Number of things can go wrong with an EFI car when it comes to bad economy or unstable starting condition. Then you need to tackle the problem wisely and eliminate and cross check each and every sensor one by one.
- Did you check the Catalyst Converter, it their is one, have it removed and throw it in dust bin.
(It can also be an issue with bad fuel economy)
2) you need to open the throttle body, clean it and readjust the throttle position sensor to factory specs.
3) Did you check the positive crank case ventilation valve, get a brand new one and have it installed. sometimes, it stops working, fails in two ways, sometimes also creates an issue of sludge and poor fuel economy in the engine.
4) Did you verify the fuel pressure.
5) did you clean the fuel injectors.
6) did you cross check the ignition coils.
7) did you check the pressure inside intake manifold.
8) Did you check the MAP sensor and compared the values with another vehicle of same model.
10) Check my thread "Bang my head" i verified and cross checked every sensor and on last was Variable resistor.
Only few, who are good technicians or have a good mind can solves this problem with patience and maintaining the diary of what has been eliminated and what yet has to be left.