I've got a 99 Si and im getting around 21 or 22 mpg's, i've gone through two different tanks of gas and its still the same, first tank I thought it was me, but second tank im shifting around 3. I do have an issue with one of my 02 sensors buts its been that way for awhile, and before I was getting 26-29 mpgs. Any one have any ideas? I did a search for bad fuel mileage but nothing came up.

 

Start with doing air filter/cap/rotor/plugs if you haven't done those recently. I was getting very good mileage prior to doing a tune up, managed an extra 1-2 mpg after doing those things.

 

I agree.

Also replace the plug wires if they are old.

Your engine has two O2 sensors, primary and secondary. Which one is throwing a code? If it's the primary, then this could definitely reduce gas mileage as signals from this specific sensor are used by the ECU to determine optimal air/fuel ratios. Fixing such a problem should greatly improve gas mileage. What CEL code is thrown?

 

Somehow missed that part; Ron's right, a primary O2 fault could explain your poor mileage.

 

I had replaced the spark plugs, wires, distributor cap and rotor not too long ago. It could use a new air filter though. The CEL i have is a 0132 which deals with voltage on the one 02 Sensor, ive had this problem for awhile and had been getting 26-29 mpg's but just as of lately my mileage has really dropped off.

 

P0132 indicates a fault in the primary O2 sensor circuit. This sensor is instrumental in ensuring that the engine is using the optimal air/fuel ratio. The failure of this sensor is a common cause of low gas mileage.

My guess is that in the past your sensor/circuit was intermittently faulty or partially operative but now it has completely failed. If you seriously want to solve your mileage problem, the first step (and possibly last step) is to fix the O2 sensor or its circuit.

 

Thanks I appreciate it. The problem that I have is the guy who had the car before me cut the oxygen sensor at the wrong place for a universal 02 sensor and now I have to re-wire into the wiring harness to get the 02 sensor reconnected. Am i best to try taking it to a shop and letting them do it? Or does anyone have any suggestions on how to fix this?

 

The idea behind using a universal oxygen sensor is that of using a universal cat converter. It's done to save money, but often leads to a much less clean install. I highly suggest you purchase a new pigtail for the oxygen sensor and solder it into the stock harness and then purchase an OEM replacement oxygen sensor instead of the universal one.