I’ve got a 2001 Corolla with ~142,000 miles on it. I’ve recently replaced the plugs (NGK Iridium about 9k miles ago), have an aftermarket SRI that’s designed specifically for my car (Weapon R Secret Weapon intake), and have had the catalytic converter replaced professionally about 6 months ago. The CEL is on (been on for ~1 year without serious issues). Recently I’ve noticed it’s idling rough. It acts like it wants to idle low, then starts misfiring to the point if I don’t press the accelerator it will die. I can hear when I drive down the road it misfire at points, but not frequently. When I had the cat converter replaced the CEL went off, but came on about 2 months later. I know it’s passing something through the exhaust (not burning completely/too rich?) and that probably means there’s an issue with not burning fuel correctly, but don’t know what’s going on. One thing that may affect it is the cold, but I let it warm up long enough the engine idles at ~800RPM before driving (which is normal for this car). Any ideas?
One more thing…it DOES eat a bit of oil (~1.5 US qts/3000-3500 miles), but I’ve found that for this engine that’s a common occurence, and I make sure to keep it topped off at all times.