My 2002 3.4 MPG is slowly getting worse. About 6 months ago, it would do about 20 average of city and highway. That's down to about 15 now. I understand the cold plays some part in that, but not 5 mpg.

I've replaced the spark plugs, PCV valve, had the tires rotated and balanced and recently switched to 10w 30 Supertech Synthetic. About the only think I haven't done is the O2 sensor. Could that be the problem? Any ideas?

 

change air filter?
clean throttle body?

 

Was this before the bigger tires? I drive kinda aggressive and I get only about 16 which is the same a few of us others have reported and mine wnet downa little with the upgrade of the bigger and heavier tires. I never saw 20mpg or even close so maybe the law of averages caught up.

 

I get 11-12mpg winter, about 17-18 summer on same roads. Just lots of short trips.

 

Mine depends on how much I use full throttle acceleration. I generally see 220-230 a tank while my mom sees always over 260 in her '99 Limited and when I borrowed her truck for a couple of weeks I was able to kill her mileage too.

 

Tire pressure plays a big role for me. My pressure dropped about 5 PSI from the fall until I checked them during the colder days of winter. And when I bought new tires 15 months ago, the newer tires had a higher rolling resistance then the previous ones.

As long as your doing all the maintenance stuff and your driving habits and routes haven't changed much, there isn't too much you can do.

 

cleaning the throttle body gave me 1mpg more. took less than 3 minutes to do.

 

Do you mean "switched" as in the type of oil, or weight, or both?

Running 5W-30, which is what my manual recommends (I don't see why your 5VZ would be any different) should give you slightly better MPG.

Did you change anything at about the time you saw the difference?

I drive pretty hard, and I get about 16.3 mixed city/hwy (my mods are in my sig). So at 15, something is definitely going on. Was this a sudden change, or gradual?

 

How many miles on your truck ? I would follow up on your O2 sensor idea.
It can be tested but if it looks fouled when you pull it (primary) I would replace it unless you have low milage.

 

My rig has 56000 miles and I've already cleaned the air filter and MAF. My tires recommend 50 lbs max and I run 45 in each. And yes, I did notice a 2 mpg or so drop with the lift and 75 series tires.

The only thing I can put my finger on and the reason I'm questioning the o2 sensor is that about the time my mileage started to drop, I seafoamed my enging and am wondering if it was clogged.

 

Seafoam can destroy your o2 sensors, maybe thats the culprit.

 

I think after a Seafoam job you should replace your plugs too.

 

I've already done the plugs so that's taken care of. Can the o2 sensor be cleaned or does it need to be replaced? Is it an easy job?

 

How much is a new O2 sensor anyways?

Bob_SR5 has a writeup in the tech sextion, it looks simple.

Hey don't feel bad man, I just got 13 mpg on my last tank!! I have something wrong though I think... I usually get more on the 15-17mpg range.

 

O2 sensors can vary so if you do have to replace it check several places. I've seen them as little as $80 and as high as $180 but on 4Runners I think it's inbetween those prices.

 

It's an easy job, though in your part of the country you'll have a lot of road salt up in there and with that comes rust. My truck spent it's first 5 years in Chicago and when it came time for an O2 sensor, it wasn't a lot of fun.

Get some PB Blaster (auto parts store) and start soaking the nuts across the next 24 hours. That'll help a lot.

You'll only want to replace the front O2 sensor, and the best prices that we've found come from URD USA:
http://www.urdusa.com/product_info.p...s_id=430006000


You may also want to try cleaning the sensor... Just soak it for 10-15 minutes in brake cleaner, swirl it around once-in-a-while. If it works, then you've saved the cost of a sensor.


edit: I just noticed you have a 2002 - you probably have an A/FR sensor versus a classic O2 sesnor. That just doubled the cost of the sensor.

 

i'm running 255 85 16 which is like a 33' tire and my fuel milage has dropped about the same from summer to winter.

 

That may be from crappy winter gas. It "should" get better about the middle of April.

 

I've seen the same drop before. My MAF had oil/dirt buildup on it. So I cleaned it. (I first removed my MAF, then I sprayed it with WD-40, then sprayed it with contact cleaner, then I doused it with denatured alcohol) I have a K&N air filter also. I called K&N and they told me that when you reoil the filter to use the oil sparingly. Too much and it will be drawn thru and into the MAF. My MAF lasted for about 80k after I cleaned it so I don't belive I hurted it. My mileage went right back up to 20 - 21 MPG instantly.

 

Thanks for the input guys! Any other thoughts?