GMC Acadia 09

You have 4 O2 sensors on your exhaust, one of which is after the cat, and assuming you hooked all 4 back up, then you are likely throwing an O2 code, or codes, and the ECU is dumping extra fuel into the air-fuel mix because it is seeing waaay too much air in the exhaust after you gutted the cat. They put an O2 sensor after the cat as that monitors oxygen in the exhaust flow, to ensure there is enough oxygen to burn the unburned hydrocarbons and CO produced by the engine. Too much oxygen after the cats then it's too lean, BUT too little oxygen results in unburned emissions in the exhaust. So your O2 sensors and ECU engage in a balancing act, just the right amount of oxygen needs to be in exhaust to prevent running lean or rich or leaving unburned emissions. By gutting the cat, your O2 reading is now off the charts and ECU is attempting to compensate by dumping more fuel into air-fuel mix, result is running rich. You can either restore a proper catalytic converter, OR get a custom "performance" programmed ECU. Keep in mind your factory ECU can only accept factory presets so to reprogram a setup for your custom exhaust would mean an aftermarket chip.