Cold hesitation/jerking/misfire issues at light load - anyone else see this?
All,
The last thing I really need to fix on the truck tune (minus the annoying TCM relearn that has me back and forth with 4th gear shift flares after reflashing) is this cold hesitation or misfire, basically a jerking feeling. I am looking to see if anyone else has noticed this in DI engines or had any luck fixing. I have talked to several others and even different makes/non-modified cars and there have been similar complaints. Most seem to be around carbon buildup, but I looked at my valves with the intake swap and they didn't seem THAT bad.
This is a 2018 5.3L Sierra with 8L90E with 9000 miles
Here is when I notice it:
-once I am in 2nd or 3rd gear at very light load, >10% TPS, 1200-1500 RPM, 0.20+0.28 g/cyl
-cold in low gear is around 30-40C ECT, in 8th at light load I can be around 50-60C ECT
-light load uphill in 1st gear at a slow crawl
-within around 2 minutes of starting
-at times, in 6th through 8th gear under 1500 RPM at the same light load condition
-seems worse after the truck sits for several days
-notice in the AM leaving, but mainly on my way home from work after cooling all day
Here is when I don't notice it on cold start:
-when I let the truck idle moving cars around or something before I leave
-if I give it more load and accelerate faster through those areas
Here is what I have seen in the logs:
-some cases, I have seen the TPS move around a little on me
-low timing from CLO and extra fueling seemed to make it worse, I had warm restart issues because of it and just disabled it completely (no more warm start issues at all)
-I can't really pinpoint a change in timing or anything when it happens vs. doesn't, though VCP spark does make some quick changes in that load area
-The SFTFs make corrections quickly to the fueling, I am not sure if they are pulling any enrichment away that it may need or what, though I have conflicting results with lower commanded fueling that seemed to help
-I have questioned or been concerned with too low of a heat range on the plug or running 93 octane and how it affects cold start idle. I do get a little rough idle warming up, but it runs fine when hot. If the plugs are too cold (6.2L runs the same heat range I think) and I am running 93 on top of it, could I be creating a tough condition for spark to ignite?
-when this happens I log misfires, random on several cylinders. Count never get over 10 but also are never consistent
-in all this troubleshooting I also put on a catch can, did seafoam and for the hell of it put on new plugs and wires just to make sure there wasn't an issue - no changes I noticed at all.
Honestly, I would like to know the next hotter heat range plug to just do a test and see. The question is how cold of a plug does it need when hot, with 93, to not become an issue of being too hot? Again, assuming that may be part of an issue. I have tried 91 octane and the issue felt just as bad. Have not gone back to 87 for any tests.
Here are things I have tried and the effect:
-Open loop IVT gain had some sub-1.0 areas so I set them to 1.0. That should command extra fueling and man it misfired bad. Odd thing was the STFTs were still doing their thing so I would have expect the fueling to be about the same. With a cold sensor, maybe the error is too large.
-I have tried to run it hard before shut down to make sure it "cleared out" the cylinder before the next cold start. I have not seen anything consistent here to make me feel like it helped.
-I added some timing to the ECT spark table and I felt like it got better. I added more and would say it got worse. Then I question if it is so inconsistent that these minor changes are only by chance
-I have tried to add and take away from the CL delay to see if that would help. Since the STFTs are active, I really didn't see any help. I did feel like at one point the extended time I did have (90 seconds vs about 15 seconds) helped, but again I didn't feel this consistently and think it was just by chance I felt that.
-I noticed the o2 switch point was quite rich in the 39C ECT area. I tried to pull it back and when I dropped the 300mV offset to 150 mV it felt the best. This confirmed my suspicion that the added OL IVT gain being richer made it worse. So adding fuel made it worse, pulling fuel made it better. So my concern right now is too much fuel
-I tried to lean out some of the OL tables with no luck, I feel it got worse. I may have reach a point of diminishing returns (assuming my improvements were real).
-I tried to mess with the idle external load without much luck, my tune had some odd humps compared to other older files. I have my idle speed at 600 RPM as well so I can let off the gas and roll in the garage without having to give it throttle. I have questioned the idle airflow/TPS effects on this based on the fact that I have seen some logs with the actual TPS varying with a constant input (I also questioned the driver demand, but my changes there were minimal and smooth).
-I tried to smooth the VCP spark, no luck. I have moved around the timing tables without huge repeatable changes. In that area I am around stock timing or at leas within 2 deg.
-MAF is all dialed in, both with STFT and WB. I have not touched the VE with minimal mods and honestly just needing to understand the VVT and how it plays into it/properly logging and tuning
Sorry for the long post, but I think that covers it :D
If anyone has similar experience, feedback, etc. Let me know. I tied to post some logs of the first 1-2 and 2-3 shift where it was happening but it will not load. I will try again later.
