Replaced Oxygen Sensors and MPG Suffered

I have replaced my oxygen sensor on my 1999 Toyota Tacoma and my gas milage dropped as a result!! My truck has 270k miles on it and the O2 sensors I pulled out were pretty dirty but my gas mileage dropped from around 25 to 20. I did not buy the most expensive oxygen sensors, but does that matter? What do you guys think? Should I buy denso brand oxygen sensors? Will that bring my MPG back up? Does anyone have experience with these cheaper oxygen sensors?

 

What about your MAF censor? Did you replace both O2 censors (up and down stream)?

I wouldn't think the brand of censor would matter that much, unless you *really* cheaped out and got a $5 one from ebay, lol.

How are you calculating your mileage? Are you just looking at your OBDII computer (if you have one) or are you dividing miles by gallons at fill-up? I'm going to assume your driving is identical between your mileage drop, but that's something you should be aware of, is that your right foot has the biggest impact on mileage, so if you were driving more in the city/stop and go for that first tank after you replaced your O2 censors, that could easily account for your drop.

It's possible your ECU is still calibrating to the new O2 censors. Might give it a couple tanks then reassess.

You could also be looking at unrelated issues, and are just manifesting by coincidence. Which is why I ask about your MAF, also clean your throttle body, etc... There are several mileage threads here, with detailed things to check/clean/fix that will (help) increase mileage.

 

The O2 sensors I bought were about $25 a piece and I have driven it for about a month now like this thinking maybe it needed to adjust like you said. The miles I have been comparing were highway miles. I have not cleaned the MAF but I'll try it.

 

I've only used Denso. I don't know if you can see anything wrong with either the upstream air/fuel or downstream O2 sensor just by looking at it.

Is the MIL on? What engine have you got?

 

What could I see that could be wrong? It isn't leaking. The sensors are tight. I did add a gasket to the rear sensor because there was not one and after I got the rusty bolt out I couldn't get it back in far enough without possibly breaking it, but I do not think a small gasket would do anything to it. I do not know what you're talking about when you say MIL, I have the 2.7L

 

Yeah, your ECU should be good by now, usually only takes a tank *maybe* 2 to adjust. The O2 censors I've seen (from rockauto) are more in the $100 range, but I still would have a hard time believing that would cause a 20% drop in mileage.

I while back I noticed a similar drop in mileage (although mine went form 14 to 12, lol) after I did my timing belt at 120k miles. Thought maybe I screwed something up with the belt replace, but it seems like it was more related to the MAF, O2 censors, and a dirty throttle body, and th emileage drop was more a coincidence than anything related to the work I did.

Curious why you replaced your O2 censors in the first place. Did you throw a code?

 

MIL is the check engine light. He's just asking if your throwing any codes. I assume your not.
I believe previous poster may be onto something about the computer getting used to new sensors and I'd certainly give a few tanks to see if it improves. Really can't see the brand making THAT much difference if they;re working well enough for the computer to recognize them.
Now if it throws codes after a few tanks.......maybe.


EDIT: I would def. look at other things as far as 'tune-up' to see make sure everything is optimum. MAF and throttle body cleaning are part of a tune u these days.

EDIT #2: Those were some awfully cheap sensors......I put one on my niece's Lexus the other week and when looking at Denso vs. others, i read some people def. didn't rec'd those Ebay cheapos because they didn't work. Period. But they also said it didn't get rid of the code so they bought better ones (Densos are avail. for around 50 at some places) and that was the problem. But again, that was a code that was being stubborn.

 

Crux is right: MIL=Malfunction Indicator Lamp, commonly know as the "check Engine" light. When lit, the computer has stored a DTC, or Diagnostic Trouble Code, which can be read with a OBD II scanner (On Board Diagnostics, version 2, for vehicles manufactured 1996 and after-which was an improvement of the earlier OBD I and OBD 1.5 systems from the early 1990s)

I didn't know what they were either when I first came across them, so I looked them up.

 

Well my check engine light is not on so I'm not throwing codes from what you have told me...should I plug the old ones up and try them out again? I've gone 3-4 tanks with these sensors on...I hate it because it was so hard to pull the rear one off...

 

25MPG? Wow. You were living large. I never get near that in a 2000 2.7 PreRunner. Cherish the good times while they lasted. 20MPG is about right, if not still pretty damn good.

 

Really? I did not realize that was so good. Lol I guess I know what I'm doing tomorrow...replacing those old sensors back in.

 

I gotta agree, if I were getting 20 I'd be happy. Only got about 22 on the last road trip I took.

 

Old ones could have had you running lean, which might get you better mileage but it'd worse for the engine, runs hotter.
Use this to find the factory part number and then Amazon or @RockAuto for best price
http://densoautoparts.com/find-my-part

 

Yea, both the O2 sensors I recently bought were in the $90 range. Not sure what kind OP got for $25.

 

Intersting, I hadn't thought about running too lean before. That's def plausible.

I would get 27mpg out of my previous Tacoma.

Of course, that was a '97 4cyl 5 lug 5-speed with tiny-ass pizza cutter tires. I get more like 14 in my current 4x4 auto 3.4L with 265/75's and a heavy right foot.

OP: what truck do you have? 4x4, pre-runner, 5 lug, etc...?? If you were getting 25mpg from a 4x4, that's pretty damn high, so you may have been running lean (better mpg, but no bueno for the engine).

I would consider getting an OBDII code reader when you hook up the old censors. I have this one

for Android Phones ONLY - Wireless Bluetooth Diagnostic OBD2 Scanner Car Code Reader and Scan Tool for All 1996 & Newer Vehicles ELM327 Compatible OBDII

(link) and a $5 phone app. Well worth it, IMO. You can hook it up while you drive, and track all sorts of stuff, like fuel mixture, engine temps, and see if there are any codes or other data that could help diagnose the issue.

 

We really do need to know what config. you got 25 in. Lean sounds plausible but i would think spark plugs would show something THAT drastic. That sounds melted electrode, hole in piston lean.

 

I had to LOL at myself but wanted to put this in here to lighten the mood

censor:

an official who examines material that is about to be released, such as books, movies, news, and art, and suppresses any parts that are considered obscene, politically unacceptable, or a threat to security.

sensor:

a device that detects or measures a physical property and records, indicates, or otherwise responds to it

 

not as much difference as i thought

 

2WD 6 Lug 2.7L SR5. I ordered an OBD2 and we will see what's going on I presume. There were no check engine lights and no reason for me to think the oxygen sensors were bad. I replaced them simply because I was told they get dirty and improves has mileage to change them if they have never been changed before.

EDIT: There was also no change in temperature when I changed them. It's always where it should be...

 

You can also take the old O2 censors and clean them with an air compressor, see if that does anything.