I went to Pepboys and have them replaced oxygen sensors (2), spark plugs, spark plug wires, fuel filter, and map sensor; performed a fuel injection system cleaning; and cleaned all engine codes. All of that for $600. The gas mileage improves from 21 to 25, and stalling, rough idling, and buring rich all go away.
However, there is still one problem that Pepboys did not detect and I spent additional $80 with that. 60% of chance after the car starts, especially when it is warm, occasionally at cold, it hesitates 5 to 15 seconds after the car is in motion. During the temporary hesitation, the rpm fluctuates from 300 to 700, then jumps to 1400, and finally falls back to 800. It is more obvious when the car is moving forward than backward. Can this be a transmission problem? It hesitated but won't stall.
I need to replace the transmission fluid and filter any way. It has been there for 3 years.