A few months ago I bought a 1993 Mustang Hatchback 4 cylinder N/A, with the automatic transmission. The problem I'm having is that the car doesn't run quite right. At low speeds(40 and below), and low rpms, the exhaust smells very gassy, and I get very poor mileage in low speed stop and go traffic. Also, when I come to a stop the engine occasionally shudders, like it's going to stall out. And when slowing down from the engine pops, like it's backfiring. I've replaced all the usual stuff, the spark plugs, the air filter(with a K&N), pcv valve, transmission fluid/filter and the fuel filter. Anyone have any ideas what to look at for improving the car's mileage/performance?