I installed a new tune on the GXP, and am experiencing pretty severe misfires at 20+psi. The tune aims for 24psi. (I'm at 500ft above sea level, FWIW.) It's bad enough that the engine starts to lope - cylinder deactivation, I'm sure, as it clears up ~30 seconds later - and I get the lovely flashing (misfire-induced) flashing CEL. The CEL goes away shortly thereafter, BUT scanning the ECM reveals P0300 (multiple random misfire) and some P0301/2/3/4, which moves between cylinders. I've never seen all four P030* codes; typically just one or two cylinders.
The tune came from a reputable vendor, but I'd rather not mention whom as I'm reasonably confident that this is an issue with my car, not with the tune per se, and I'd rather not hurt their business.
I've swapped the coils from the blue car, replaced spark plugs - they needed it - checked for vacuum leaks as well as I can without the assistance of a hair band roadie and their equipment.
If I keep my foot out of it and stay below ~15psi, everything is fine.
With the GMPP tune vs this new one, I hit 14 PSI very quickly, but it doesn't try and go higher than that, and it works well (as in, no misfires).
I went ahead and did a compression test, just for fun and grins, and got a respectable 165/158/164/165. The slight drop on #2 is a little odd, but well within spec.
(Yes, I know that driving with a severe misfire condition is a Very Bad Thing. I went back to the GMPP tune for the time being.)
Any suggestions on a diagnostic approach?
Some more data:
* I also recently installed DDM's charge pipes and CAI (it's nice to be able to hear the turbo now), and PWerk's intercooler. Aside from that and the GMPP MAP sensors, the car is stock other than the tune(s) and brakes / suspension bits.
* While installing the pipes, one of my charge pipe brackets (driver's side) went missing. I haven't got around to making another one. I can't imagine how this could possibly be relevant.
* This is on an '07 with just over 100k miles on it. The battery is from 12/2014, and appears to be in great shape.
* I replaced the HPFP a month or two ago due to a hard start condition, which instantly disappeared after replacing the pump.
* My windshield is still cracked following a rock encounter at Black Hawk this year.