Crappy Fuel Economy

[BADan]

Hey guys,

As you can guess by the title, my car (EFII 4.0l) is, and has been getting crappy fuel efficiency for a while. City driving, at the most I will get 350k's out of the tank. I see many others on here easily getting 450k's +. Also, I am not a leadfoot at all.

I have a new fuel filter, fuel pump and it has been recently serviced (new plugs, air filter, oil).

I don't know how accurate the fuel information displayed in the car is, but on acceleration, the numbers (instantaneous) jump to very high numbers: 60,70 sometimes. When cruising, it seems to be ok, judging by the dash estimations (7-8 on the freeway). It says that the average is around 15.1L/100k, this is well below what the actual usage is IMO. Other things to note is that is does not idle very smoothly, and jumps up a bit when sitting. On a cold day last week, I noticed that the amount of air the excaust was spitting out looked fairly excessive.

What reasons could be causing this? I read a few posts on dodgey o2 sensors that can be a problem. Is taking it out, and seeing if the car runs the same the best way to test, or a multimeter like I read? Could blocked injectors cause this, or other reasons?

Sorry about the long post, but with petrol prices continuing to rise, I want to try and get rid of this problem.

Cheers.