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Would be best to get a live data scanner.
There are lots of things that could be wrong.
Does the fuel trim say it's running rich (abve regular, positive percents, however it's displayed on the alero)???
Or is fuel trim normal???
If it's fuel trim, the comp is getting a reading that it needs more gas for some reason..
is your coolant temp sensor working?
intake air temp sensor working?
If those read cold then it'll dump nasty rich in.
If you have an exhaust leak (pre sensor), your o2 sensor will pick up lots of extra o2 on it's reading and dump in fuel to compensate.
Get a scanner and look at live data... if any of the sensors read funny those are prolly your culprit.
If fuel trim is regular something is holding the car back...
Dragging brakes/ ebrake
Tires
wheel bearing making it hard to drive
etc.
Also, as they said, plugs and wires are always a good idea. I gained about 4mpg when I changed plugs in the Aurora.... Gotta see how much voltage each plug is pulling (gonna need a spark analyzer)... Should peak around 8-15kV. If it's more than that you need plugs and or wires.
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