I have an 05 LQ4 in a '88 k2500. I'm running a FiTech Ultimate LS 70050 kit on it. It has run ok until randomly a few weeks ago it started spitting and sputtering REALLY bad. It's undriveable at this point. It will misfire seconds after starting it cold, and will get worse as it warms up. Revving it to 2k+ will mostly clean it up in neutral, but under load it will fall on its face. I have replaced the spark plugs, wires, coils, injectors, Crank senor, Cam censor, wideband O2, IAC valve, air temp sensor, fuel pressure regulator, cleaned the throttle body and checked the grounds. I even checked my valve springs for Christ's sake. I haven't found ANYTHING that would cause such a horrible running engine. It doesn't knock, tap or tick. When idling you can smell unburnt fuel in the exhaust. The FiTech isn't throwing ANY codes. I'm at a loss with this POS.