Even though the CEL is not on, have it checked for codes.
Normally the only reason it could get that bad and have no CEL is if it was not reaching closed loop operation (ie. bad coolant temperature sensor or MAF not plugged in).
Vacuum leaks, IAC, MAF, or intake air temperature sensor could be responsible too, but also check the plugs and wiring.
Running diagnostics on the car ($100) will show if the computer is getting bad information from one of the sensors.
