Deathly low idleing KA

[DJ_Sunrise]

So i was driving to school today, i got on the highway, and as i was in neutral coming up to a stop light, my tach was bouncing around between 100 and 500 rpm. It still drove, and refused to stall on its own. its S13 with 138xxx miles. The car has a recent tunep, all vacuum lines, fuel lines, oil and fuel filters, spark plugs, spark plug wires. I replaced my O2 sensor and coolant temp sensor and still nothing. btw, i get about 10MPG if that has anything to do with it. I do smell a weird smell coming from my car, hell several different ones, but no discolored smoke. Could my cat be melted inside? I hate being a total but how much does it cost to have a cat replaced?

I was thinking.. maybe the exhaust is clogged up, and when i let the engine run a little, all the shit slowly flows outta there, because my rpms to clear up into the low 700s once in a while.. anyone got a spare cat laying around???


~Sunrise