mine had the same prob. it was the main 02 sensor (the one in the turbo 02 housing

My upstream o2 was bad as well as the gauge. My scanner showed the fuel trim though.

But if it was the o2 sensor wouldn't there be a light on

No not really. When my o2 sensor went out, mileage decreased and it ran rich. Of course every situation is different.

My CEL never illuminated when my O2 went out. The only way I was able to tell was with the scans from my scanner and knowing what the readings meant.

Try a tune up if nothing else.
Boost leak test,
Spark plugs and plug wires,
Maybe cleaning throttle body and whole intake would helped a little(helped me)
More than likely O2 Sensor if you don't have a CEL.
I'm in the same situation trying all this stuff. Filled my tank and was down to 1/4 of a tank in 80miles.

I'll check the o2 sensors

I have a 98 GSX automatic. I'm getting under 200 miles per tank which to me is horrible gas mileage considering its a 4-cylinder. Not sure if something is wrong with my car or what? Anyways I was wondering what are some upgrades or anything I can buy for my car to increase gas mileage

When's the last time it was tuned up? You have any check engine light? Could be a falling O2 sensor or bloost leak.

No tune, not sure where I should go to get one, no check engine

I would start with changing. Plugs, plug wires, airfilter and oil filter, and Pcv valve. Pretty basic tune up stuff.

I've changed the oil, new plugs and wires I think I'll change the pcv valve as you said

I'm fwd but,
How are you driving, when I drive hard or I'm boosting more then other times I can get about 175-200 miles to full tank. When I drove 487 miles 2 weeks ago at 63 mph, I got 38.3 mpg. I didn't think would get that good. On the way back I wasn't so conservative and got little less then 12 mpg, 13 psi 14b. Stock fuel at that time. Saved only 30 min also not too worth it . Food for thought.

A lot of conservative city driving your going to be around 20mpg give or take a few, probably take, Would be my guess. So reset your trip when you fill up and divide miles by how many gallons you put in and see where your at.

But I hope this gives you an idea. Sounds kinda normal to me

Thank you so much that's a great idea, I'm probably gonna fill up today and do that and drive extra conservative to see what kind of mileage I can get, steady throttle best way to go?

Make sure your upstream O2 sensor is working properly or you will stay in open loop mode and get terrible mileage. http://www.gnttype.org/techarea/ecmsensors/O2sensors.html

That's another question, how do I check an o2 sensor? Also where is that one located

Get a scanner that can stream the O2 signal. Then you can watch it and see if it's stuck or moving.

Sorry for all the questions but where can I get something like that

A simple palm pilot logger setup would tell you. It's a good diagnostic tool. You can see what the check engine lights are saying and see the outputs of senors, water temp, ect.

Its a 2g so its an obd2 car i would suggest to you a elm 327 scanner you can get one for under 10$ on ebay its a bluetooth adapter that plugs in and get on your phone assuming you have a smartphone torque pro its i think 4.99 and you can log everything you need with it and check trouble codes i have 13$ into my setup for logging granted i dont have my dsm anymore but i used it to tune my 2g with an safc and worked great one of the best investments i made for that car and i still use it for engine lights in curtent car or friends cars so on

jbeaton9292 said:

Its a 2g so its an obd2 car i would suggest to you a elm 327 scanner you can get one for under 10$ on ebay its a bluetooth adapter that plugs in and get on your phone assuming you have a smartphone torque pro its i think 4.99 and you can log everything you need with it and check trouble codes i have 13$ into my setup for logging granted i dont have my dsm anymore but i used it to tune my 2g with an safc and worked great one of the best investments i made for that car and i still use it for engine lights in curtent car or friends cars so on

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That sounds perfect thanks for the help guys I will do this for sure!

MasterMatt209 said:

That sounds perfect thanks for the help guys I will do this for sure!

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No problem its a great tool to have for these cars

Mine gets about 200 miles per tank