Alpine114

I have a 99/ 4runner, and currently i am getting rather poor gas milage. Its not like im hard on it at all, especially with gas prices today. I have 81,000 miles on it right now, and dont know if its possible to have a upstream O2 sensor going bad, yet not throwing a check engine light??? I have changed plugs, wires, air filter,fuel filter, etc. I dont know what else it might be? ANy suggestions....

RustBucket

iTrader: (1)

Your TPS maybe? Is this change in mileage a recent occurance, or did it just slowly degrade to 13 mpg?

Alpine114

Somewhat recent......Could be degrading??? Its just a pain...and i thought i have done almost everything to get it back to snuff. At the parts store i work at, a buddy of mine has a 98 runner just like mine, and says hes getting close to 20. Thats a huge difference, and hes all stock.

MTL_4runner

I still get close to 20 on my 1996.......you do have taller tires so that would lower the mileage a bit but you should still be getting near 18. I would give the O2 (actually A/F) sensor a shot (the one in front of the cat). They can degrade and cause bad mileage without throwing a CEL. Make sure your tires are properly inflated as well (32-35 psi typically) for the best possible mileage.

Alpine114

yeah jamie, ill give it a try. IS there any better price than $89.00 for a o2 sensor from www.urd.com.....is bosch that bad? thanks

LittleRedToyota

i've always hear that www.sparkplugs.com is a good place to get o2 sensors for yotas...haven't ever needed one myself, so i don't know personally. i think denso is probably the OEM brand.

i would also check to make sure your e-brake isn't partially frozen so your rear barkes are dragging. and make sure you are not leaking fuel anywhere.

also, are you correcting for the fact that your odometer is off because you have larger than stock tires? your odometer will tell you that you have driven fewer miles than you really have, so you are getting better gas mileage than indicated by your odometer.

you can estimate how much it is of by doing calculations based on the difference in tire diameter...4rawler has a calculator on his website that will do it for you, i think.

www.4crawler.com

a more accurate way is to just go out on an interstate or other highway that has mile markers and drive for ten miles according to the mile markers. note what your odometer says at the first marker and at the last marker.

for example, assume your odometer says you have driven 9 miles when you have really driven 10 miles (as measured by the mile markers). in this case your odomter would be off by about 11% (1/9).

to figure out how many miles you have really driven, you would multiply the mileage shown on you odomoter by about 1.11 (which is 10 / 9). so, if, according to your odometer, your were getting 17 mpg, you would really be getting 18.8 mpg.

so, your problem might not be as bad as you think.

MTL_4runner

I think the best place to get them is URD (they are OEM brand and great pricing).

You might also take a look at your MAF and clean that as well if it looks excessively dirty.

Alpine114

I have been working a ton this week, trying to pay some late bills, so i havent been able to work on my truck. I will check out the MAF sensor. I beleive i can just clean it with electric parts cleaner by CRC??? im pretty sure thats how it was explained. A while ago, i had a check engine light on, with PO171 which is fuel running lean. Took it in to the local toyota dealer, and they said it was the MAF, and replaced that. If im not mistaken, wouldnt the upstream O2 sensor make the fuel run lean??? Maybe the problem wasnt fixed. Also, my idle is around 850-1000. This is after i cleaned my throttle body, it started idling sorta high. ....Possible vaccume leak???? or can you adjust the TPS??? thanks

MTL_4runner

P0171 is usually the sign of a dirty or malfunctioning MAF so if they replaced it, chances are it is still pretty clean. You can pull it out and hit it with some CRC cleaner too just for good measure. When the MAF misreads the incomming air (as in P0171) then the system will run lean and force the O2 sensors to try and compensate for the addidtional air....so when you replaced the sensor the system should have returned to normal loop mode.

Now if your vehicle is idling high even after the MAF was replaced and you have no CELs, then I would start looking for a possible vacuum leak (also look at the throttle body to ensure it is no overly worn...thus letting more air through) and also check the IAC valve located just under the front of the throttle body.

DoubleZero4x4

I am experiencing the same issue and the CEL is on and throwing a P0171 code. The local AutoZone said its a bad O2 sensor (front). The problem started about 800 miles after installing a K&N air filter and has come back about every 1/4 tank of gas after a reset.

I also was calculating the gas mileage and I am seeing 13mpg if i am lucky. At $3/gal this is stinging and I am thinking that replacing the O2 sensor is mighty cheap when $50+/tank of gas is being sucked out.

I cleaned my MAF tonight so I will see what happens and if that rectifies the situation. I am probably going to just order the O2 sensor from URD and change it out anyways unless someone can tell me that the code is something other than the O2 sensor.

Anyone with any other hints, tips, suggestions, or ideas...please respond, I would like to know.

MTL_4runner

99% of the time P0171 is a MAF sensor not an O2 sensor (there are very rare cases that are something else). I would bet money your K&N air filter is fouling the sensor (replace it with an amsoil, sell the K&N on eBay and clean the MAF well). If you reset the ECU and the CEL comes back I would either get a new MAF or try to get one from a bone yard before I changed the O2 sensor.

Link to clean the MAF:
http://www.4x4wire.com/toyota/mainte...tenancep5.html

BTW a malfunctioning MAF will produce the horrible gas mileage you are talking about.

ebelen1

Doublezero and Alpine, did you guys ever get this worked out. I'm really confused b/c some people have the same gas mileage as myself but there are others that claim 20+ mpg. I do mostly stop/go driving so I understand that mpg will be lower but by how much? Also, I may be reading my odometer wrong but I'm not sure what the stock tire size was when the truck was new. I'll do some research and try the calculator and see how big the mpg difference is.

BTW, I have a '99 4runner.

MTL_4runner

Stock size is 265/70-16.

Yes, stop and go will reduce mileage quite a bit (hence the city/highway numbers on the window at the dealer).

I get about 21 highway most of the time (slightly less in winter).

rimpainter.com

And for comparison, I have a 96 also (well maintained) with the mods in my sig - and I drive it pretty hard, and I get 16.62 mixed over the last 1000 miles or so. If I drive it easy and on the highway, I get 17 - 18 all day long. I have also programmed in a tad bit of additional fuel in my map, so that might influence things a bit too.

13 is out of control. What are you getting ebelen1?

ebelen1

I'm getting 15-16 mpg on street driving but sometimes can go as low as 14 mpg. I can get about 17 highway but have only done this a few times since I got the truck 4 months ago. I do step on it off the line quite a bit and with some of the mods, I can't really help it. I think I lose a ton of gas b/c the freeways that I do get on are always at an incline and the tranny struggles to find 4th gear. I always feel like I have to punch it to shift gears on these inclines. Once it's in 4th gear, all is good as long as I don't let it downshift.

On regular stretches, the tranny works great. Any thoughts? I will do a full blown tune up soon but do have new plugs and wires.

Alpine114

ebelen1 have you figured out what is wrong with your runner? I have been still getting around 13. My runner is almost all stock, and it just is ridiculous to think some people are getting 20+. I barely touch the pedal, and im getting these numbers. I have heard diff. stories regarding po171, but i tend to wonder if im getting these numbers. Working at a parts store, there is a good fuel injector cleaner that is made by BG. It supposed to be top of the line, BG44k. But i didnt see much of a diff in my gas milage. I dont want to order a 80 dollar O2 sensor and have it not be the problem. Is this something i can take to a mechanic, and determine, or by trial and error?? let me know what you guys have done

rimpainter.com

You might want to have Autozone do a code scan (its free) just for the heck of it and see if any codes are stored. Otherwise I tend to agree with what some other people have posted, such as the taller tires and checking regular maintenance items. Looks like you might have a decent amount of weight added too. If you combine the weight with taller tires, a heavy right foot, and some maintenance items that need to be addressed, you could easily be looking at 13MPG.

Yes, there is a testing procedure for 02 sensors, but they have to be "hot" so it tends to be rather inaccurate. By the time you pay a shop to do the test, you could have a new 02 sensor anyway.

ebelen1

I'm not sure there's anything wrong with it. I do have a lift and bigger tires so I'm sure this affects my mpg. But, others including Lee (posts a ton on this board) gets similar mpg as me. He and I have the same size tires and my lift is a little higher than his.

My truck has never thrown a code so I don't know. I will be doing things in the near future such as some cleaner like Seafoam, a full tune up including cleaning the MAF, and we'll see. Even if the truck is not running efficiently, I can't see a jump of 3+ mpg after all this; I could be wrong.

DoubleZero4x4

I went on a road trip at the end of July from Gilbert, AZ to Oshkosh, WI and we ran an average of 75 - 80 MPH fully loaded with camping gear, food, water and two adults.

Before the roadtrip - posting about 17.5 MPG in stop and go city driving.

Here are my MPG numbers during the roadtrip:

18.74
20.36
18.83
20.33
18.08
20.69
17.93
16.59
14.70
17.94
17.02
15.96
18.10
16.31
19.49 - New Air filter
17.68
18.65

Check Engine light came on (P0171, P0130, P0133) at this point and now I am seeing 13.07 to 13.9 MPG city and 14.7 Highway.

I cleaned the MAF ordered a new O2 sensor from URD so we will see if the CEL comes back and what my MPG is after that.

ebelen1

Using a tire size/speed calculator, I think my odometer is off by 11.8%. My rationale is the stock tire size that came with my truck was a 225/75/15 and I now have a 265/75/16. When I calculate the speed difference between the two tires sizes, my odometer is off approximately 11.8%.

When I add an additional 11.8% to my mileage, I get about 17 mpg doing mostly street stop/go driving. If I take this to the highway calculations, the few times I've run my tank on mostly highway, I've gotten 17 mpg, which is roughly 19+ mpg. I guess I'm not too bad.

I will probably replace the 02 sensor, clean the MAF and hope for the best.