Don't know why but it started a few months ago with my headerback exhaust only putting out 147hp and today my engine is misfiring and the exhaust tone changed. Probable causes: spark plugs, bad injectors, vacuum leak, ingition wires fkd up, o2 sensor defective, gasket leak, bad installation, and probably a lot more. You see what kind of hole I am in. Any advice will be GREATLY appreciated. I am almost inclined to just throw on a extrude honed manifold with the LSJ downpipe but still trying to consider what to do with a cat since the stock is extremely restrictive. But I replaced the plugs today and it sounds the same and I plan on getting it flashed it back to stock on Friday, and I'm taking to a guy on Monday that apparently knows a lot about 4 banger tuner exhausts that works at Midas. Wish me luck please, this REALLY sucks.

 

well, i would start by checking the things you listed and then take it from there.