Gas mileage loss

[urbex]

Starting a few weeks ago not long after I got my alignment and tires done I've noticed a large decrease in gas mileage. Instead of the usual 30 or so on the highway I've been getting between 23 to 25mpg. No lights are on, and there's no stored codes. Any ideas what I should check first?

 

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[RobertISaar]

tire pressures?

spark plugs?

wires?

O2 sensor?

brake hanging up?

any out of the ordinary behavior?

any way to check fuel trims? or stored codes that aren't lighting up the SES?

 

how quickly did the drop occur? and has the route/conditions changed at all?

[urbex]

I used my scanner, and there are no stored codes and the CEL is not lit up. Spark plugs and wires are less than 2 years old (actually the wires are 6 months old). I'm not 100% certain what I should be looking for in the fuel trims, but I think they're normal. I mean, they stay pretty consistent around 0, for the most part. I have noticed the brake pad is chirping when I drive, and it stops when I apply the brake. Took it apart today, but didn't seen anything loose or rubbing, but we'll see if that noise goes away now, but I don't think that'd be it hanging up.

 

I don't really know what to look for to check the o2 sensors.... Is there certain voltages I should watch for? I can see all the live data with my scanner, but it doesn't help if I don't know what it should be.

 

Really, I first started noticing the drop after I put my new tires on. I figured I may see a slight change in milage beings that they're a size bigger, but I didn't think it'd be noticeable and I still dont' really think that would cause it. My drive has been the same for quite a few months now, but it's harder to notice on day to day driving as much as a long trip. I just know after driving nearly 200 miles at 70mph my average has been about 26 and 24mpg.

 

Of course, everything in the engine is clean from when we did the head/intake gaskets, and I'd think if something wasn't hooked up from that I would have had issues a long time ago. As of a little over a month ago I did the same highway drive and got 31mpg.

 

 

[RobertISaar]

with fuel trims near 0, i am hesitant to look at anything engine(or engine sensor) related then, especially if the misfire counters don't have any odd looking data in them.

 

tires generally won't be that much of an effect on MPGs, when they're new they do have a higher rolling resistance, but it's on the order of where it wouldn't likely effect 1MPG, let alone several.

 

so what tire size did you start at and what are you running now? the odometer will be off by a couple of % due to that alone, but it's not going to be much.

[urbex]

I originally had 205/70-15, and now I went up to 225/60-16 (which is the stock size for those wheels). My odometer has never been entirely accurate, it's always been about 2mph off. Well now, it's about 1mph off, according to my GPS).

 

Also, I did check to make sure that Overdrive was working, and it was...so that's not the issue.

[RobertISaar]

there's only a 1.26% difference between those sizes, so that can be ruled out.

[skitchin]

Try going to a different gas station

[urbex]

Gas was a thought too, but I tried that and still no difference.

[RobertISaar]

still have the old wheels/tires?

[urbex]

I do. I had considered putting them back on to see.... Maybe I should.

[RobertISaar]

that's what i would do. easy enough to test.

[Schurkey]

I used my scanner.... ... I'm not 100% certain what I should be looking for in the fuel trims, but I think they're normal. I mean, they stay pretty consistent around 0, for the most part.

 

with fuel trims near 0, i am hesitant to look at anything engine(or engine sensor) related then, especially if the misfire counters don't have any odd looking data in them.

Shouldn't the fuel trims be hovering around 128?

 

What is the engine coolant temperature?

Is the knock sensor active?

 

 

 

 

I've lost about 3 mph highway on my Luminas, and I'm pretty sure it's because the piston slap is so bad it's got the knock sensor working overtime to pull timing advance.

[urbex]

Coolant stays at 200 once it's warmed up. As for the knock sensor, I have no idea. How do you check that?

 

 

Another thing... At around the same time I did start using the a/c more consistently...but I've never had that much of a loss from a/c.

[RobertISaar]

Shouldn't the fuel trims be hovering around 128?

 

depends on how the data is interpretted. most OBD2 scanners seem to take the LTFT and STFT values(which transmit the same as BLM and INT, 128 being 0%, higher being fuel added, lower being fuel removed) and do a little bit of math to make them a -100 to +100 % range.