1600 RPM Bucking Bronco

[tazinhawaii]

I have some pretty bad bucking at 1600 RPM and only with VERY light throttle. If I accelerate through 1600 RPM, there's no bucking and if my foots off the accelerator at 1600 RPM there is no bucking. I have roadrunner and have tried changing base spark, high/low octane spark, desired airflow, TC, VE table in this area (1600 RPM and around .2 g/cyl). All with no effect. The logs do show about 10 degree timing swings when this happens. I tried logging every timing related PID and the numbers just don't add up either... Base spark or run spark with smoothing doesn't equal actual timing (all other PIDs were 0). I am attaching my tune and a log of this event. It was bucking pretty good at the end of the log (and a few times in the middle of the log). Anyone find something that works who has experienced these conditions?

Derrick

P.S. This is SD tune and no factory O2's.

[tazinhawaii]

I think I found the problem somewhat. It's the drastic drop in spark advance in the .08 g/cyl area. I don't think GM intended the car to operate down here, but due to having to "trick" the PCM to keep air flow <2.5 g/cyl and setting a p1514 while running 14 psi boost, I end up operating this low. I bumped those spark advance values up to the rest, and it's much better. I little bucking, but livable and I'll keep tweaking...

Derrick

[hquick]

Yep...I discovered my bucking at low speed (40kph or so) and around 1900-2000rpm as caused by not enough spark in those area's.
I was pretty much just hitting boost and it would start to buck. If I pushed it hard...it'd push through it and be fine.

[tazinhawaii]

Well, I think I finally found the problem... Crank position sensor relearn of all things. Somewhere in the past few weeks I used an old tune from when I had a bad reluctor wheel and had P1336 code turned off. I forgot that for some reason my roadrunner looses it's crank learn when I disconnect the battery and didn't reperform the crank learn. I reactivated the 1336 code and did the relearn. Loaded the stock spark tables and presto. No more bucking really. Some very slight bucking, but liveable... Just wanted to update this thread with my results in case someone else needs the info in the future...

Derrick

[ForcedC5]

Congrats!!!:thumb_yello:

[PRAY]

How did you do the relearn? I just put a 383 in a guys car and I have this very issue with the crank learn thing. I had turned off the light and code and now the car is running like crap. I guess I have to turn it back on now.

[tazinhawaii]

In the scan tool there is a crank position sensor relearn function. A bunch of other codes can't be set (i.e. 0335, 0336, etc.) and then you just warm the car up to the required temp, and follow the instructions. It's actually not that hard.. I just wish EFI Live told you afterwards if the relearn took. The only way you have of knowing is to turn the car off and on again and see if the P1336 is gone... I'm pretty sure detailed instructions are contained in the EFILive manual on how to do this.

Derrick

[SSpdDmon]

This has always been a point of interest and confusion for me. I read articles and see 'pro' tunes that say more timing down low is what you need because a leaner mixture at low air flow levels (i.e. light throttle) takes longer to burn than that of a richer mixture. To some extent, I agree. This is why we see 35+ degrees of timing when cruising around at 3000rpms and only ~27 degrees at WOT for the same 3000rpms.

Now, here's where I get lost. I don't know if it's because of the design of our Gen III V8's, the cylinder heads, or what. But, I've tried jacking up the timing in the low regions only to find that the bucking I'm trying to get rid of is anything but gone. The only way I've been able to cure the problem with my old cam'd LS1 and my friends' cars is to reduce timing and limit the rate at which it changes. In other words, hooking up the bi-di controls and commanding 18 degrees of timing at light throttle during a 1600rpm parking lot cruise in first gear was a hell of a lot smoother than say 38 degrees.......and the thing that is so irritating for me is, I can't for the life of me figure out why!?!?!?!

http://www.rodandcustommagazine.com/techarticles/0604rc_ignition_timing/index.html

[PRAY]

My car is bucking bad now with the big cam. Timing doesn't do any thing for it. I have gone down to 18* and up to 40*. I think it is all in the fuel. I am going to put in a COS and see what happens. The funny thing is it was a big change in going from a 244/248 114 cam to my current 247/254 112 cam. It was drivable at 1,500 with the smaller cam. Not it is unbarable under 1,900 with the bigger cam. But there was only about 6-8* of duration change.

I will look in the system for the crank relearn. I just found out that I have another car that is doing it as well.

[tazinhawaii]

I haven't really updated this thread in a while, but I finally got the bucking to a reasonable level. But it took setting the timing at 43 degrees and getting the A/F ratio to 13.0:1 or even 12.5:1 in those cells. As soon as I lean it out or reduce timing, the bucking comes back and the more I reduce timing, the more violent the bucking... I don't know why. So basically, my cells between 60-75 kpa and 1600-2000 RPM command 12.8:1 and 43 degrees now. All the rest are 14.7:1 (except WOT) and stock timing...

Derrick

[PRAY]

That's what I figured. I am going to COS3 tonight with the partial CL option. I will try increasing the timing first and see what happens. I think it is at 35* right now. I would just hate to run down the highway at 13.00 afr.

[VTC_WS6]

I read articles and see 'pro' tunes that say more timing down low is what you need because a leaner mixture at low air flow levels (i.e. light throttle) takes longer to burn than that of a richer mixture. To some extent, I agree.

Kinda odd isn't it? Conventional wisdom tells us leaner hotter mixtures generally promote flame propigation much quicker than cooler mixtures of the same density, and when you consider the increase of the natural EGR function of the bigger cam (excess duration + lengthy overlap) and it's cooling trend of the charge, it makes sense as to why you'd increase timing in an area with such little flow. Make any sense?

[SSpdDmon]

Kinda odd isn't it? Conventional wisdom tells us leaner hotter mixtures generally promote flame propigation much quicker than cooler mixtures of the same density, and when you consider the increase of the natural EGR function of the bigger cam (excess duration + lengthy overlap) and it's cooling trend of the charge, it makes sense as to why you'd increase timing in an area with such little flow. Make any sense?
Yes and no. How much cooling, or change in temperature, from a duration/overlap change do you expect to happen in such a short amount of time? From what I understand, air is a very poor thermal conductor. So, why would there be such a significant delta in comparison to say a stock cam?