Questions: O2 Sensors, MAF Cleaning, fixing poor mileage, etc. etc.

[Leksikon]

Been getting terrible mileage in my 97 for a while and haven't had the time to tear into it and figure out what's up until recently. Originally I thought I was getting some drag from improperly adjusted/worn rear drums so I had them replaced and found out the axle seals were shot as well so that's been fixed. Front brakes are new as well and the e-brake is now properly adjusted so I can rule out drivetrain drag as a cause to my mileage woes.

Now I'm looking at what else could be the problem / should be addressed.

I've cleaned the throttle body already and have been tossing around the idea of seafoaming it (155k or so on the odometer and it's never been done). I've got new plugs and wires sitting in the garage so they'll go on eventually but not in a huge rush, they were replaced about 3 years ago in which time I've only put probably 25-30k on the truck. I have some MAF cleaner (spray) in the toolbox as well that I'll use this week while I'm sitting around doing nothing, and the only other thing that comes to mind is replacing O2 sensors. To the best of my knowledge it's never been done on this truck so it's probably well past due. The truck's not throwing any codes by the way, but I know something's wrong, I'm only getting a little over 200 miles per tank and it's all stock.

So which O2 sensors should I be replacing (and OEM or Autozone brand, does it matter)? And should I even try to clean the MAF sensor or should I pick up an OEM replacement (or neither)?

I'm pretty handy but honestly I've never had to mess with this truck much aside from routine fluid changes and brakes. It's been rock solid since I got it in 99 so the need has never been there for any of the above.

Also as to the obvious, I will be changing the oil after a seafoam (if I do it, it's due for an oil change in about 500 miles) and I'll be picking up a new air filter as well.

Anything else I might be missing that could conceivably help with my mileage loss? I do appreciate, in advance, any input or advice.

Thanks guys

 

[PitbullRescuer]

You pretty much have the right idea. Stick with Denso for the O2 sensors. Definitely clean the MAF and replace the air filter. I'd do a fuel filter too.

 

[MCObray]

Been getting terrible mileage in my 97 for a while and haven't had the time to tear into it and figure out what's up until recently. Originally I thought I was getting some drag from improperly adjusted/worn rear drums so I had them replaced and found out the axle seals were shot as well so that's been fixed. Front brakes are new as well and the e-brake is now properly adjusted so I can rule out drivetrain drag as a cause to my mileage woes.

Now I'm looking at what else could be the problem / should be addressed.

I've cleaned the throttle body already and have been tossing around the idea of seafoaming it (155k or so on the odometer and it's never been done). I've got new plugs and wires sitting in the garage so they'll go on eventually but not in a huge rush, they were replaced about 3 years ago in which time I've only put probably 25-30k on the truck. I have some MAF cleaner (spray) in the toolbox as well that I'll use this week while I'm sitting around doing nothing, and the only other thing that comes to mind is replacing O2 sensors. To the best of my knowledge it's never been done on this truck so it's probably well past due. The truck's not throwing any codes by the way, but I know something's wrong, I'm only getting a little over 200 miles per tank and it's all stock.

So which O2 sensors should I be replacing (and OEM or Autozone brand, does it matter)? And should I even try to clean the MAF sensor or should I pick up an OEM replacement (or neither)?

I'm pretty handy but honestly I've never had to mess with this truck much aside from routine fluid changes and brakes. It's been rock solid since I got it in 99 so the need has never been there for any of the above.

Also as to the obvious, I will be changing the oil after a seafoam (if I do it, it's due for an oil change in about 500 miles) and I'll be picking up a new air filter as well.

Anything else I might be missing that could conceivably help with my mileage loss? I do appreciate, in advance, any input or advice.

Thanks guys

Denso products, when it comes to o2 sensors. I got mine front and rear from Amazon for a total of $162 for my '00 4Runner. I have Cali Emissions so I purchased the following:
Denso 234-4153 Oxygen Sensor
Denso 234-9002 Air Fuel Ratio Sensor
When replacing o2 sensors, do yourself a favor and spray some PB Blaster or Kroil to help get those bolts off. My o2 requirements may be different than yours, so make sure to check and make sure you get the right one. Use the following guide to determine what part number you need for your vehicle:
Oxygen / AFR Sensors - Underdog Racing Development
My plugs (Denso), wires (NGK), Throttle Body Cleaned, and MAF cleaned has lead to a 10+ mile improvement in gas tank range so far, and my Fuel Light has yet to even come on. In the coming months, I plan to change my Fuel Filter, PCV Valve & Grommet, and Fuel Filter to garner a baseline for maintenance, as well as an increase in Fuel Mileage. If you can stick with OEM Toyota parts. I'll be ordering mine from local dealership or MidAtlantic Toyota. Best of luck!

 

[Leksikon]

Thanks for the responses guys. I forgot to mention that I did replace the fuel filter a few months back with an Autozone or Advance Auto (I forget) filter, but of course I've read since then that they really don't work as well as the OEM for some reason. It was just a quick mention in another thread here, i can't recall the information behind it. Would rather not do that again but we'll see where I'm at with the rest of the maintenance.

 

[LittleCaesar]

Stick with OEM sensors, or at least the manufacturer of them.

I ran 1/3 can of sea foam in the tank, then changed the oil. It did seem to help. If you are putting it through the PCV valve, I'd wait to change the 02 sensors until afterwards.

 

[Leksikon]

Cool, and good idea. I'm going to seafoam it tomorrow and order the O2 sensors tonight. Should be able to get the new sensors / plugs / wires / and MAF cleaning done this weekend.

Thanks again guys, much appreciated.

Anything else I should be doing while I'm at it let me know, I'm all ears.

 

[Dannyboltz]

Im having the same issue with my 97 12mpgs:wtf:. I just changed the plugs,air filter and fuel filter. I wanted to hold off on the 02 sensors till i had some more $$$. Post after you replaced them and let us know if your mpgs jumped back up.

Just a FYI you shouldnt base fuel economy off of tank mileage. Fill your tank, reset your tripometer, drive to atleast a 1/4 or more, fill up again and divide mileage by gallons used to fill tank.

 

[JMat]

I was having the same issue with my 1998 and as it turned out it had nothing to do with the engine or electronics, it turned out to be a combination of 2 things... The pinion bearing(which was starting to get noisy) and the axle bearings.

After replacing the pinion bearing(which I knew was starting to go) my mileage got better but it wasn't until one of the axle seals went that my mechanic checked the axle bearings and sure enough, they were real sloppy. As it turns out, the axle seal went out because of all the slop in the bearing and once they were replaced my mileage went back to normal. What surprised me was how little noise the axle bearings made especially compared to the pinion bearing but when my mechanic showed me the axle bearing you could tell they were in bad shape.

And by the way, this all happened at about 225k.

Something else to note is the 96-98 MAF sensor is different than the other 3rd gens. and is much more protected... I recently pulled mine after 230k and it was clean as a whistle, I sprayed it with MAF cleaner any way but the small wires in there didn't really appear much different afterward and after running a couple tanks of gas through it my mileage hasn't changed at all.

Hope this helps.

 

[Leksikon]

Im having the same issue with my 97 12mpgs:wtf:. I just changed the plugs,air filter and fuel filter. I wanted to hold off on the 02 sensors till i had some more $$$. Post after you replaced them and let us know if your mpgs jumped back up.

Just a FYI you shouldnt base fuel economy off of tank mileage. Fill your tank, reset your tripometer, drive to atleast a 1/4 or more, fill up again and divide mileage by gallons used to fill tank.

So I haven't gotten around to doing anything yet, just been busy with one thing or another and every time I'm ready to get into the truck it either rains or something else pops up.

But I'm glad you posted about the fuel economy calculation vs going off of the tank mileage... so simple and yet I never bothered to think of it that way. Well now I'm super confused because I just filled half a tank after driving about 120 miles of pretty evenly mixed street and highway and it calculated out to 17.987 MPG.

Now I'm starting to wonder if my issue hasn't been something stupidly simple like the gas gauge, sending unit or fuel tank. 18 MPG mixed on a 15 year old truck with almost 160k on it ain't bad.

Anyway we're supposed to have good weather next week so hopefully I'll be able to sort some of this out and post up some updates then. Just figured I'd share the MPG number because at least this last tank was far better than I would have guessed. I'll run it lower before the next fill up and see what it does on a full tank, wouldn't be surprised if there's sludge and other crap at the bottom of the tank that might be hurting the last half of it's mileage?