Gas mileage plummeted no codes??

J

Johnnie

New member
my 99, 3.4, auto religiously gets about 20 MPG only varies about 1 mpg for the last 20K , miles. the last 2 tanks are 13.? MPG. no codes have been kicked out. and the engine is running great. smooth and responsive. what could possibly be that wrong that it wouldnt throw out a code. driving habits are the same, same city/hwy combo of miles????? thats over 30% drop in gas mileage. I have not seen a tank with less than 18 miles per gallon in the last 2 years. any thoughts?
:confused:
 
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What have you looked at already? Any maintenance you've done or get fuel at a different gas station? Do you have a Scangauge or the Torque App by chance?

Sent from my SM-G920I using Tapatalk
 
I assume your talking about an APP for reading the engine parameters, no I don't have one, same gas station as I always use. I did recently have the O2 sensor go bad, engine running rough blowing black smoke out of exhaust. it would blow a code within 1 mile of resetting it. installed new Denso sensor and it been running smoother than any time since I bought it. its got 3 or 4 hundred miles since the sensor replacement and no codes. maybe its time to spring for something to read the engine parameters
 
Johnnie said:
I assume your talking about an APP for reading the engine parameters, no I don't have one, same gas station as I always use. I did recently have the O2 sensor go bad, engine running rough blowing black smoke out of exhaust. it would blow a code within 1 mile of resetting it. installed new Denso sensor and it been running smoother than any time since I bought it. its got 3 or 4 hundred miles since the sensor replacement and no codes. maybe its time to spring for something to read the engine parameters
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My first thought was the 02 sensors. You mentioned you changed one and on our rigs, it is best to replace BOTH at the same time. Also, if you haven't already, clean the MAF and throttle body....disconnect the battery for 10+ minutes to reset everything.
 
Don't think an O2 sensor could go so bad that it would blow black smoke out from being too rich. Maybe also check your IAC valve and check that the spark plugs are not fouled from the rich mixture? I'm not a mechanic but that sounds weird to me.
 
Tapatalk freaking out... sorry if this is a double post.

Updated my Torque Pro App yesterday to better monitor a few things. Have a few more to add but you can see its potential.
138e749e91df02557a3576d28da5d8f1.jpg


Sent from my SM-G920I using Tapatalk
 
I did clean AF sensor and throttle body when I replaced the O2 sensor, the crazy thing is everything is running great, idle, power, hot and cold running,its never runner better in the 20K miles that I've driven it. I think its definetley time to get an reader to hook up to my Ipad any recommendations on where to buy one Amazon?
 
so when my runner started blowing black smoke out the exhaust and kicking out codes within a mile or two of driving , I changed the upsteam O2 sensor and the codes went away immediatley but it took awhile before I noticed the gas mileage had dropped so much. I think maybe the sensor I replaced it with was better than the one installed after it failed but not working as good as the last one before it failed. the new sensor was a Denso but thats the only scenario that makes sense to me
 
I might as well bring this post to an end. My gas mileage continued to climb and my last tank was 22.3 MPG. I don't know if this idea is correct but I did change the O2 sensor and not long after installed a new battery. did I reset something and the system had to relearn. I went on an 1100 mile trip and had to refill the tank 5 or 6 times. and the mileage slowly climbed on each tank until the last one at slightly over 22MPG. I guess I can't argue with success even if I'm not completely sure of what I did right
 
Johnnie said:
I might as well bring this post to an end. My gas mileage continued to climb and my last tank was 22.3 MPG. I don't know if this idea is correct but I did change the O2 sensor and not long after installed a new battery. did I reset something and the system had to relearn. I went on an 1100 mile trip and had to refill the tank 5 or 6 times. and the mileage slowly climbed on each tank until the last one at slightly over 22MPG. I guess I can't argue with success even if I'm not completely sure of what I did right
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I read your post and it was a classic symptom of a failing O2 sensor. That's what would do it without throwing a code.
 
i'd sure like to know how some of you are getting that kind of mileage. i am barely getting 16, and i don't do anything crazy.
 
Leaving my trans in ETC power mode all the time I get 20-22 in the city and can get 30 highway if I try but 28 is normal for highway for my driving. Keeep in mind a few thing factor into what your mpg will be. Driving style in a major one, being easier on the throttle instead of flooring it all the time saved a lot of gas. Tire size and pressue, gearing, extra weight will also effect mpg. So if you are lifted, steel bumpers, steel roof rack, spare tire swing out rack with full size spare, 2 5 gallon gas tanks full of gas, hi-lift, winch, with 35" tires your mpg will be $hit.
 
^^^ Even with a 4 cylinder and 4x4 manual 22MPG in the city is impossible. A camry (96-99) with 2.2 L would do 20 in the city.

For my V6 4X4 I received the followings
At 70 MPH cruise 17.7 MPG
In the city 12.5 to 13.5 MPG

Forgot to mention I use ethanol blended gasoline: AC ON 80% of the time, stock tires, and city driving is 3-4 miles each way.
 
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I can get 17MPG in the city with my 3.4L 4x4 33" MT tires if I'm lucky. The most I get on highway going 65MPH is maybe 20MPG. Having the AC off helps a lot, but thats hard to do in Texas haha
 
psraff said:
Tapatalk freaking out... sorry if this is a double post.

Updated my Torque Pro App yesterday to better monitor a few things. Have a few more to add but you can see its potential.
138e749e91df02557a3576d28da5d8f1.jpg


Sent from my SM-G920I using Tapatalk
Click to expand...

This pic just motivated me to buy an OBD2 adapter for my tablet. I bought the tablet to use as an offroad GPS and Spotify player, but that looks pretty awesome. What kind of mount did you use for the tablet?
 
02gixxersix said:
This pic just motivated me to buy an OBD2 adapter for my tablet. I bought the tablet to use as an offroad GPS and Spotify player, but that looks pretty awesome. What kind of mount did you use for the tablet?
Click to expand...

Sorry man, I'm up on the Slope right now, with bad internet and a new laptop... not getting the pic you're referring to.... But if you're referring to the suction cup mount for my tablet, it's this one:

https://www.amazon.com/gp/product/B004M8ST36/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

I've had it for four years not and it's held up great. Only issues is very hot days it can come loose (so for me like once a year!)
 
turbo8 said:
Do you feel like you have lost any power at all? Make sure your ebrake isn't dragging.
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It looks like you solved the problem later in this post, but I'd like to throw this in please. I had very similar issues. Loss of power. Bad gas mileage, and no codes...ever. My solution was the fan clutch.
I got this diagnosis from the mechanic that did a diagnostic. I saved $400 and changed the clutch myself. Remove the shroud and 8 nuts later I was back to full power and resolved mileage. Good luck all!:bounce:
 
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