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95.5-2004 Tacomas & 96-2002 4Runners 4th gen pickups and 3rd gen 4Runners

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Old 02-05-2008, 11:56 AM   #1
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Sudden Gas Mileage Drop

I have a '97 4runner 2WD 3.4L V6 engine with 260k miles that's been running great. no real problems, but then all of the sudden i'm getting about 20-30 miles less than I used to per gas tank. i was getting about 21 MPG and now it's about 19 and i think still dropping.

This started two fill ups ago and i thought it was just due to worse than normal traffic and some weather for that week. but the next fill up same thing.... and then this 3rd fill up it's the same thing again. i fill up the same amount each time and just can't figure it out.

i ran seafoam in this car about 3-4 months ago, but i really didn't need it at the time. i had about 21 MPG before running seafoam and it may have bumped it up 0.5 MPG after running the seafoam. Prior to that time.... i've never had a mileage problem in all the last 50k miles i've had this car for.

in the middle of writing this post i just remembered that i did add half a quart of mobil 1 5W30 for SUVs to my 4runner as the oil was getting kind of low. i think i may have been at the middle of my first tank of gas that gave me low MPG when i added it. could that have any affect? i didn't add too much and i doubt it would affect the mileage. right?

any thoughts? what's the deal?! i need to change out some of my fluids... coolant, transmission, brake, etc, but those shouldn't affect the mileage right?
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Last edited by toynewb; 02-05-2008 at 01:56 PM.
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Old 02-05-2008, 12:10 PM   #2
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after remembering that one point about adding the oil i went back to my car just to check on it. i know i need an oil change, but it looks pretty dark... and the level at which i filled up the oil is back down again to the bottom. my car isn't overheating, but not enough oil and dark oil can't be good. i'm going to get an oil change this weekend.

anyone think this is the main cause of why my mileage dropped?
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Old 02-05-2008, 12:51 PM   #3
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Pick up a can/bottle of BG44K and run it through the gas tank. See what happens...
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Old 02-05-2008, 01:07 PM   #4
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I had some ty MPG on a tank, best I could figure is someone syphoned it. Don't know if it help didn't read all your post and no pics Good luck.
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Old 02-05-2008, 07:23 PM   #5
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well i just took a bunch of pics of my engine.... but i can't really tell of anything that's really wrong with it. i'll post the pics up tomorrow since i don't have a card reader here and can't connect to computer. My valve cover gasket is leaking and has been leaking for awhile, but my mechanic said i should wait a little before i really did change it out (said no reason to waste money early on). the oil is coming out of the sides of the engine as a result, but it's not bad.

oil level is alright, but then i realized that my coolant level is low... i didn't notice that before. i may try to run BG44K or seafoam through my tank. i'm going to get the oil changed tomorrow. checking hoses and such i didn't see anything leaking. any other ideas?
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Old 02-05-2008, 07:32 PM   #6
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whens the last time the o2 sensor was replaced? that will cause your mileage to drop.
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Old 02-05-2008, 07:38 PM   #7
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just so you guys can feel good, the last 2 tanks of gas On 14 gallons I got 100 miles.............
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Old 02-05-2008, 08:01 PM   #8
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Yeah I recently suffered some weird gas mileage drop. I had a o2 sensor code so installed a URD o2 sim and think its all good but the mileage has kinda come back to normal though my roof rack doesn't help.


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Old 02-05-2008, 08:52 PM   #9
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yea i did think that it might be a sensor. I replaced the mass airflow sensor in the spring of last year, but have no idea about the o2 sensor. which o2 sensor would i be looking for? how often should i check it/clean it/replace it? also.... how would i know that it's an o2 sensor problem.... or any sensor problem? I don't have a CEL going...

thanks for all the help!
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Old 02-06-2008, 03:51 AM   #10
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take it to advance auto or auto zone and see if they can pull some codes. here is a list of things to look into. its cheap maintenance and vastly affects gas mileage

-spark plugs (ngk or denso dual electrodes)
- pcv valve
- clean throttle body
- clean MAF sensor
- check air filter and change every 10-15k miles ATLEAST


O2 sensors do not require to be cleaned AND sometimes seafoam or other cleaners will mess the O2 sensor up. the O2 sensor in question would be the first one in line before you get to the Caty.. Do you know if it has ever been replaced?
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Old 02-06-2008, 05:17 AM   #11
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Alos check the throttle position sensor since those can screw with MPGs too.
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Old 02-06-2008, 06:27 AM   #12
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Where do you live? Is it cold there? In cold climate locations, gas stations usually change gas formulations in the winter to one that has higher volatility. This also leads to lower gas mileages in the winter. I get roughly 17 MPG in winter and 20 MPG in the summer.
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Old 02-06-2008, 06:41 AM   #13
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Well I dunno what your complaining about...

I have a '98 Limited with the 3.4 v6 and I get ~15-16mpg with 70% city 30% highway.

I havent had the car during the summer so I don't know if its winter gas. I live in Oklahoma, so it doesn't get super cold here. I really hope I can improve my gas mileage somehow.

Where are/how do I clean the throttle body and MAF sensor? What is PVC value? haha. The person who owned it before me was kind of an idiot so I can see things being dirty.
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Old 02-06-2008, 07:05 AM   #14
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Quote:
Originally Posted by GSGALLANT View Post
Where do you live? Is it cold there? In cold climate locations, gas stations usually change gas formulations in the winter to one that has higher volatility. This also leads to lower gas mileages in the winter. I get roughly 17 MPG in winter and 20 MPG in the summer.
thats true too even in places that dont change the gas.. cold weather = poorer gas mileage

another question do you warm your truck up in the mornings?? if so that maybe a reason.
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Old 02-06-2008, 08:04 AM   #15
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take it to advance auto or auto zone and see if they can pull some codes. here is a list of things to look into. its cheap maintenance and vastly affects gas mileage

-spark plugs (ngk or denso dual electrodes)
- pcv valve
- clean throttle body
- clean MAF sensor
- check air filter and change every 10-15k miles ATLEAST


O2 sensors do not require to be cleaned AND sometimes seafoam or other cleaners will mess the O2 sensor up. the O2 sensor in question would be the first one in line before you get to the Caty.. Do you know if it has ever been replaced?
i don't know if there would be any codes to check since there's no CEL. Haven't seen any errors, would there still be codes if there is no CEL?

i changed out the spark plugs and the wires with ngk brand about maybe 3 months ago. i'll look into cleaning the throttle body, i got the MAF sensor early last yr about march/april... can it get dirty that badly? I never had to clean out the MAF prior to changing it. i just changed the air filter when i added some mobil 1 oil, but i forgot to change the filter awhile back so the last air filter was pretty dirty.

i don't know if the o2 sensor has ever been replaced. i'll try to look into it. this car was a hand down from the parents.
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Old 02-06-2008, 08:15 AM   #16
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i live in socal and i did think that the weather might affect the mileage a little. it has been around the 40s recently, but a few weeks back it was just just as cold and i didn't see any huge dip in mileage. i do let the car warm up in the morning for a little and drive it out slow. i know i do have to change the pcv valve since it's kind of old so i'll try that as well.

here are some pics. 1st pic is just the throttle, 2nd is showing the result of a leaky valve cover gasket, 3rd is just the otherside of the engine. I'm not really sure what else i can take a pic of...

now that i'm thinking about it... i think i'm just being paranoid about this whole thing.....
let me make some changes and we'll see if i'm just paranoid....
Attached Thumbnails
Sudden Gas Mileage Drop-1.jpg   Sudden Gas Mileage Drop-2.jpg   Sudden Gas Mileage Drop-3.jpg  

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Old 02-07-2008, 09:51 AM   #17
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well i went in to change my oil and the mechanic told me my rear differential fluid was practically out and the gears were grinding some. There were metal shavings on the cap. He filled up the rear diff fluid for me and now my 4runner seems to drive better (i only have rear 2WD).

do you guys think this could be a reason why my mileage dropped? less oil means more work for the rear wheels to take on to turn. only thing is... it would take a good amount of time for the fluid to leak out wouldn't it? what do you think?

on my 4th tank and still at the same low MPG. But it doesn't seem to be going below 19 MPG...
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Old 03-14-2008, 03:58 PM   #18
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Well last weekend the CEL came on and I thought well ! I disassembled the pass side glove compartment and made certain the O2 simulator from URD was still correctly and firmly attached. No problem there, I scratched my head and thought hmmm no code reader and CEL is on. I pulled the air filter and while it was dirty it wasn't bad. I then remembered what else can set the CEL and VSC lights a blaze. That damn MAF sensor must have been covered in something, so I pulled out the brake cleaner and started spraying, the truck died and I started it again and proceeded yet again. I then pulled the - batt. terminal left it for 2 minutes and put it back on. I then drove 300 miles the next day then another 300 a couple of days later. Not only did the power come back the gas mileage is through the roof 17.5-19 mpg.


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Old 03-14-2008, 04:33 PM   #19
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Originally Posted by xcmountain80 View Post
Well last weekend the CEL came on and I thought well ! I disassembled the pass side glove compartment and made certain the O2 simulator from URD was still correctly and firmly attached. No problem there, I scratched my head and thought hmmm no code reader and CEL is on. I pulled the air filter and while it was dirty it wasn't bad. I then remembered what else can set the CEL and VSC lights a blaze. That damn MAF sensor must have been covered in something, so I pulled out the brake cleaner and started spraying, the truck died and I started it again and proceeded yet again. I then pulled the - batt. terminal left it for 2 minutes and put it back on. I then drove 300 miles the next day then another 300 a couple of days later. Not only did the power come back the gas mileage is through the roof 17.5-19 mpg.


Aaron
With 295s??? Is that an adjusted figure? I am impressed. I need to clean my PCV valve and see if that helps anything. My MAF is super clean and my oil is fine. Newish plugs. My O2 is maybe 3-4 years old and 55K on it. I get 16 MPG pretty steady. I am pretty sure my truck has 4.30s though and I know I need new tires but I am running 265/70/16 Pirelli Scorpion ATRs. Done the Seafoam thing a lot too.
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Old 03-14-2008, 06:52 PM   #20
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Oh yeah I changed the pcv a month ago and got nothing but if the damn MAFS has been building up crap then I would effect this. I have re-geared to 4.56's and the nitto 295's are 33.something I buy tires by the spec size not the stamped size. I was happy to get the good mileage again it's been bumming me out for a couple weeks.

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Old 03-14-2008, 06:52 PM
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