Breaking Up Consistently at 4,000 RPM

[Mr. Church]

Hi guys, I'm working on an LQ4 that I swapped into a friend's 87 K10 with a TH400. The engine has stock 30lb 8.1 injectors, a BTR Stage 2 V2 truck cam and their LS6 springs. I've had an bunch of issues with the truck, but slowly getting them sorted out. Currently our issue is that it pretty consistently is breaking up and cutting out right at 4,000 rpm +-100. The truck is running great right now at part throttle, idle and cruise, and even first gear seem to rev up fine but once it hits second gear and the engine is under more load, it breaks up.

The engine is an 05 LQ4 but the pcm was gone from the pick n pull so we snagged one from a 2005 LM7 Suburban. I've compared the tube to numerous others that i've got saved from tuning other vehicles and can't seem to find the issue. The issue is noticeable at 15:58 in the log. It runs through first gear fine then when second hits, at 3970ish rpm the injector duty cycle shoots up, delivered engine torque dips, injector PW jumps 3 ms and the commanded AFR goes from 12.45 and then begins dropping to 11.31. I've even tried disabling power enrichment to eliminate a variable yet the problem still persists and the AFR drops the same. Any help would be appreciated guys! Thanks!

[anniversaryss]

Try disabling COT (cat over temp).

[Mr. Church]

Thanks for the reply, I will give that a shot next!

[JoshuaGrooms83]

I was having the same issue with mine tuning adventure as well. still haven't smoothed it all out but I was hitting that rich wall at about 3500. Then I found that my IFR was not set right, partially my fault, mostly the previous tune shop. My tune is 2bar SD OS, and I have 60lbs injectors with a boost referenced FPR, but the IFR was set to 60lbs, which under normal driving and massive boost leaks, is fine, however with a sealed hot side and incorrect IFR, this caused massive rich conditions in my new turbo configuration. Big thing for sure is make sure you do not have vacuum leaks, especially from an uncontrolled source. this will wreck hell on tuning, and throw stuff all over the place.

I'm not sure if you condition is similar to mine, but it would be an easy insurance to make sure your injector fuel rate is set for 58psi if you are running the stockish fuel pressure, as injectors are rated at 43psi, so some adjustment needs to be made. For example my 60lbs injectors at 58psi, are more like 72-75 in the IFR table. checkout injector flow data for your specific injectors or close to, floating around one of these forums that should get you going if this is the case. once I bumped those up to just a generic 72, it was night and day difference and finally could get the car to get up to 4500rpm and pull strong, until I popped off a coupler lol
I gotta tighten some clamps and double check my wastegate, but this is one area that seems to be an overlooked item on occasion. Again, cant say exactly your conditions, as every setup is different and you aren't pushing boost. however, you initial tuning should be similar to most to get your tune sorted out.

[Mr. Church]

Our setup is running an external Walbro 255 pump and C5 Vette filter) regulator. At idle and part throttle it runs around 68psi, but wide open it's dead nuts at 60, so I just used the stock injector data from an 8.1 tune that I had done previously

[Mr. Church]

Well I gave the COT disable a try and to no avail. I even zeroed out the enrichment tables for COT just as a precaution and now it seems to run worst if anything. Hopefully I can snag a fresh data log tomorrow

[JoshuaGrooms83]

I found that looking and feeling where exactly it is that you are hitting this wall can help to smooth out these issues if it is indeed a fueling issue.
I noticed that in my VE that i was consitantly hitting a wall of over fueling conditioning at about 3500 to 4500 at 150-175 kpa and my AFR would dump to 9.0afr.
I also started noticing that my idle was starting to turn to shit and lean out excesivly everytime i was initially logging and copying & pasting that data. So what was happening was two fold. The table wasnt moving enough in the rich areas to notice much change due to excessivly too rich, and then the areas that were showing change was doing so too much, such as my idle area, that after about 3 or 4 datalogs, had my idle at 20 afr and soundling like shit and even almost dying and even surging.

This is when i started to pay attention to targeting tuning these regions. So it isnt just a matter of copying and pasting and then adjusting, its also paying attention to which way the afr is going as well. After about 3 sessions of just idling in my driveway, i leaned out my idle area in the VE which was around the 800-1200rpm range and rougly 70kpa and slowly added fuel to it, roughly 4%, and finally got it to 15-16afr, which is much better and the car was purring like a kitten again.

Then knowing that, i finally was understanding the colors in the VE table too now, as well as actually analyzing some graphs and data to specifically target that rich area i know i kept hitting by looking at rpm vs kpa vs afr, which led me to lean out one section and smooth around 160kpa by about 2% and then again after another run. Ive finally started seeing my afr in areas i have not seen before, +/- 5% if not less in many of the cells that conastasntly were around 20 to 30% afr error before.

So this method i feel can target trouble ares that seem to not be moving fast enough, with all things running correctly and reading right. This also now shows me the correlation between commanded and actual afr as well. This also demonstrates the need for patients and developing your own way of approaching tuning, depending on what you are presented with.

Im still working on it but now my entire tune is starting narrow the afr spread, and im actually seeing PE and BE afr where rhey are suppose to be. I still need to work on areas that are over 5500rpm and 170kpa as my afr is still rich, but now its around 10.5 instead of 9.0. Now i can sneak up on leaning it out better. Hopfully this weekend, i can finally clean up the rest of the tune, and then work of spark timing, and then finalize my boost settings.

[smokeshow]

Well I gave the COT disable a try and to no avail. I even zeroed out the enrichment tables for COT just as a precaution and now it seems to run worst if anything. Hopefully I can snag a fresh data log tomorrow

Your problem is that you haven't calibrated your MAF properly. And you're being fooled by a noisy MAP sensor preventing dynamic airflow from entering steady state. Your VE table airflow is much lower than your MAF, so I'm sure fueling seems OK...until you hit the dynamic air cutoff at 4000rpm and you're forced into filtered MAF only. At 4000rpm in your file, I calculate about 0.62g of air in the cylinder. The MAF is almost double that.

Need to calibrate your airflows...