My truck is opposite, I swear. I've been getting 11.5 MPG on the last fill up. I decided to disconnect my pre-cat O2 sensor. I remember I've done this before when I cut the wires when I was trying to get a box end wrench to loosen the sensor, which had no luck. I drove around with that disconnected for a month and my MPG improved. Well, today is no different. I disconnected it before I filled up, and I went on a good 50 mile drive. Went back to fill it up again, and only 2.5 gallons filled it (same gas station). So if you do the math, the trip was somewhere a good 19.5 MPG with an average speed of 45 MPH all city. I'm sure that the MPG is unrealistic at this point since I've noticed a full tank does better at first for reasons I don't know.

Anyway...is there something I'm missing here? Do I need to dig up the O2 circuit and see if there's something wrong with the wiring? The post-cat did in fact trigger a CEL. What are my options on eliminating the CEL?

 

they sell a fake o2 sensor so it always has a perfect reading it may help idk

 

Do you disconnect both Pre-Cat Sensors? Has anyone else heard of that getting better fuel economy with the Pre-Cat sensors disconnected? This would be very interesting with fuel up in Canada at 1.33 ltr or 5.50 gallon.

Speak up if it's true, I thought it was the other way around poor economy!

 

I only have 2 sensors, one before the cat converter and one after, at least that's all I can tell. And no, this is unheard of, which is why I don't understand what my truck is doing. A new O2 sensor is supposed to adjust the air fuel ratio properly. Having it disconnected, at least I assumed, would be the equivalent of a bad O2 sensor, which dumps fuel and can lead to bad emissions. Turns out the computer dumps less fuel with it disconnected so I'm looking into it.

 

The 4.7 has 2 banks one on each side. There are O2's before and right after each cat. Could be different in Canada. Post up if you find out if there is a difference between a unplugged O2 and a broken O2

 

i have no knowledge of an unplugged o2, but i get a p0155 code every once in awhile, when i do i get great milage. i do beleive that cel code is an upstream (pre cat) o2 senser. i never posted it cuz i thought yall would think im smokin something. i prob jump 3-4 mpg when my cel is on. and yes i have (4) o2's

 

yea this is a weird topic because on my 85 4runner the wire that runs to my O2 sensor got frayed so it obviously would not read. if running at the same rpm (like on the highway) for a few minutes, it would go into open-loop and just throw fuel into the engine so i would get terrible gas mileage.

so do you guys think that if the O2 sensors in our trucks were replaced, would you get better gas mileage?

 

When the forward (pre-cat) sensor is not functioning, the PCM will go into open loop, which basically means it's going to get the A/F mix from a pre-set parameter table programmed into the PCM. This usually will mean a richer mix than when in closed loop as the PCM "thinks" the vehicle is warming up.

Very rare to get better fuel economy without the pre-cat O2 functioning properly.


As far as the rear one goes, you can purchase an O2 simulator to get rid of the nuisance CEL, but they are getting harder to find and the cost is around $100 when you can find them. Most guys just simply use a couple $5 non-foulers...

 

what do you mean by a non-fouler? and what effect will removing the post -cat O2 sensor have on things like fuel mileage?

 

as far as i know, the post-cat has no effect on the A/F...it just makes sure the pre-cat is doing it's job. correct me if im wrong.