poor mpg. what's wrong?

[greekman]

the last 2 times i've filled my car up i reset the trip odometer, to check my mpg. the first time i got around 250 miles and the second time around 265 miles. this works out to only 15 or 16 mpg (15.8 gallon tank, right?). i've driven in city and highway, and usually punch it on the highway. a/c hasen't been used that much. so bottom line, what's my deal? why am i getting such shitty gas milage?

[P10DET]

Originally posted by greekman
the last 2 times i've filled my car up i reset the trip odometer, to check my mpg. the first time i got around 250 miles and the second time around 265 miles. this works out to only 15 or 16 mpg (15.8 gallon tank, right?). i've driven in city and highway, and usually punch it on the highway. a/c hasen't been used that much. so bottom line, what's my deal? why am i getting such shitty gas milage?

First of all, you cannot go by the miles driven. Did you pay attention to how many gallons you put in your tank when you filled up? If not, you cannot say what your mileage is. If you did indeed note the gallons your car took, and divided it into the miles driven and got that kind of mileage, almost certainly your O2 sensor is kaput.

[greekman]

thanks geo. i'll check it out when i fill up in the next week here. i'll get back to you once i get my new results.
if my O2 sensor is kaput, where do i get one, how much, can i install?

[P10DET]

Originally posted by greekman
thanks geo. i'll check it out when i fill up in the next week here. i'll get back to you once i get my new results.
if my O2 sensor is kaput, where do i get one, how much, can i install?

I get them from the dealer. They come with the electrical connector attached and are easy to swap. You can get a Bosch O2 sensor for less, but you have to cut and solder. The prices have gone up and I'm not sure what they run right now. More that $75, but less than $100 IIRC.

They aren't that hard to replace.

[greekman]

okay, i did it right this time. i put 11.217 gallons in and noted where the gauge was. then i drove 245 miles until the gauge was where it was before i filled up. that is 21.84 mpg. much better than my previous estimations. is 21 mpg good. i did a lot of hard driving on that tank so i would guess i'm in the range of 21-25 mpg. is this average?

[P10DET]

That's still not right. You have to fill the tank, zero the trip odometer, and drive for 3/4 of the tank or more. Then fill up again, noting the gallons pumped in to fill the tank. Divide the miles driven by the gallons pumped.

[greekman]

okay hopefully i can get it right next time. i have to fill up tomorrow so i'll go from there. i'll keep you posted.

[howard_w13]

Speaking of the O2 sensor, where is it and how could I tell if I need to replace it?

[Anthony]

It is screwed into the exhaust manifold, although it maybe partly obscured from view by the heat shield.

There is unlikely to be any visable damage to the O2 sensor - when they go bad, you usually get either crap fuel economy, or your car will run like a dog.

[greekman]

finally. i was at 3/4 empty so i filled up, zeroed the trip odometer and drove. i got 275 miles till i was back at 1/4 full. i filled up with 10.070 gallons. that makes 27.34 mpg. this sounds more correct to me. geo-please confirm that this is right for me, thanks.

[P10DET]

Originally posted by greekman
finally. i was at 3/4 empty so i filled up, zeroed the trip odometer and drove. i got 275 miles till i was back at 1/4 full. i filled up with 10.070 gallons. that makes 27.34 mpg. this sounds more correct to me. geo-please confirm that this is right for me, thanks.

That is the correct way to calculate it. Yes, it's correct. It's also very good mileage. I never get that in either my G20 or my SE-R (before I took the engine out). My usual mpg is around 24-25. My wife always got that in the G and now that we have the DET in it, she's getting 29-30.

[greekman]

good to hear that i finally got it right. yeah i imagine 27 is high for me also. i was doing mostly "nice" driving, so 27 is probably high. but, i'm not complaining.