Question about gas mileage. HELP!!
 

1997 4Runner 2WD SR5 3400 V6 Automatic. I havent done any significant work to it in a while. 2-3 months ago, over 3 tanks of gas, mpg went from 16city 18hwy down to 10city or less, hwy unknown. No noticeable change in engine performance or rough idle. No unusual sounds. No engine lights. No evidence of gas leak. Brake Check replaced fuel filter, plugs and wires(I asked for the more expensive ones), uv joints and shocks (which one of the rear ones fell off a week later). Another local shop did an invasive fuel system clean of everything from before air filter to just before the gas tank. I replaced both O2 sensors, recharged my K&N air filter, bought a Fuel Doctor that you put in the lighter plug that supposedly helps fuel mileage, wheel bearings are good, was running 92 octane but was told to run mid gas, tires are over inflated some, not running the AC and have been driving "nicer". After all that, my last 3 tanks have been less than 13city and 14.5hwy. Anybody have any idea what could be causing this cause I am out of them.

Mileage usually goes down in the winter because they put more additives in the fuel. Since you just cleaned your K&N you might want to check and clean your MAF sensor that would also effect your mileage if it's covered in K&N filter oil.
Hope that helps

FYI on this:bought a Fuel Doctor that you put in the lighter plug that supposedly helps fuel mileage

it is junk, just so ya know. there is no geegaw or performance chip you can put
on any modern vehicle that will magically do anything. especially something plugged into the lighter or power port. go get your money back asap.

what does work is real engine tuning with a programmable chip, 2nd ecu you can open up and work on, toyota techstream, and a dyno...but you also need to change injectors and other hardware...and some engineering...all called 'tuning'

your mileage problem could be transmission problems, binding of the propeller shaft or
anything connected, winter gas (usually winter gas takes me from 22 mpg to 19 mpg highway, and 15-17 city to 14-15 city) or you have other issues as simple as a bad sensor somewhere, or clogged cat

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test your mileage for real:

1) fill tank until it stops at first autoshutoff, with 87 octane, close to a highway
2) get on highway and drive 50 or 100 miles at 65 mph pinned on cruise control. count your miles,
get your arse back to the gas pump (make it a round trip)
3) fill up tank again at the very same gas pump, parked in the very same spot
facing the same direction...stop at first autoshutoff...measure the gas you put in.
4) go drive the 50 or 100 miles again at 65 mph pinned on cruise control. count your miles,
get your arse back to the gas pump (make it a round trip)
5) fill up one more time until it clicks. how much gas went in ?


do all the math, what actual mileage are you getting ? should be 17 easily on winter gas
with stock size tires. my 1999 4wd 3.4 gets 19.5-20 in winter using the above method

less than 17 something is hosed, clogged, stuck, broke, or you have fat tires

Where is that MAF sensor? I assume its the mass airflow sensor. And how do I clean it? I know when i was getting my O2 sensors tested, they guy at the dealership told me that i could soak them in seafoam for a while which luckily, i told a friend about it before i tried to clean them and the friend told me you cant submerge them in fluid or it will ruin them.

quote--

your mileage problem could be transmission problems, binding of the propeller shaft or
anything connected, winter gas (usually winter gas takes me from 22 mpg to 19 mpg highway, and 15-17 city to 14-15 city) or you have other issues as simple as a bad sensor somewhere, or clogged cat
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How can I know if there is a transmission problem with out taking it out? Is there a test that can be done or should i just flush it and put new fluid in? I havent noticed much except maybe a slightly louder than usual sound when shifting from reverse to park.

Do the diagnostic computers they use at AutoZone for free checks work for testing sensors that may be working but not at their best? Im sure my light will come on if something fails, but not if less than max efficiency.

How can I test the cat? Need to go to a muffler shop?

Thanks for the help.

do the 50 mile drive as I wrote up and see what your REAL mpg is

other than that, I cannot help

It's the black box just up stream of the airbox, it is the thing connected to the only electrical plug. You can take it out with a screw and a light pull. You can get a MAF cleaner at an autoparts store, but brake cleaner will do the same thing, spray carefully on the tiny wires, you brake them your out $180 for a new sensor. Reinstall it and reset your ecu by disconnecting your negative terminal on your battery for a couple minutes. Drive it normally for 50 miles and it will reset.

My 2cents