3800 SC stumbling under heavy throttle

[srf13]

Hi all,
I'm new to tuning and haven't modified anything on my PCM yet. I'm still learning how to read the charts and figure out what needs to be done.

Here's the problem: 3800 with mild changes (bigger throttle body and 3.5 pulley, ported exhaust) stumbles (KR appears most of the time) under heavy throttle input.

Any help with reading the scans would be appreciated. I'm mostly looking for some direction on finally clearing up this irritating problem

I've attached the latest scan. It was the first run with a new PCM thrown in. I didn't have an issue until around 5:45 where KR appears. At 6:17 the chatter in the RPM line is pretty noticeable, but there's no KR (confounding).

Thanks for any help!

[louvered97gtp]

Hi, welcome to the community. Seems you need to log more parameters, not enough info. Tuners on this forum use similar methods but look for different tell-tale signs depending on their experience. You are not logging TPS, MAF or AFR. I see your RPM's fluctuating, but how are we supposed to know if it's throttle related (tps) or air flow (MAF hz)? Maybe some of the pro tuners on here can figure out your problem with less logged parameters based on their vast experience, if you peak their interest enough for them to respond to you.

For now I'd suggest begin tuning your MAF, and your stumbling should start to go away. you can't change your engine parts to aftermarket without expecting a computer controlled car to freak out. Bigger TB means slower velocity though your TB since you've in creased the inlet size, this affects your MAF readings, and changes your fuel curve try adjusting to your commanded AFR.
Good luck.

[srf13]

Thanks for the reply.
I hadn't thought about tuning the MAF. That will definitely give me something to chew on for a bit, thanks.
For the log - I actually had all of those parameters being recorded. At least I thought I did... I see all of it when I open it with my scanner. Must have missed a checkbox somewhere

Thanks for the help!

[matt5112]

Define bigger TB.

Without knowing that, we can't suggest a maf calibration.

Changing the cross section of the point where the MAF sits is what would most greatly affect your maf readings, not the TB blade.

[Phantom]

First thing you need to do is adjust the MAF I would multiple the whole table by 1.1 or 1.05 to increase the table by 10-5%. After that clear the fuel trims with the scanner be going to VCM Controls>Fuel & Spark>Reset fuel trims (scanner needs to be connected to the car).

Warm the car up and do some scanning low load and cruising do not gun it (under 7500hz), if the fuel trims are +10 or more all over the table multiple the whole table again.

When the table is with in 7-5 you can start fine tuning the table and try getting it with in 2-4.

[GeoJB]

Do the VE table first.

[GeoJB]

Your car seem to be stuck in 16.4 lft = lean. Probably from the, pulley, TB and freer flowing exhaust.
Do you have a rear Cat/O2 sensor?
What kind of car?
I'll second the notion of better scanning parameters.

[srf13]

Everyone, thanks for the help so far. I've gone through multiple iterations of MAF tuning, and I think I finally got that down. I've managed to get the AFR error down to +-3%. I think that might have taken care of the stumbling issue. I've been constantly reading and trying to learn what to work with, and I think I might need to start adjusting the spark timing. Is there some guide on spark tuning? I don't want to just go randomly changing numbers. Thanks!

[Phantom]

For spark tuning after fueling is delt with you will scan run a histogram for KR. I use a cell count on 0 for KR and use that when reduceing spark if you have any KR reduce spark by 1. Then use a a histo with the cell count between 3-6 if the cells have no KR you can then increase that cell by 1. I also keep an excell file to log the cells I have reduced so I will not increase them if you eliminate the KR.