I have a tuned G6ET that runs fine when driving normally but when I put the boot in up a hill or to overtake etc it will start running really rough and blow black smoke at around 3500rpm. Once I back off it takes a couple of seconds before it runs smoothly again. It's not logging any codes. It was running great for 18months after HP tune was done. So far I have replaced all plugs and gapped at .7, new Bosch coil packs, 02 sensor, Walbro 460,
Scan tool shows its getting to 35psi (absolute) boost, so that's about right for tune at 18psi.
Mods done before tune injectors, intercooler, cat delete, CAI, exhaust.
Like I said, it ran great after tune, 389kw
I'm at a loss.
Any help would be greatly appreciated