Another_User

I don't know if it is actually progress, but I was doing some testing and managed to create/amplify bucking under normal throttle conditions by changing the O2 error, adder, and pretty much all the other O2 related stuff. It freaked my STFTs out real bad. So now I need to figure out why, and reverse it. Kind of like turning the Hulk back to a normal guy...hmmm. For those that are interested, I was intrigued by the difference in Corvette tables, so I did some quick math and saw that generally (after applying the multipliers) they were in the same ballpark, and that IFR doesn't actually seem to have any impact on how GM tuned things, so I replaced my 98 tables with those from a 2000 C5. Interesting note: I wanted to use the tables from a 2002 Z06, however there is a limitation in the earlier PCMs that won't allow multipliers as high as the ones later on. Interesting stuff, but I don't know if it will help me in my surging/bucking pursuits. Back to my logs...

God Forgives I Dont

I created the surging and bucking on my car one time. I've been trying to remember exactly what I changed because I only changed one or two things that I had read somewhere helped with Idle or IFR or something and first run out I felt all that surging and bucking when I was low RPM driving. It did help with what ever it was supposed to, but I wasn't about to put up with the surging and bucking over what ever the problem I originally had. So after a couple of days and messing with the tune trying to fix the surging I finally gave up and went back to the tune before the change. I'm going to go back through some of my earlier tunes to see if I ladled it and then I can go in and tell you what I had changed maybe it will help you isolate your issue.

Another_User

I have actually eliminated almost all of the surging/bucking, but it still lingers a little at really low rpms. I am being picky. And stubborn. I have narrowed it down to being primarily fueling, and amplified by some IAC and spark tables here and there...it's just a matter of figuring out where the PCM is not agreeing with me...

luv2spd

iTrader: (1)

Is the bucking a product of being rich or just messed up 02 readings from cam and LTs?

Another_User

First you have to get all your VE and MAF fueling right. Then I think (still speculating) that the bucking is a product of the base adder being too high in a specific airflow, which is causing the PCM to overcompensate. I have had my eye on Airflow Mode 8 for a long time, because the airflow usually overlaps with where my problems are. It is also the highest value on my base adder table by a lot. At least it is something new for me to try. I had tried changing whole tables before, but I never really isolated that airflow mode and made changes...

Longtubes cause errors with the O2s that are probably outside of the range for normal tuning. At least with the stock sensors.

luv2spd

iTrader: (1)

Yeah, I figured that most of it was fueling. What have been your experiences with adding or subtracting spark advance in regards to improving the bucking? I'm pretty sure it helped my car when I added some spark to the low rpm, low load areas of the table. I have heard others say to pull spark to help with bucking though too. thoughts??

Another_User

I have never found any significant impact (and I have made some really drastic changes). The only thing timing related that I have seen impact bucking is if your car starts bucking and the "bucks" make you jump between two areas on the timing table where there is a significant different in timing between the two areas. That causes the "bucking" effect to be amplified. I have seen the same thing occur with the throttle cracker table, if the bucking causes you to jump between two RPM areas on the throttle cracker table and it starts opening and closing the IAC. Adding timing has only really helped me idle smoother, and even then a lot of that is actually fueling.

I have started doing some testing tonight with O2 proportional base adder changes. I reduced the value in the area where I am experiencing bucking, and it seems to have had an impact. I will need to take a longer drive to be sure.

pkincy

iTrader: (1)

I suppose that fueling will also be a factor, but I think you can control it more with spark. I add a TON of timing to my idle and very low load spark tables for a big cam. And I mean a TON of timing. The overlap a big cam has bleeds off a bunch of cylinder pressure. You need to get that pressure back. I have about 40 deg of timing in the idle cells where the stock pcm tune was in the teens/low 20s.

The surging is a feedback loop getting to a resonant state. It moves from one cell to another (generally do to MAP changes) and tries to control the surges and the changes the pcm tries to make simply makes it move faster from one cell to another. Log those surges or set up a dashboard with a prominent MAP guage. you will see wide swings in the MAP readings when the surging occurs. Smooth out the idle fuel and spark curves in the idle area and you should be able to control it.

I can drive my car in first at 1500 rpm without surging now and I still need to tweak the tuning some. (mid 230s at .600 lift on a 112 with the cam)

Perry

Another_User

I have thrown up to 50 degrees total timing at it at low RPMs with minimal difference. It affected my trims more than anything else. I imagine throwing a few degrees of timing at it now that I have everything else ironed out would not hurt though.

jimmyblue

iTrader: (11)

I think you could also have the surging from discontinuities
in the VE table (MAP or RPM steps in mixture). But you don't
need discontinuities or gross steps to make the idle control
unstable, simply having slow enough response to the IAC
position (over-rich / under-advanced) is enough for a lot
of folks. Also bad O2 sensor responsiveness makes for a
grossly wandering mixture.

Another_User

I have seen some ugly spots where the car suddenly becomes rich and the STFTs shoot down to negative 40+. I think that is related to having a ported MAF. Can anyone confirm?