check engine light on. burning smell from heat vent. rough cold start. on 1999 Mercury Mountaineer

check engine light is on. no increased heat per temp gauge. slight burning smell from the heater vents but no smoke. car is progressively getting worse about needing gas peddle applied until engine is warm or it will die when idle. I just had transmission flushed, oil changed and applied fuel inj cleaner to gas tank multiple times to try to correct this. what am I missing?

Asked by for the 1999 Mercury Mountaineer