REALLY bad gas mileage

lately I've only been getting 11 or 12 mpg from my 3.4l. in the past, even on the worst days it would still get 16 in the city with lots of short trips throughout the day. i thought it was my new commute but my 100 series landcruiser gets about the same and it weighs double and has a v8. pretty sure something is wrong.

right now I'm thinking it could be old o2 upstream sensor. mine is original with 325k miles. also have p0420 code(cat efficiency below threshold) so that might be related.

might also be a vacuum leak as i have a p0450 code recently as well. either vacuum leak or sensor is bad.

might also be coolant temp sensor or rather ect sensor. though the engine idle seems normal after warm up.

new spark plugs and wires about 10k ago.iridium plugs and aftermarket wires/ boots.

MAF I'll clean but that usually results in better mpg and less power when it's dirty.

power seems normal to me but i don't hardly ever get into it anymore with my short commute with 1 mile of freeway.

i thought it was bad gas so i put in chevron and it's the same story.

any ideas on what could cause such a huge loss in mpg in a stock truck?

 

Yep sounds like you’ve got some things you already know about that are pretty obvious culprits of that issue. Just take to a reputable shop and figure out which things wrong is what I’d do. Worth it versus getting 11mpg . A smart investment !

 

well my scanner(which i really don't know how to operate) shows that my engine coolant temperature is 201 after warning up such I'm inclined to believe. So probably not that.

this scanner can show two live graphs at once so i tried to get it to show me the upstream and downstream sensor values at once. i THINK that's what i have here but not sure. anyone who knows how to read these things please let me know.

 

Get a bigger gas tank and it will even out .

 

Could be a tank of bad gas. Maybe run some fuel system cleaner through it as well! Also, winter blends of gas will return lower MPG as well due to higher conditioners to keep the fuel from freezing present.

 

same tires and air pressure as before you noticed?

I have currently lost nearly 50 miles on a full tank and am waiting for spring to troubleshoot and do experiments ... other than different style of tires than i had before i dunno
I actually once had a bit over 400 miles on one tank to experiment on what the max I could go

 

i did just put some of my old 265/75r16s on which are probably a bit heavier and an inch taller than my street friendly tires. maybe that plus short city commute plus winter gas? I'm also on cali so we have our own "special" blend.

 

i watched this video and i realize that my o2 sensor is NOT doing what it's supposed to. it switches but pretty slowly and on the freeway at higher rpms it isn't switching using the full spectrum of voltage. it sometimes just fluctuates a few milivolts.

so im gonna order a new o2 and see what that does!

this scanner was a couple hundred bucks and i bought it back when i was diagnosing my wife's car with the same code. i wasn't sure if it was worth it then but i think it's paying off now. i confirmed it wasn't a ect sensor which is like 50 bucks for a denso. maybe if i haven't killed my cat with a rich mixture, this new o2 sensor will save me from replacing that too.

 

Replace the spark plugs with OEM ones from Toyota that can help a little, not a lot.

I can tell you right now the P0450 has little to no effect on MPG. I've had that issue before and it didn't have no lower milage effect.

That P0420 code is the problem or at least a big part of the problem. The Catalytic converter does affect MPG by a long shot. You need to service it and replace if needed. It's pricey but if you replace it you'll be saving gas money.

 

If your Cat is getting plugged up you definitely will get bad gas mileage.

You can always take the cat off and look through it to see if it’s getting plugged up.

You definitely have something serious wrong to get 10-11 miles per gallon.

I get easily 20-25 miles per gallon highway and that’s with a Supercharger too

 

actually 2 years ago i pulled the cat off and washed it out. it wasn't plugged up then.

 

2 years ago. A lot could've happen since then. How many miles do you drive it a day?

 

2 years was probably 20k miles. i guess i was just judging that if it wasnt plugged up after 18 years, another 2 would be fine. though i suppose as the truck ages, something could happen to cause it to plug up quicker. what does it get plugged up with? carbon?

 

From everything you've mentioned. Get a new O2 sensor (upstream). It has been 325k miles which is far longer than what mine lasted at about 150k miles. Also before replacing your catalytic converter, make sure to take it out, diagnose it, and if really dirty then clean it. I've cleaned mines before and it's pretty easy. Just let it sit in soap and water for the night and afterwards rinse it thoroughly at least 3 times to make sure none of the soap and water stays there. Then let it dry a bit and re-install it. Do not use lacquer thinner to try and clean it because it will mess with a lot of stuff. Especially since the gas tank is not made of metal like the cars of old were. Best of luck OP

 

Just wanted to add that this winter, with short stop and go stuff, the colder weather for the last 2 months, mine (with almost 150k) is getting 4mpg less than normal: 12mpg. Previous to this year, 16mpg was the average around town. It's running nice and smooth, no problems. I think its the cold and the ethanol.

 

i live in CA on the coast at the moment so it barely gets into the 30s overnight. it's basically been 50s and 60s this "winter"

 

As my sig says, I'm in SoCal, on the coast, too. Lived in the Berkshires on the MA/NY line for a decade, so I am aware of "winter."
I think the refineries botched it...

 

If the rear O2 does not cycle it could be bad... with a small propane torch introduce propane into the intake AFTER the MAF and preferably after the throttle body and watch both 02 sensors. The A/F sensor is going to change pretty quick then the rear should start to move. If it doesn't, the rear o2 could be stuck rich. The sensor not switching fast enough is more than likely the very limited sampling rate of the tester and not the 02 sensor. The front A/F sensor is the input to the PCM to change the duty cycle of the injectors for a rich/lean condition, the rear 02 monitors the cat.
With my 96 ext cab 4X4 auto, as soon as I switched to 10 31 15's tires the acceleration went to poo and the gas mileage dropped. My 98 PreRunner, i.e. no 4X4 with the same tires gets 2 mpg more than the 96. My 2003 2.4 manual stripper gets 28 mpg... stock as stock can be.

 

Probably the 02 sensor then

 

@thenodnarb any update?