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Turbo LQ4 Torque "Flatlines". Feels Like Being Held By A Bungee Cord?

PCM Diagnostics & Tuning - LS1TECH -
Hi All,

I've been struggling with an issue that I can't figure out for a couple weeks now. My truck shifts great and pulls very well under less than 60% throttle or maybe at less than around 4,000 RPM. The problem is that when I try a WOT run it will hold back at around 4200 RPM so It won't reach my shift points. It will do this in first or second gear for sure and you can see it in the log I've attached. The delivered engine torque just goes completely flat. I've posted on the HPTuners forum to no avail and was hoping someone here might have run into this issue.

The feeling is like being held back by a bungee cord or like trying to break though a balloon. It doesn't seem to be dropping timing, at least by what I've logged. In fact, I've effectively disabled Burst Knock Retard in my latest attempts just to test if it got better since I saw some BKR in that area. I'm not seeing any knock and I'm running at 11 degrees base.

This doesn't seem like a mechanical issue but I haven't been able to fix it with anything I've done with the tune yet.

Below you'll find the build and an attached log file. I'm also attaching a tune but it's not my most current (BKR is still enabled and other small stuff). The same issue is there in both that tune and my current so it shouldn't matter.

I'd really appreciate some insight on this. I just don't know where to turn with it at the moment.


Here are the specifics of the build:

01 GMC Sierra 2500HD 2WD

LQ4 6.0 L engine

Swapped from P01 PCM to P59 PCM

HPTuners Custom OS 3Bar Speed Density.

MAF Deleted

PAC 1218 Valve Springs

Siemens Deka 80lb injectors

Flex Fuel Sensor

Walbro 450lph in-tank fuel pump.

Stock feed and return lines.

VS Racing 78/76 T4 Turbo

44MM Wastegate plumbed back into the exhaust post turbo.


Log manifold for the hot side made from flipped drivers side manifold.

2" crossover from drivers side to passenger side.

AEM Wideband

AEM Tru-Boost boost controller.

4L80e transmission

Transgo HD2 Shift Kit

Circle D 2900 RPM Stall Converter

11.5" AAM 3.73 ratio G80 optioned Rear

265/75/R16 tires

Stock exhaust minus the CAT

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