Just like the title says. I recently noticed that my highway mileage is down about 2 miles per gallon. Was getting 28 highway and 20 city. Now am at 20 city and 26 highway. I have replaced plugs, cleaned k&n as well as injectors. o2 sensors are probably stock with 190k on them. I know they usually only last 50k, but up till 2 weeks ago mileage and power was good, so why mess with it. Could they not react fast enough and cause less highway mileage. Let me know

tia

Andy Schuck

 

with 190k on them i would change them. i think thats pretty obvious. i think you answered your own question....

 

You said you cleaned the injectors. Did you do this with a pour-in-the-tank fuel additive? If so, go to a shop and really get them cleaned. That stuff works a little, but it doesn't to as good a job as it could.

O2 sensors going bad should throw a code. I'm not entirely sure that's true on an ODB1 car, but it's definitely true on an ODB2. Maybe someone else can chime in here?

 

well on my 97, my o2 is throwing a code...p0151, meaning low voltage...and my mileage has gone to crap, thinking when we extended the harness we might have crossed a wire or something...but yea i'd def change them if i had them in for that long

 

well, on obd2 cars...as long as the sensor is still sending the flop signal (high to low and back high) the ecm/pcm wont throw a code. but...if it stays high or stays low...or is shorted/open then yes it will throw a code...

problem with mileage in an old ox sensor is that it doesnt flop fast enough after about 80k miles or so. then you need new ones.

and clean the injectors, not the 'fuel system'...those in-tank additives are not fuel injector cleaners. i like seafoam myself...or if you can get a can of wynn's or BG chem stuff you're gold...they work wonders. my dad just picked up 3 mpg on his 125k mile truck, 5.3 vortec.

 

OBD-II also sets a code for slow switching (>100mSec), and for a stable reading in the range of 352-552mV.

But its still possible for a sensor to be putting out a faulty signal, without setting a code. If that "false" signal is indicating leaner than the mixture really is, its going to cause the PCM to add fuel the engine doesn't need, and the MPG will go down.

 

Like most things on a modern car, a code thrown means "this part is completely fd up" but performance/efficiency was going down long before it got to that point.

O2 sensors are good for about 50-75k if treated kindly.