O2 sensor and gas mileage

[Osiris]

Does the O2 sensor have a lot to do with gas mileage? I read elsewhere that it can account for 5-8 MPG, but I don't know if that is just like 'headers gives you 40 HP' (for a 3.0) talk or if it does effect gas mileage that much if at all.

Could a bad O2 sensor cause that much of a difference? How can you test an O2 sensor? I don't get a check engine light so the ECU is not triggered for anything being wrong. Could the O2 sensor be malfunctioning and not throw any flags, but adjusts the amount of fuel to run rich when it shouldn't and thus give very bad gas mileage?

I am going to look at the FSM online and see what that has about the subject, but I like to get your good thoughts on the subject.

Osiris

[MTL_4runner]

The answer to your question is yes, the front O2 sensor can be going "bad" and hurt you fuel economy, but not throw a CEL for quite some time after. This happens especially when the sensor becomes sluggish (ie the response time of the sensor has slowed down significantly), but not to the point where the ECU has flagged it as being out of spec and needing a CEL. If you are having MPG issues, then you might consider replacing the sensor based on mileage rather than just waiting until it throws a CEL. I'd also urge you to use some Seafoam in the gas tank (2 cans in 1/3 of a tank or 3 cans in 1/2 a tank and drive it like that until the "E" light comes on) to be sure the fuel injection system and combustion chambers are clean.

[Osiris]

did Seafoam on last tank. 2 cans in 1/2-3/4 tank until light came on and then put 2/3 can in manifold while running, shut her down and poured the other 1/3 in manifold, let sit for 30 minutes and then cranked her up and after a few minutes drove her to get gas.

[bamachem]

I had MPG issues on my 99 at about 130k miles. In a one-month timeframe, it dropped from about 16 around town to the 12/13 range. I bought a new front O2 sensor from http://urdusa.com and put it in and pulled the ECU fuse to reset it. The MPG's immediately went back up to almost 17 MPG in town. It never threw a CEL on mine either.

[MTL_4runner]

did Seafoam on last tank. 2 cans in 1/2-3/4 tank until light came on and then put 2/3 can in manifold while running, shut her down and poured the other 1/3 in manifold, let sit for 30 minutes and then cranked her up and after a few minutes drove her to get gas.

Ok, great. :thumbup: Then your next step would definately be to get a front O2 sensor from Northridge Toyota or the local dealer and see where that gets you. Unfortunately URD only carries sensors for most trucks 1996 and up because they usually can beat almost anyone's price on those. :(

PS You might want to put some specs for your truck in your signature. ;)

[Osiris]

PS You might want to put some specs for your truck in your signature. ;)


I will do that so I won't be this guy ==> :tease: !

I noticed URD only went back to '96. There is no way to test it? Maybe run some resistence testing? I'll look at FSM again to see if I see anything. Tough at work though, but here goes!

Osiris

[MTL_4runner]

There is no way to test it? Maybe run some resistence testing? I'll look at FSM again to see if I see anything.

There are ways to test it but honestly they usually aren't worth the effort involved because the O2 sensor is an oscillating voltage and needs to be tested hot. Alot of people in the past have asked the exact same question and a few have tried to actually test it, but even then sometimes the tests may give false readings (ie you might think it tested ok, but the sensor in reality is still boarderline and hurting your fuel economy). The other fallback is often OBD II but trucks before 1996 only had OBD I so that makes it alot more limited for being able to check the sensor via that route. If something is blatantly bad like the heater circuit, a simple DMM test will work (but it also should have thrown a CEL). Checking the response time requires an oscilloscope to check reliably and that is way beyond what most people have at their disposal. You can try the DMM method but the only one I'd really consider reliable is the oscilloscope. Check how much the sensor is at Northridge, you might be surprised at the price and just replace it.


Here's a good explanation of the two types of tests:

"Testing Oxygen Sensors

For the ECU to control the AFR and keep it within tight limits, the oxygen sensor must be working properly. Failed or worn out oxygen sensors cause problems such as poor fuel economy, failed emission tests, failure of the catalytic converter and poor driveability. Therefore it is important that you can read the signs of a failed or worn oxygen sensor and have the ability to check their performance.

On Car Test: Before you can test the operation of the sensor, you will need an oscilloscope. You should first check that the basic engine set up is to the manufacturers specification, then thoroughly warm up the engine - remember that the sensor will only function once it has reached its operating temperature.

Two methods of testing an oxygen sensor are using an oscilloscope or a multimeter. An oscilloscope is the best method for testing. This will give you the exact output of the sensor along with its response times. A multimeter can also be used but this will only give an indication if there is an output or no output. The sensor will be switching too quickly for any response times to be measured.

Oscilloscope

Using an appropriate connecting device, connect the sensor output to your oscilloscope; do not disconnect the sensor from the ECU. Run the engine at approximately 2000 rpm. A properly functioning oxygen sensor will show a rapidly fluctuating output voltage between approximately 0.1 and 1.0 volts. The time taken for the voltage to change from 0.1 V to 1.0 V (referred to as the lean to rich response time) should be about 300 milliseconds. A similar time should be measured when the voltage changes from 1.0 V to 0.1 V (rich to lean response time).

Multimeter

For testing with a digital multimeter you will also need to connect the sensor output to the multimeter using an appropriate connecting device. Run the engine at approx. 2000-2500 rpm. The output will be a DC voltage, oscillating between approximately 0.1V and 1.0V. Although the sensor output is technically an oscillating DC voltage some multimeters may require to be set on AC voltage measurement to correctly read the sensor output. Also, the response time of the multimeter must be better than the response time of the sensor. If the multimeter is too slow then a constant output will result even though the sensor is actually switching.

If the sensor output is constant or the response time is too slow the sensor should be changed. It is a good idea to check the oxygen sensor function at every tune up and before submitting cars for emission tests. A slow sensor will effect fuel economy. A new sensor will pay for itself by cutting fuel bills."

http://www.ngksparkplugs.ca/tech_osinstall.cfm