My 2010 Sierra (E38, 5.3) has been giving me an odd buck under load. I attached some screen shots of what I'm feeling and seeing. My log it too large to upload.
I can feel a buck on steady state/cruise and under heavy load like climbing hills or pulling into traffic with a longer acceleration period. It registers as a loss in engine torque, but what should I be looking for in the tune? It started or became noticeable after I added MBRP cat back, and got worse after going to LT headers and ORY. Retuning MAF and VVE didn't help.
Only other mods besides exhaust are UD pulley, CAI, and wires.
log all your timing PID and see if your ecu is pulling timing, base advance vs actual advance. And see if timing is being pulled in those areas, as well as knock retard. I had the same issue around 1600 rpm, found it to be timing due to knock
Here's everything I'm logging for pulling and adding timing. I'm pretty sure that's all the parameters that can pull timing and be logged (?). All knock parameters were 0's through the entire log, even when there's a dip in torque. Timing isn't affected, or enough to make a 40 lb/ft difference in the logged torque. Timing goes down or up by .3* through the entire dip.
It did get better today. Less often and less noticeable, but still registers as a 30-40 lb/ft decrease in torque. I flashed a different timing table, as well as increased my torque converter offset to 90. Only 2 changes.
I struggled with this damn jerk, buck, or whatever you call it also for the last 6mos. When I started VE tuning I saw this small sharp dip in the same rpm range where the buck was happening. After smoothing out the VE table the buck went away.
Make a histogram that shows your ve value cells when the drop happens, I’m thinking there is a discrepancy with ve in that area that is dropping timing to accommodate the ve value.
Post a full log of the issue if you can. What does the histogram show in that area of torque drop, fuel discrepancy? Do you have ability to log ignition advance and base advance. One will show what you are commanding (base) and the other will show what is actually being applied (spark advance) with all variables in affect.
Circling back. Finally got my wideband installed (AEM X obd model), and remodeled the MAF transfer and VVE. I was shocked that after using STFT (LTFT turned off), in OL MAF only I was commanding 13 afr, and 10 at WOT, but in OLSD I was commanding 15.5-18.5 and 14.7-15 at WOT. Tuned both to a -2 to 0 error. No buck whatsoever in OL.
Loaded the final blended tune with CL enabled, and the buck is back, just nowhere near as severe or often. Also, in CL STFT are running +10 -15 across the board, though WB is showing .99-1.01 lambda average in scanner, and hanging around 14.7 on the gauge. Of course, my logs are 45 minutes long and too large to upload.
Here's what I'm seeing from the scanner. When I feel the buck, I see the NB's both literally drop out, flatline, and the WB go lean to 1.21 lambda..
Ran it about 35 miles yesterday, and another 60 so far today. Same tune but CL disabled. Buck is still there, just not as often in blended OL. Also, I still have O2 MV readings that sometimes move around in OL, but no stft activity.
Discovered there's a link between the dip in torque and NBO2s dropping out. Wideband reads lean. Logged DFCO status and is almost always on. Reverting to stock settings and will report changes back. Took some shorter logs today that I will post when I am on my tablet.
Quick update- I did 2 things with the same tune when I changed it back to stock DFCO settings.
I noticed that my TQ model showed a difference from stock, which was probably an accidental save since it read lower and I had only played with once prior Put that back to stock, and DFCO settings to stock, and it went away. I've since tweaked both TQ model to more accurately reflect what the computer says for delivered, and DFCO to be more aggressive, and no longer have the buck in CL.
Reply With Quote