MPG all over the place

[MStudt]

All these issues started on our Christmas trip when the fuel gauge started acting up. My driving style I feel is pretty normal, nothing more than 5 over, no fast starts, and no burn outs. I've been watching the mileage each time I fill up the tank, and there is no consistency with the mileage. I've been trying to fill up right around 150 miles since I no longer trust what it says I have. I filled up this past weekend when I was at 1/4 of tank remaining, and put in just under 14 gallons. The thing is that I only had 167 miles on 3/4 of a tank. Another time I filled up at 110 miles, just to add around 6 gallons of gas. Over the past month I've been bouncing around from around 12 to 19 mpg. All of my driving has been 55+. I don't do the stop and go city driving since 98% of my driving is highway driving. I've read some threads about gas mileage and it seems to be a guessing game. I haven't been able to come across any threads with solid evidence with what could be wrong.

 

[Ol'Blue]

Just after getting my 4Runner I was having the same issue. I was thinking it was a full framed 4x4 etc...

Soon after I got a code saying AF sensor. I got one off amazon (100 bucks) and soon after I was right at 19mph/20 on the hwy and about 17 in town.

I'm not saying it is going out or anything but that helped me out.

Blue

 

[Gerdo]

Probably your upstream oxygen sensors are bad (the ones between the engine and the cats)(most have only one upstream, some have two). Replace with Denso sensors.

 

[MStudt]

When we first bought it a couple codes came up, and they were P0440 & P0442. I started with just replacing the gas cap since the o-ring was pretty much falling apart, and then I erased the codes. I've had no check engine light since and close to 3k miles later. If I remember correctly, I think those codes had something to do with the auxiliary emissions control. The O2 sensors have not been replaced as of yet, but might be in the future. Also, wouldn't a safety inspection catch bad O2 sensors?

 

[UTEP]

Probably your upstream oxygen sensors are bad (the ones between the engine and the cats)(most have only one upstream, some have two). Replace with Denso sensors.

Hmmm

I have similar issues, 15-18 mpg recently but used to 17-23 mpg.....

Meticulously tuned 2000 4R, I had not considered O2 sensors as I'm not throwing codes...

I chalked it up to winter blends....but I had a 2000 Tacoma 22R and while a 4 popper the mileage was always consistent over the same seasonal trips.

I'm basing my mpg on fwy miles Tucson/El Paso to San Diego about 4 times a year. Chevron 89 and recently trying 91 octane.

Like to here more about O2 sensors being the culprit before the expense and hassle...


Thx

 

[eplanajr]

I know that my 4R is definitely that I'm in the 15-18 on winter blend and 17-22 on summer stuff.

 

[Gerdo]

The upstream O2 sensor tells the computer how rich/lean to run (more or less gas). The downstream O2 sensor tells the computer if things are really out of whack. You won't always get a CEL with an upstream O2 sensor that is going bad (as in not reading perfectly). And no, a safety inspection will not catch bad O2 sensors. The vehicle is running fine, just a little rich and wasting gas.

 

[MStudt]

The upstream O2 sensor tells the computer how rich/lean to run (more or less gas). The downstream O2 sensor tells the computer if things are really out of whack. You won't always get a CEL with an upstream O2 sensor that is going bad (as in not reading perfectly). And no, a safety inspection will not catch bad O2 sensors. The vehicle is running fine, just a little rich and wasting gas.

Thank you!

I'll be adding this to the list of parts I'm replacing.

 

[Grenvilleter]

The colder it is outside, the more gas my 4R uses.
With the sub-zero temps recently, my mileage is half of normal summertime consumption.
With temps below freezing but average, I lose approx 25% of summertime consumption.
Bottom line recently, my mileage is also all over the place except "good".

 

[MStudt]

The colder it is outside, the more gas my 4R uses.
With the sub-zero temps recently, my mileage is half of normal summertime consumption.
With temps below freezing but average, I lose approx 25% of summertime consumption.
Bottom line recently, my mileage is also all over the place except "good".

This might have something to do with it as well since we were -13 for a couple days last week. However, this does not explain why it was bad in southern Utah when it was in the 40-60 the entire week I was there.

 

[paddlenbike]

The ethanol they are putting in our fuels are killing O2 sensors early and producing poor economy. There is less energy in ethanol, and a 5-8% reduction in economy is typical. 8% off 20 MPG takes 1.6 MPG right off the top.

At startup, when the coolant temperature is under around 40-deg farenheit, the air fuel ratio will be very rich (13.5:1) for 30-45 seconds to get the catalytic converter(s) warm. Above those temperatures, the air fuel ratio starts out at 14.7:1. I.e., cold starts and short drives kill mileage.

As others have mentioned, the upstream O2 sensor can start getting lazy and produce poor mileage, even without a check engine light. I replaced my O2 sensor last week and saw an improvement. Buy a Denso O2 sensor on amazon for 1/2 the price of the dealership. I paid $57 with free shipping for mine.

I generally get over 20 MPG during summer months, but barely get 18 MPG on the winter blend fuels.

Hope that helps.

 

[UTEP]

As others have mentioned, the upstream O2 sensor can start getting lazy and produce poor mileage, even without a check engine light. I replaced my O2 sensor last week and saw an improvement. Buy a Denso O2 sensor on amazon for 1/2 the price of the dealership. I paid $57 with free shipping for mine.

I generally get over 20 MPG during summer months, but barely get 18 MPG on the winter blend fuels.

Hope that helps.

I could be persuaded to change mine…how much of a PITA is it really? I imagine tight bolts in tight spots! PB Blaster in liberal amounts.

Come to think of it when ever I tackle things like this, those that aren't yet "broken" I open a can of worms… Is O2 generally straight forward?

Thanks,

U

 

[paddlenbike]

Not sure about 4Runners with the automatic transmission, but on a manual, the O2 sensor plug is located on top of the transmission and appears to be completely inaccessible. The trick is to take the two nuts off the O2 sensor, pass the O2 sensor (still connected to the wiring) over the top of the transmission and pull the harness toward the driver's side. Then the plug becomes accessible. The harness won't move toward the passenger's side where the O2 sensor is located, but does move the other way. With that tip, replacing the O2 sensor should be a 5 minute job.

Someone else with an automatic transmission can hopefully confirm this is true with the autos as well...

 

[Gerdo]

I could be persuaded to change mine…how much of a PITA is it really? I imagine tight bolts in tight spots! PB Blaster in liberal amounts.

Come to think of it when ever I tackle things like this, those that aren't yet "broken" I open a can of worms… Is O2 generally straight forward?

Thanks,

U

On my 2000, it's fairly easy swap. The two bolts are easy to get to but I do PB them ahead of time. Be gentle and don't force them. PB again if needed. The connector is another thing. Its on top of the transmission and held there with a clip. It's doable but just takes patients. I put anti-seize on the bolts when installing.

 

[nissanh]

I have OEM air filter/spark plugs, NGK wires. 267 75 16 tires with 30 PSI, New Denso pre-cat O2 sensor. Synthetic blend 10-40 Valvoline.

Truck did 16 to 17 MPG at 70-78 MPH on Free way all the time (5500 miles trip). I am planning to replace the fuel injectors. This is all left for me to do.
Any thoughts to improve the MPG?

 

[slomatt]

The ethanol they are putting in our fuels are killing O2 sensors early and producing poor economy. There is less energy in ethanol, and a 5-8% reduction in economy is typical. 8% off 20 MPG takes 1.6 MPG right off the top.

At startup, when the coolant temperature is under around 40-deg farenheit, the air fuel ratio will be very rich (13.5:1) for 30-45 seconds to get the catalytic converter(s) warm. Above those temperatures, the air fuel ratio starts out at 14.7:1. I.e., cold starts and short drives kill mileage.

As others have mentioned, the upstream O2 sensor can start getting lazy and produce poor mileage, even without a check engine light. I replaced my O2 sensor last week and saw an improvement. Buy a Denso O2 sensor on amazon for 1/2 the price of the dealership. I paid $57 with free shipping for mine.

I generally get over 20 MPG during summer months, but barely get 18 MPG on the winter blend fuels.

Hope that helps.

Hi Ken,

Would you mind sharing what part number you purchased for your 99? The Denso website lists different parts for CA vs federal trucks, and also has part numbers for "Air Fuel Ratio Sensor" and "Oxygen Sensor", with upstream options for both. Prices seem to differ greatly between the various models ($102 for CA air fuel ratio sensor vs $56 for Federal oxygen sensor).

My mileage has dropped a bit recently, and since the truck is at a whopping 109k I might as well replace the O2 sensors at some point.

- Matt

 

[TheDurk]

I just hope no one in this thread is using their fuel gauge to measure MPG. These gauges are flat out useless for that purpose. The only thing you should be doing is filling the tank to full the same way every time--from the same pump is the best if you can, and then dividing the number of miles driven until the next fill-up by the number of gallons needed to refill the tank. Do not involve the gauge in these calculations.

Comments like "I only get 170 miles to a tank" make me nervous that people are doing it the wrong way.

 

[MStudt]

I just hope no one in this thread is using their fuel gauge to measure MPG. These gauges are flat out useless for that purpose. The only thing you should be doing is filling the tank to full the same way every time--from the same pump is the best if you can, and then dividing the number of miles driven until the next fill-up by the number of gallons needed to refill the tank. Do not involve the gauge in these calculations.

Comments like "I only get 170 miles to a tank" make me nervous that people are doing it the wrong way.

I really don't know what to see to this. Gauges are designed to be read, and that is the main reason cars come equip with them. If they designed to fail from the start, I highly doubt those manufactures would still be in business.

The thing that you should really be concerned about is helping people fix the issue, not telling them to blackout the fuel gauge. I've had mpg's range from 110 at a 1/4 tank, 186 at a 1/2 tank, to 176 at 3/4 of a tank. This mileage has all been based on the trip mileage and how many gallons to fill the tank at that given time. This is more than a gauge issue at this point, and it needs to be fixed.

 

[Canuckrz]

I have OEM air filter/spark plugs, NGK wires. 267 75 16 tires with 30 PSI, New Denso pre-cat O2 sensor. Synthetic blend 10-40 Valvoline.

Truck did 16 to 17 MPG at 70-78 MPH on Free way all the time (5500 miles trip). I am planning to replace the fuel injectors. This is all left for me to do.
Any thoughts to improve the MPG?

Switch to P245/75R16 with low rolling resistant tread like the M/S2, up pressure, if you've got a roof rack and don't use it remove it. People have also reported better mileage using the ECT in PWR mode, but that could come down to the lay of the roads. Brake less, coast more in town, anytime you have to use the brakes is a waste of energy.

The list goes on and on but at a certain point there's only so much one can do to a brick someone rounded the edges off of.

 

[paddlenbike]

Hi Ken,

Would you mind sharing what part number you purchased for your 99? The Denso website lists different parts for CA vs federal trucks, and also has part numbers for "Air Fuel Ratio Sensor" and "Oxygen Sensor", with upstream options for both. Prices seem to differ greatly between the various models ($102 for CA air fuel ratio sensor vs $56 for Federal oxygen sensor).

My mileage has dropped a bit recently, and since the truck is at a whopping 109k I might as well replace the O2 sensors at some point.

- Matt

Matt, it never hurts to replace it. Some sources say O2 sensors start to get lazy around 60,000 miles, but when 10% ethanol is used, the O2 sensor may need to be replaced much sooner.

Mine is a federal emissions model since it was purchased new in Arizona. I used 234-4161.

1999 & 2000 49 State manual V6 4WD - Front: 234-4161; Rear: 234-4154
1999 Cali manual V6 2WD/4WD 4Runner - Front: 234-9003; Rear: 234-4153
2000 Cali 4WD manual V6; Front: 234-9001, Rear: 234-4153
2000 Cali 2WD manual V6; Front: 234-9001, Rear: 234-4155

99+ Cali 4Runners generally use the air fuel ratio sensor, while fed models still generally use the same O2 sensor from 96-00. I believe all the 01-02 4Runners use the air fuel ratio sensor. Most of the part number differences are wire harness lengths to account for different transmission/transfer case options.