Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

98 Toyota 4Runner Getting Poor Gas Milage

I am all of a sudden getting really bad gas milage on my truck and have no power. The “check engine” light was on and I changed the O2 sensor, and the light went off, but am still getting about 15 miles/gallon as opposed to the 22 I was getting. Any suggestions? Changed my spark plugs about 1 year ago.

Is there any particular reason you changed an 02 sensor due to this problem?

There’s a lot of things it could be including low tire pressure but enough info to work on needs to be provided.

The first thing to check for is a partially stuck open thermostat. You may be getting heat in the vehicle, and that temperature could be 140?. But the OBDII engine management system doesn’t go into the closed loop mode until coolant reaches about 155?. So if the system stays in the open loop mode the computer thinks the engine is cold so it uses more fuel.

Another thing to check for is a defective coolant temp sensor for the computer. If this sensor has failed where it’s telling the computer the coolant temp never reaches operating temp, the computer stays in the open loop mode and uses more fuel because it thinks the engine is still cold.


Do you only have one O2 sensor on this vehicle? You need to check your tire pressure, air filter and O2 sensor integrity I am fairly certain you have more than one O2…Pre and post Cat at least…

I had computer tested at an Autozone and they told me it was the rear O2 sensor that was causing the “check engine” light to be illluminated, so I changed that and the light went off, so I don’t think I need to change the other one. I changed the air filter recently also. I will look into all the other suggestions, thanks.

There is no code that says replace the O? sensor. The codes consist of one letter and four numbers in the format: “P1234”

 That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

If you have the V6 and 4WD, then you should be getting 15 MPG city and 18 MPG highway per the EPA’s revised estimates. If you’re getting about 15 or 16 MPG overall that sounds about right, especially if you do more city driving than highway drive.

Does anyone think it could be engine compression, maybe not running on all 6 cylanders? Compression test reccomended? What about fuel injectors?

Tune up time…Air filter, plugs, oil change and make SURE your tires dont have like 10psi in them…I just checked my girls Civic and she had 12psi in every tire! I filled them up and what a difference the vehicle actually ROLLS now…lol. It was surprising…they looked a “little” low, but nowhere near what you would think with that little pressure. Could be something that silly…silly but very important.

The O2 sensor was an excellent decision. Also find your MAF sensor and CLEAN IT…they sell specific cleaners for this or you can just use carb and choke cleaner to do it…

With this issue you want to start with all the basics…tire pressure, tuneup, air filter cleaning MAF… You never said how the engine ran…does it run funny? Any strange smells? How does your tailpipe smell while running? Like raw fuel? Gassy? Let us know so that we arent overlooking something obvious.

Wow, thanks for all that info. Yes, I need to check my tire pressure, that is a good idea. Other than that, new air filter, spark plugs were changed about a year ago, and i’m good on my oil change. I will check out the MAF as well. Engine is sluggish, other than that, all seems good, no funny smells or noises. Thanks so much for your advice.