MPG all over the place

[paddlenbike]

"Oxygen sensors (O2) used in most of today's automotive gasoline engines are failing at an ever increasing rate. There can be one to five sensors per vehicles and having them replaced can cost a consumer $100.00 to over $300.00 each. This can happen in brand new vehicles with low mileage or older high mileage ones.

Interestingly, most of these "failed" sensors are not actually defective or even worn out. What has happened, is that a small amount Ethanol in the gasoline (gasohol) will get past the piston rings and into the motor oil. The Ethanol with agitation and heat liberates some of the phosphorus from the motor oil. This phosphorus is vaporized and sucked into the Positive Crankcase Ventilation (PCV) system and burned in the combustion chamber. This burned phosphorus on its way out the exhaust coats the O2 sensor(s) building up in layers.

This phosphorus coating acts as a insulation causing the sensor to react slower than normal. The engine computer reads this slow reaction time as a failure of the sensor forcing its replacement.

Note: This is the same material that coats catalytic converters causing them to go "cold" or cease to function."

Tell your legislators you want ethanol removed from our fuels. It is degrading our car's fuel systems and reducing life from our vehicles, reduces power and economy, and the quick degradation of oxygen sensors and catalytic converters harms the very thing it is intended to help, air quality.

 

[TheDurk]

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.

Maybe for you, but I am certain that the lousy gauges on these trucks are deceiving some people into thinking their mpg's are out of whack when they are not. You have verified your findings with the pump; not everyone has. The fact they shouldn't be lousy does not mean they aren't. They are. Meanwhile, a refresher course in 3G sensors appears to be needed here:

1. There are only three types of actual sensors: O2 flange (front and back), O2 screw-in(rear only), and A/F flange (front only). All other differences are wire length only. A/F's and o2's have different plugs. Wire length and specific part number varies by year, auto/manual,4wd/2wd, V6 or 4cyl, Cali./Fed. See #5 below.

2. Failing front sensors can degrade MPG's without throwing a code; if MPG's are out of whack, clean MAF and TB, then change front sensor if over 75k, change plugs if over 30k, change air filter if it needs it (visual). Resolve any DTC codes if MIL is on (check-engine light).

3. All '96-'98 4Runners had the one-cat system. Cali'. systems with A/F sensors in front and two cats existed only in '99 and '00 and were sold all over the country, especially in 2000. Where the truck was sold means nothing except all trucks sold in Cali. were Cali.s in '99 and '00. Then, all trucks sold from 2001 on had the "Cali." two-cat system that then became the Federal 50-state standard.

4. To tell if you have a Cali., count cats or look at emissions sticker. Look at top left under ENGINE FAMILY: if it says ...AF, TWC(2), HO2S....it's a two-cat Cali or '01-'02 Federal. If it says ...TWC, HO2S(2).... : it's a one-cat, '96-'00 Federal.

5. Use Denso or Toyota sensors (made by Denso) only. You will pay a lot more for the exact same thing from Toyota. Get your Denso part number here:Find My Part | DENSO Auto Parts

6. In salty states, the nuts will rust away and have to be replaced. They are stainless steel M8-1.25 nuts, part #90179-08059 at the dealer. Two per sensor. Denso/Toyota sensors come with the gasket included.

7. Denso sells "universal" sensors in the o2 flange and o2 screw-in types, but not the A/F type. Universal means it is just a sensor with pigtail and no plug. I, and lots of others, do not recommend this. It is just a very aggressive environment to rely on solder and heat-shrink to maintain a connection for ~100k. Save a few bucks somewhere else.

 

[slomatt]

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.

Thanks Ken, of course my CA (purchased in Napa) 2000 requires the twice as expensive air fuel ratio sensor.

- Matt