98 Mustang GT will not idle

So my car (98 Mustang GT) for the longest time had problems running in general, and especially idling. The guy who did my exhaust cut the cats out for whatever reason without telling me So I've changed the IAC, the plugs, wires, fuel pump, and now injectors. The injectors makes the car RUN like a dream. However, it's been hard to turn over lately? It'll go to start but won't ignite for a little bit. The car misfires quite a bit as well. When I'm sat at a red light or push in the break hard enough, it'll go try and die QUICK unless I rev it. I'm really not sure what else to replace. Some other are saving it could be related to the EVAP, not sure. It does have an exhaust leak in the right header I haven't fixed yet.

I did have to replace the tranny. It was an auto, now has a T45.

Maybe related? My AC always blows hot exhaust smelling air into my cabin. Any idea why?

wmburns, you know I've been trying to narrow down that misfire issue but I can't figure it out. I guess I'm going to start trying to fix the exhaust leak on the header but it keeps burning and spitting out the gasket. Like I've gone through two already. Also would the no cats cause my O2 sensors to be wonky or something?

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GDawg said:

This may be obvious but is your "check engine" light on? Is your car throwing any codes? If so, what are they?

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The only things it throws is O2 Sensor Circuit and O2 Sensor Heater Circuit. However, after running it for a couple months, it also said system too lean and Cylinder 4 & 5 misfire detected.

I cleared those codes and drove the car for about a week but the only thing that came back is the O2 sensor codes.

I also just swapped the coil packs and I didn't seem to fix anything.