Extremely low gas mileage

[SpidermanGeek]

Disclaimer: I can have a lead foot at times but even being gentle, fuel economy is still low.
I have '00 3.4L. I check my fuel economy pretty much every time I fill up (out of habit). I get anywhere from 12-14 city, 16-18 highway. I run 265/70/16 Goodyear Wrangler Duratrac tires and know that the LED light bar doesn't help with drag but I was getting the same numbers prior to its install. I also have a cold air intake.

I installed NGK iridium plugs less than 25k miles ago (rather than the OEM laser platinum plugs) with NGK wires.
I replaced the MAF sensor a Cardone one from Advance Auto (used to work there). The idle lowered from 11/1200 to 7/800.
I have a live data OBDII scanner that allows me to see O2 readings and a friend of mine, who's a certified mechanic, says the numbers look good.
I used to run 87 octane but switched to 93 shortly after to try and help clean up. I periodically use Techron Complete Fuel System Cleaner, which states it can treat up to 20gallon tanks (tried numerous fuel system cleaners on my '00 Camry , when I had it, and it gave me a 3-3.5MPG increase per tank it was used on.

Honestly, my MPGs are terrible whenever I use higher octane than 87. In my experience, I don't think a 15+ year old 5VZ-FE is efficient at burning higher octane gasoline. High Octane doesn't "Clean" anything up in your cylinders. A higher octane fuel is actually harder for your engine to burn (it requires more heat to ignite). So if your engine can't efficiently burn the fuel, you'll be sending unburned fuel through to your catalytic and it won't be able to properly do its job as well. It might even be playing tricks on your upstream sensor.

Also, the NGK plugs that you used.. are they specific for the 5VZ-FE's Wasted Spark Ignition System? It should have twin ground prongs, rather than the usual single.

And how long ago did you replace the MAF? Did you pull the EFI relay from the fuse box to reset it when you replaced the MAF? If not, your EFI system could still be "learning" the new MAF readings.

I honestly don't know what else to look into, other than replacing the upstream O2 sensor (knowing its expensive because its an air/fuel ratio sensor). Any input would be greatly appreciated.

Is yours Cali emissions specific (2 catalytic converters)? If not, then you don't have an A/F sensor for upstream, it's just an O2 Sensor.

If it's not cali-emissions, you'll want:
Denso 234-4162 Oxygen Sensor for Pre-Cat (Upstream)
Denso 234-4154 Oxygen Sensor for Post-Cat (Downstream)

 

[SpidermanGeek]

While your MPG isn't great, it's not "terrible" per say. I usually get about 16-18mpg but i drive mostly highway. My rig is also pretty stock like yours.

I'll agree with this too.

My combined (but 90% city) MPG is less than 13 MPG, but I'm lifted and riding on 285/75R16s, with no skid plates.

 

[nissanh]

"Honestly, my MPGs are terrible whenever I use higher octane than 87. In my experience"

Right at the change to high octane gas, the gas mileage is not so good because the ECM is doing it's job to get the readings right. I think 3-4 tanks will show bad gas mileage. After that things should be alright.

I get 12-13 MPG in the city and 16-17 on highway. Not so good for a DD.

 

[spazzyone]

"Honestly, my MPGs are terrible whenever I use higher octane than 87. In my experience"

Right at the change to high octane gas, the gas mileage is not so good because the ECM is doing it's job to get the readings right. I think 3-4 tanks will show bad gas mileage. After that things should be alright.

I get 12-13 MPG in the city and 16-17 on highway. Not so good for a DD.

I've been getting roughly 10-12mpg city this winter. I changed my tires right before the winter to Kumho Road Ventures (stock size) and have been autostarting at least 5-10 minutes before getting in it so I don't freeze to death at least twice a day. My last tank I got 150miles before the gas light went on... Before the winter I was getting around 16 combined, tops. I've never gotten any better than 230-40 miles out of a tank before. My SR5 is basically stock minus the tires. I don't ride it hard or gas it hard since it's not fast to begin with. I tried the ECT and that made it worse actually.

I looked through the maintenance records yesterday and I'm due to change the spark plugs, air filter and pcv valve soon. I plan to once it's slightly warmer. I've had the truck for roughly 1 year / 8k miles. I have no idea if the MAF was cleaned ever but I'll be doing that also. I'm going to try resetting the ECU once all said and done.

 

[SpidermanGeek]

"Honestly, my MPGs are terrible whenever I use higher octane than 87. In my experience"

Right at the change to high octane gas, the gas mileage is not so good because the ECM is doing it's job to get the readings right. I think 3-4 tanks will show bad gas mileage. After that things should be alright.

I get 12-13 MPG in the city and 16-17 on highway. Not so good for a DD.

This is a good point. Maybe do en EFI reset if planning on switching to higher octane. Might speed up the ECM learning process.

 

[mckennar]

I've been ... autostarting at least 5-10 minutes before getting in it so I don't freeze to death at least twice a day.

This, winter blend, colder outside air temp = worse fuel economy in the winter.

My commute is by mileage 62% highway, 38% surface streets. In the summer I get 18-19 MPG. In the winter (what passes in Arkansas for 'winter'), same commute, I get 16-17. When I lived in Maine it was much more drastic - 9-10mpg in the winter in my 5.2l Durango, 15mpg in the summer.

Take a look here for a more betterer explanation than I could cough up:
https://www.fueleconomy.gov/feg/coldweather.shtml

 

[KingSwingles]

While your MPG isn't great, it's not "terrible" per say. I usually get about 16-18mpg but i drive mostly highway. My rig is also pretty stock like yours.

I'm not sure anything is technically wrong, unless you used to get much better MPG before. There are a lot of variables like driving habits, where your driving (hills ect), elevation, temperature, fuel quality and so on.

Appreciate the input. I've been checking around for different ideas. I know they're not the greatest on gas but I was hoping for better than what I'm getting. I'll keep trying things one by one.

 

[KingSwingles]

Honestly, my MPGs are terrible whenever I use higher octane than 87. In my experience, I don't think a 15+ year old 5VZ-FE is efficient at burning higher octane gasoline. High Octane doesn't "Clean" anything up in your cylinders. A higher octane fuel is actually harder for your engine to burn (it requires more heat to ignite). So if your engine can't efficiently burn the fuel, you'll be sending unburned fuel through to your catalytic and it won't be able to properly do its job as well. It might even be playing tricks on your upstream sensor.

Also, the NGK plugs that you used.. are they specific for the 5VZ-FE's Wasted Spark Ignition System? It should have twin ground prongs, rather than the usual single.

And how long ago did you replace the MAF? Did you pull the EFI relay from the fuse box to reset it when you replaced the MAF? If not, your EFI system could still be "learning" the new MAF readings.



Is yours Cali emissions specific (2 catalytic converters)? If not, then you don't have an A/F sensor for upstream, it's just an O2 Sensor.

If it's not cali-emissions, you'll want:
Denso 234-4162 Oxygen Sensor for Pre-Cat (Upstream)
Denso 234-4154 Oxygen Sensor for Post-Cat (Downstream)

I might go back to 87 and see if that helps. When I replaced the MAF, he pulled the relay to reset it and we've checked the numbers periodically since. It's only been in for 2 months.
As far as the plugs go, I was talking to a friend who has the 5VZFE engine in his '00 Tundra, and he did the same plugs with no issues.
No, it's definitely Cali emissions.

 

[phattyduck]

I might go back to 87 and see if that helps. When I replaced the MAF, he pulled the relay to reset it and we've checked the numbers periodically since. It's only been in for 2 months.
As far as the plugs go, I was talking to a friend who has the 5VZFE engine in his '00 Tundra, and he did the same plugs with no issues.
No, it's definitely Cali emissions.

You may also want to define 'highway' speeds. 65 or less = much better gas mileage than 70+. I set the cruise at 67mph and get 20-22 on the highway. 16-18mpg on my short commute (traffic filled freeway).

-Charlie

 

[KingSwingles]

You may also want to define 'highway' speeds. 65 or less = much better gas mileage than 70+. I set the cruise at 67mph and get 20-22 on the highway. 16-18mpg on my short commute (traffic filled freeway).

-Charlie

Only do 5-10 over speed limit. I haven't noticed any significant difference when I've driven faster for some aspects (as in driving 15 over but still getting same MPG).

 

[Trip_84]

same issue

When i bought my 98 in july i was getting about 240-250 miles on a tank. Now i have to stretch to get 160, like fume it into a gas station. not sure why. I have done 2 oil changes on it, air filter, tranny drain and fill. There does seem to be a manifold leak. you can hear it during acceleration. Co worker thinks it might be a manifold leak so i am going with that for now

would a manifold leak cause a drop in mpg? i only have 9 mile commute to work with 3 lights

 

[JayRolla]

"Honestly, my MPGs are terrible whenever I use higher octane than 87. In my experience"

Right at the change to high octane gas, the gas mileage is not so good because the ECM is doing it's job to get the readings right. I think 3-4 tanks will show bad gas mileage. After that things should be alright.

I get 12-13 MPG in the city and 16-17 on highway. Not so good for a DD.

Higher octane does not change the AFR so no learning is needed when changing octane. Newer cars (mainly forced induction) will auto tune when octane is changed because they need the extra octane to eliminate detonation. Our ECU could care less.

 

[JayRolla]

"Honestly, my MPGs are terrible whenever I use higher octane than 87. In my experience"

Right at the change to high octane gas, the gas mileage is not so good because the ECM is doing it's job to get the readings right. I think 3-4 tanks will show bad gas mileage. After that things should be alright.

I get 12-13 MPG in the city and 16-17 on highway. Not so good for a DD.

All higher octane fuel does is ignite at higher rate allowing it not to detonate or knock as easy. No learning is done since the AFR does not change.

 

[DK327]

I'm glad I visited this thread... In the summer I've been consistently getting around 190 miles before the fuel light would come on, then I'd drive another 50 miles easy before filling back up 16.5 gallons. Right now, the light comes on at 150-160 and I'm barely touching 200. I believe this is due to cold temps coupled with short drives to and from work. The latter hasn't changed but it's probably not helping in cold temps when engine is trying extra hard to warm up. By the time I turn on the vehicle and let it warm up to 130F/60F (water/trans) by the time the water temp reaches 190, I'm already home. It's less than 3 miles commute each way! I've always thought it was the additives they put to fuel in winter so it doesn't freeze, but maybe that's not the case... I have a scan gauge and not showing any codes but will probably change the O2 sensors when I get a chance, when it's warmer out.

 

[zanes71]

I feel a lot better after seeing this thread. My 99 4wd automatic with the first gen supercharger gets about 14.33 mpg and about 200 miles till the light comes on and then i go to around 225 before filling. I usually put between 15 and 16 gallons in, and I do 50/50 highway and city or driving in the mountains, which I know really kills it. I've tried different gas stations and pulling the EFI relay. My mom had a 98 with a manual and the V6 and got 280 to a tank so I've been wondering where my 60 extra miles are... guess they're in the supercharger and auto trans

 

[Tyler James Inc]

I typically use 87 from Fred Meyer. This winter, I dropped from 15-16 mpg to 13-14.
Tried two tanks of Chevron Plus- still only 14.7ish mpg. Took a 134 mile(each way) trip to Bend last weekend, and filled up with Fred Meyer 87 before we left Bend. On the way home, I had 400lbs over normal, was in 4WD 50% of the time, downshifting often, slamming the throttle, over the mountain, through snow... 16 mpg on that tank * The two tanks I used Chevron Plus on, I was SUPER light on the throttle, and it definitely didn't help my mpg. I always have ECT PWR on, and am running 315/70R17 BFG KO2 tires.
I swear these engines like to be ran hard.

 

[redstar]

My past 2 tanks were both at a horrible 10 MPG. Before those two tanks, I was getting 15-16 MPG consistently. Tried pulling the EFI relay to reset it, but no change.

No codes, but my VSC Off and battery lights have been on for years(VSC trac light used to be off too). Never bothered to check it since I hate the traction control on these things. Wondering if this might have something to do with it?

Spark plugs, wires, pcv valve and fuel filter were changed 18,500 miles ago.
Did the deckplate mod some 2500 miles ago.
Oil and oil filter 1200 miles ago.
Timing Belt/Water Pump/Tensioner/Seals Engine Coolant Temperature Sensor 3000 miles ago.

I took some data from the Torque OBD2 app, not sure what I should be looking for... I've attached the Mode $06 report below and posted screenshots here: Torque screenshots - Album on Imgur . If anyone is willing to take a look, it would be greatly appreciated!

I'm 100% sure that the AFR(Bank 1 Sensor 1?) and O2 Sensor(Bank 1 Sensor 2?) have never been replaced.

I'm going to clean my MAF, IAC and throttle body... Anyone have any other ideas?

 

[spazzyone]

My past 2 tanks were both at a horrible 10 MPG. Before those two tanks, I was getting 15-16 MPG consistently. Tried pulling the EFI relay to reset it, but no change.

No codes, but my VSC Off and battery lights have been on for years(VSC trac light used to be off too). Never bothered to check it since I hate the traction control on these things. Wondering if this might have something to do with it?

Spark plugs, wires, pcv valve and fuel filter were changed 18,500 miles ago.
Did the deckplate mod some 2500 miles ago.
Oil and oil filter 1200 miles ago.
Timing Belt/Water Pump/Tensioner/Seals Engine Coolant Temperature Sensor 3000 miles ago.

I took some data from the Torque OBD2 app, not sure what I should be looking for... I've attached the Mode $06 report below and posted screenshots here: Torque screenshots - Album on Imgur . If anyone is willing to take a look, it would be greatly appreciated!

I'm 100% sure that the AFR(Bank 1 Sensor 1?) and O2 Sensor(Bank 1 Sensor 2?) have never been replaced.

I'm going to clean my MAF, IAC and throttle body... Anyone have any other ideas?

I'm in a similar situation as you. I went for inspection this morning and passed (including emissions). I filled up today and calculated my mpg to be 11.552. That number included half highway. I did a tune up a few weeks including MAF, throttle body, etc. The car runs great and I don't believe it to be brakes dragging either.

I'm going to be changing the AFR and O2 sensor once it's a bit warmer and pull the EFI. When I did the tune up, I just left the battery unplugged for like an hour or so while I was working. I know for sure the sensors have never been changed (121k) so I'm hoping this helps.

 

[redstar]

I'm in a similar situation as you. I went for inspection this morning and passed (including emissions). I filled up today and calculated my mpg to be 11.552. That number included half highway. I did a tune up a few weeks including MAF, throttle body, etc. The car runs great and I don't believe it to be brakes dragging either.

I'm going to be changing the AFR and O2 sensor once it's a bit warmer and pull the EFI. When I did the tune up, I just left the battery unplugged for like an hour or so while I was working. I know for sure the sensors have never been changed (121k) so I'm hoping this helps.

Well, let me know what happens. I'm not sure what I'm gonna tackle first, but as soon as I figure it out I'll post it here. Maybe I'll do what brian2sun did here:

http://www.toyota-4runner.org/3rd-gen-t4rs/63042-engine-running-horrible-but-not-any-more-hopefully.html

 

[spazzyone]

Well, let me know what happens. I'm not sure what I'm gonna tackle first, but as soon as I figure it out I'll post it here. Maybe I'll do what brian2sun did here:



http://www.toyota-4runner.org/3rd-g...ning-horrible-but-not-any-more-hopefully.html

Nice find. Never heard of this before but looks like there's a shop a few minutes away that offers it. Going to call in the morning and see.


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