97 sunfire poor mpg

[Basstender10.2]

So I purchased a 97 sunfire as a commuter vehicle to drive back and forth for college. It’s the base model 2.2 5 speed. It’s got 184,000 miles. But it’s been kept up great. There’s no leaks no oil usage. Nothing. The only thing is the fuel milage rating for the car is 33 for the highway. I’m on the highway speeds at 60 for 90% of my driving. And the best I’ve gotten is 25. I’ve got a code that pops up from time to time for the rear oxygen sensor but it comes off within a hour or so. But the downstream oxygen sensor doesn’t effect fuel efficiency, right? I’ve heard mixed things on that. I plan to do a full tune up using only AC Delco parts. Spark plugs, oil and filter, fuel filter, etc...would anyone have any kind of a guess at to why the milage is so far off? The car runs great and has plenty of power I just don’t understand why the fuel milage is so far off it’s claim. Any help is appreciated.

 

[melsg5]

Starting with a tuneup is a great idea.
The rating was for a new vehicle not a high mileage vehicle.
Exactly what is the code?

 

[Basstender10.2]

Starting with a tuneup is a great idea.
The rating was for a new vehicle not a high mileage vehicle.
Exactly what is the code?

I’ll have to wait until the code comes back. It’s not showing now. But it’s saying the bank one sensor two heated oxygen sensor high voltage.

 

[CjruelasPonyUp]

Id say before you start replacing parts give the old ones a good cleaning. Run some seaform in the oil and gas and treat the intake. I have a 99 Sunfire with 250,000+ miles and I’ve taken real good care of it. Also made some upgrades.

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[Basstender10.2]

Id say before you start replacing parts give the old ones a good cleaning. Run some seaform in the oil and gas and treat the intake. I have a 99 Sunfire with 250,000+ miles and I’ve taken real good care of it. Also made some upgrades.

Any idea of what fuel milage you’re getting?

 

[CjruelasPonyUp]

Any idea of what fuel milage you’re getting?

Right now poor, no cat and O2 is just throwing fuel until I get a dummy plug. Before the cat delete my Mpg was great