timing or something else.

your trims will be screwy if your timing is out to lunch, or if there's some other problem...

are you sure it's running evenly on all cylinders? perhaps there's some other reason it's idling rough?

I have found 26-32 seem to work best for me on even a stock cam vortec engine.

[BLG355]

Barry - what valve springs you running and what are your open and closed pressures?

I have Howards 98213 springs, they are 120# on the seat and 300# open.

[BLG355]

I'm still messing around with this, slowly. So I figured I'd give an update. I've been able to the BLM at idle down to +/- 1 or 2 from each other. I've messed around with the timing from 22 to as high as 32 at idle, just trying to see how it changes everything. My next step is to try it at lower settings. I'm also messing with the idle speed. It doesn't seem to like to idle really good below 650rpm with higher timing, and I am going to mess with it more when I try a bunch of lower timing values and see how it does. The smoothest it runs is right after it fires off, it's silk smooth for a few seconds then starts to roughen up...but I'm thinking a lot of it is in my head...I'm comparing it to a 2002 6.0 LQ4...which is smooth as silk from idle to redline. But the whole reason for this thread was the BLM's @ idle and I did get them in line after much effort in messing with the tables again. Now I just have to mess with idle speeds and timing some more and I think I got it nailed. Thanks to everyone for their input.

BLG

[Rocko350]

I have always tuned idle for the lowest, leanest , stable idle possible with the least amount of timing. This is of course modified slightly with the cam and combination being used. I have to get more creative with tuning a build that someone else put together. I know it sounds cryptive, but the method i've reverted back to is just what we used for older carbed cars. Points, timing, then carb. It has served me well going in that order. Usually set to open loop and have a vacuum gauge hooked up and drop timing back until it starts to get thready. Then start leaning it out until is stumbles a little and then add that fuel back. Drop it in gear and feel the transition. Hopefully no surge or stumble. Quick, small throttle move in gear, foot on the brake and it should settle down slowly to idle, and not drop like a rock. Give it what it wants. That's the trick.

As far as idle fuel trims, Idle should be one of the last subroutines fleshed out after the rest of the tune is in order. SO if there is some learning out at 5 minutes, maybe increase the prop gain a little and see if the trims come back into line quicker.

[BLG355]

maybe increase the prop gain a little and see if the trims come back into line quicker.

Is this what you are referring to? I haven't messed with this at all but I could always give it a shot and see what happens.

Attachment 9082

Thanks,

BLG

[Rocko350]

Yes. The values in the 800 rpm line. Set those to the cells around them in the 30,40, and 50 kpa lines. Look at your over speed and under speed tables. Are they stock? How does the vehicle react during the transiotns from closed hrottle in gear to c/t in park. Any surge or stall?

Chris