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Here is what is going on. I have a auto 2008 FJ with 137k miles, for the last 3 years I have been averaging 220 miles per tank. Last month I dropped to 190-200 after blowing a rear seals/bearings...or so I thought this was the cause.

while tearing down the rear I noted both rear calipers had pretty good grip on the rotors while removing them even though they were soaked in gear oil (I could also smell the gear oil burning when coming to a stop). Installed new rear bearings, seals, parking shoes, pads, and rotors. Then dialed in the parking brake cable which I regret not replacing as its fairly stretched out.

The end result is now 175 miles per tank.

Power feels about the same, though it feels like I do need more throttle input to maintain the same speed.

Here is some telemetry:
water temp 185-193f
Air intake temps average between 40-67f
Transmission fluid temp 140-155f
tires 36 psi
Washington state with 92 fuel


within the last 8k miles I have done:
rear end bearings brakes as mentioned above
front and rear gear oil
front wheel bearings
all new suspension parts (same ride height)
spark plugs
cleaned maf
cleaned air filter (k&n, nearly no oil sprayed on it)
new battery group 31


other things to mention:
No engine codes
rear hub temps are sub 100 after stopping, rotor pad surface is 180ish, rotor hub is a little less (170ish but cant get a good reading through wheel), outside pad/caliper 200-220ish.
front are a little warmer, but not much.
oil changes between 3k/4k
transmission does have a bit of a whine when center console removed, and a little "chirp" on up shift when cold between 2k-3k rpm slight uphill.
I let my truck hit at minimum 120 water temp before rolling off
extended the factory battery cables with hydraulic crimper/solder to fit group 31 battery so this should be solid
replaced serpentine pulleys about a year ago
rear calipers pressed into bore very smoothly
no trouble starting cold or warm


Thinking of throwing a few new parts at it:
ignition coils
new plugs
fuel pump? (starts fines, revs like a high compression 4.0, idles very smooth around 600)
o2 sensors?


and again this is very recent... my tires are the same, lift is the same. I don't think it is related to rear wheel bearing job... just a really good timing coincidence.


Any ideas would be appreciated


Thanks in advance.
 

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Impressive list, and grasp of temps on so many parts/systems! Did you clean your throttle body? No other idea other than fuel blend...my mpg always takes a hit in winter. If you don’t figure this out, and it improves when summer blend return you’ll have your answer :wink
 

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I know this sounds too simple, but your latest loss is between 1-2 mpg, which, although frustrating, could have a simple cause. Any changes to tire pressure or truck weight due to mods or cargo increases? Don't discount that the mere onset of cold weather season could be a factor as well as the seasonal fuel blend and telemetry you have presented.

36 PSI is lower than I like to roll for highway driving. Cold weather, altitude changes, or heavy mods and cargo can cause you tires to a have lower PSI than you intended. Sometimes my TPMS light will come on in the mountains and then automatically return to normal at sea level. Airing down even 5 psi could conceivably reduce efficiency by that much. Even adding winter snow chains and other cold weather supplies as cargo or taking an occassional passenger more often can reduce my mileage efficiency by 10%.

Try increasing to 42 psi if your tires and configuration allow for it. I would also drive stripped down of cargo for one refueling cycle just for a scientific baseline and piece of mind. This might help recoup 1 or 2 mpg's until spring or until you find another root cause for lagging performance.
 

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I read years back that FJs were losing mpg because the thermostat was going bad. Now I`m not sure how accurate this is but just throwing it out there if you want to check it out :)
 

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Maybe consider switching the thermostat as it's a mechanical spring/disc that lets coolant through. Switch back to a Toyota filter. K&N won't help and more than likely to cause gunk down the line. Minimize your idle times. Not sure how long it takes to get to 120* water temp (2 minutes?), but if it's freezing temp you can do a longer start up procedure, but on regular days, shorter idle times have helped (20-30 seconds) and take it easy until your gauges show your up to temp. Bad spark plug gaps might not effect mileage on a whole, but may help the performance in the end. Give it a go! Lastly, make sure to grease any thing in the drive train that needs it. I get about 16mpg year round and 14mpg with A/C on and I'm on heavy wheels and tires. 14 to 11mpg is definitely out of place.
 

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This has been covered here many times in the past, but the #1 cause of poor fuel economy in vehicles with fuel - feedback mixture control systems is fuel-air sensors and oxygen sensors that have reached the end of their useful lives, which is typically around 85k to 120k Miles.

The fuel-air sensors and the mass airflow sensors work together to maintain a 14.7:1 fuel/air ratio. A contaminated MAF will not provide accurate data on the mass of air entering the engine, and worn-out fuel-air sensors will not provide accurate data on the volume of fuel that needs to be injected to achieve the magic 14.7:1 ratio.

1. Make absolutely sure that you have cleaned the tiny, well-concealed MAF sensor elements, and not just the match-head looking intake air temp sensor.

2. Replace both fuel-air sensors, using only OEM Densi parts.

Then, meticulously record your actual fuel consumption over at least four complete tank fills in miles-per-gallon, and not just some nebulous “miles per tank” number.

Post your results here.
 

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Discussion Starter #9
Impressive list, and grasp of temps on so many parts/systems! Did you clean your throttle body? No other idea other than fuel blend...my mpg always takes a hit in winter. If you don’t figure this out, and it improves when summer blend return you’ll have your answer :wink
No I have not. I can give it a look over. Will be posting some more info shortly.
 

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Discussion Starter #10
I know this sounds too simple, but your latest loss is between 1-2 mpg, which, although frustrating, could have a simple cause. Any changes to tire pressure or truck weight due to mods or cargo increases? Don't discount that the mere onset of cold weather season could be a factor as well as the seasonal fuel blend and telemetry you have presented.

36 PSI is lower than I like to roll for highway driving. Cold weather, altitude changes, or heavy mods and cargo can cause you tires to a have lower PSI than you intended. Sometimes my TPMS light will come on in the mountains and then automatically return to normal at sea level. Airing down even 5 psi could conceivably reduce efficiency by that much. Even adding winter snow chains and other cold weather supplies as cargo or taking an occassional passenger more often can reduce my mileage efficiency by 10%.

Try increasing to 42 psi if your tires and configuration allow for it. I would also drive stripped down of cargo for one refueling cycle just for a scientific baseline and piece of mind. This might help recoup 1 or 2 mpg's until spring or until you find another root cause for lagging performance.
chains, booster pack, survival gear, ammo, air comp, medical supplies, and recovery gear are in my rig year round. I have about 300 lbs added.

Did 400 miles since original post, brought psi up to 40 since it was mostly highway... yet me fuel economy dropped even more. tires are rated up to 80 psi, I think I have something else going on. I will post more below shortly.
 

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Discussion Starter #12
Maybe consider switching the thermostat as it's a mechanical spring/disc that lets coolant through. Switch back to a Toyota filter. K&N won't help and more than likely to cause gunk down the line. Minimize your idle times. Not sure how long it takes to get to 120* water temp (2 minutes?), but if it's freezing temp you can do a longer start up procedure, but on regular days, shorter idle times have helped (20-30 seconds) and take it easy until your gauges show your up to temp. Bad spark plug gaps might not effect mileage on a whole, but may help the performance in the end. Give it a go! Lastly, make sure to grease any thing in the drive train that needs it. I get about 16mpg year round and 14mpg with A/C on and I'm on heavy wheels and tires. 14 to 11mpg is definitely out of place.
takes about 3 to 5 minutes for 120 from bone cold (Seattle Washington). I run the K&N because its easy to clean, factory bumper cover is gone which exposes the airbox inlet and it collects dust like a vacuum cleaner. I normally clean the filter every oil change along with hitting zerk ports. I will post some more info below shorly.
 

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Discussion Starter #13
This has been covered here many times in the past, but the #1 cause of poor fuel economy in vehicles with fuel - feedback mixture control systems is fuel-air sensors and oxygen sensors that have reached the end of their useful lives, which is typically around 85k to 120k Miles.

The fuel-air sensors and the mass airflow sensors work together to maintain a 14.7:1 fuel/air ratio. A contaminated MAF will not provide accurate data on the mass of air entering the engine, and worn-out fuel-air sensors will not provide accurate data on the volume of fuel that needs to be injected to achieve the magic 14.7:1 ratio.

1. Make absolutely sure that you have cleaned the tiny, well-concealed MAF sensor elements, and not just the match-head looking intake air temp sensor.

2. Replace both fuel-air sensors, using only OEM Densi parts.

Then, meticulously record your actual fuel consumption over at least four complete tank fills in miles-per-gallon, and not just some nebulous “miles per tank” number.

Post your results here.
o2s ordered, coils ordered, and new plugs ordered. Maf cleaned every oil change or maybe every other (k&n oil can cause issues).
 

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Discussion Starter #14 (Edited)
So sorry to reply individually to everyone vs one large post.

edit here again... removing this since I clearly dont know what I am doing.

Anyway here is an update.

Did about 400 miles since Friday with increased tire pressure to 40. 95% of the drive was freeway and I ended up dropping to mid 10's. truck has very smooth idle around 600.... but think its time to do a compression check, leak down check, injector leak test, fuel pressure test, and probably need to pull some vacuum on emission and evap systems.

I ordered new coils, o2s, plugs... This is going to suck for a while...
 

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That is concerning. Mine has an efficiency sweetspot between 40 and 60 mph. Going any slower or faster than that results in decreasing efficiency regardless of cargo weight or tire pressure. I look forward to seeing how this gets resolved for you because I plan to keep mine long term.
 

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That is concerning. Mine has an efficiency sweetspot between 40 and 60 mph. Going any slower or faster than that results in decreasing efficiency regardless of cargo weight or tire pressure. I look forward to seeing how this gets resolved for you because I plan to keep mine long term.
I will post back results when I get some tests done on my rig... with snowboarding season here I need to have this resolved. I will easily do 600 miles a week.
 

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If you are seeing anywhere near 10-15 MPG, you are either driving like a maniac, have a slight fuel leak (or someone siphoning a few gallons of fuel every week), have severely dragging brakes (binding caliper pistons), or there is truly something wrong in the engine or fuel system.

(This is assuming that you aren’t running 37” tires, or carrying 500 lbs of armor, etc.)

If you don’t already have one, an inexpensive ($24) Bluetooth OBD-II interface running the Torque Pro app ($5) on any Android device (phone or tablet) will allow you to continuously monitor almost all the system parameters that affect fuel economy: coolant temperature, commanded and actual air-fuel ratios, ignition timing advance, fuel trim values, mass airflow rate, instantaneous and average fuel consumption rates, etc., etc.

Being able to watch all these parameters real-time, under various driving conditions, can be a huge help in diagnosing problems before you start blindly replacing parts (like ignition coils).
 

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I know you may think you've ruled it out, but I'd still consider changing the thermostat. My 2010 was getting 10.5 to the gallon when I bought it. I was desperate to increase my mileage. I read here that the a faulty FJ thermostat is a main cause for bad mileage and it appears others feel this way as well.

Stock thermostats are super cheap and take literally 10 minutes to change on an FJ. I swapped mine and within 2 tanks of fuel my FJ jumped to almost 16 miles per gallon and is holding strong! The way it was explained to me was: there is something about the FJ warm-up process where a seemingly good, but actually bad thermostat tricks the FJ's computer into thinking the engine is still cold even when the gauges say it's warm. This causes the computer to make it use more fuel than needed to get to operating temperature.

Sometimes when all the logic fails, part-swapping things you feel simply couldn't be the cause actually fixes the problem. Swapping the thermostat is worth a try considering how simple and cheap it is to do it.

Good luck!
 

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I know you may think you've ruled it out, but I'd still consider changing the thermostat. My 2010 was getting 10.5 to the gallon when I bought it. I was desperate to increase my mileage. I read here that the a faulty FJ thermostat is a main cause for bad mileage and it appears others feel this way as well.

Stock thermostats are super cheap and take literally 10 minutes to change on an FJ. I swapped mine and within 2 tanks of fuel my FJ jumped to almost 16 miles per gallon and is holding strong! The way it was explained to me was: there is something about the FJ warm-up process where a seemingly good, but actually bad thermostat tricks the FJ's computer into thinking the engine is still cold even when the gauges say it's warm. This causes the computer to make it use more fuel than needed to get to operating temperature.

Sometimes when all the logic fails, part-swapping things you feel simply couldn't be the cause actually fixes the problem. Swapping the thermostat is worth a try considering how simple and cheap it is to do it.

Good luck!
A few thing to remember regarding the "bad thermostat" myth:
1. The typical failure mode for a wax-expansion thermostat is to fail "closed". This will result in an overheating condition, not a "running too cold" condition.

2. About the only way a wax-expansion thermostat can fail "open" is if a piece of hard material circulating with the coolant gets trapped in the thermostat disk when open, and thereby prevents it from closing when the cooling system cools after engine shutdown. This can happen (very rarely) if a plastic water pump impeller starts to disintegrate. The OEM water pump impeller in FJs is stainless steel, and thereby can't disintegrate and lock the thermostat open.

3. The OP clearly stated that his coolant temperature was being regulated within the expected temperature range, and that his coolant warm-up time from a cold start was rapid. This is solid proof that his thermostat is operating correctly.

4. As long as the coolant temperature and intake air temperature sensors are providing accurate temperature data to the ECU, there is no way for the thermostat to somehow "trick" the ECU into creating a richer than desired air/fuel mixture, regardless of what some self-styled expert may have told you.

5. The "bad thermostat" myth seems to have originated from the crude emissions-control strategies of the '60's and '70's, when hotter than typical thermostats were used by the car makers to improve fuel vaporization and reduce hydrocarbon emissions. Shade-tree hot-rodders were certain that REMOVING the thermostats would make their cars go faster, but in reality caused the engines to run so cold that they sludged up their crankcases and never properly vaporized the fuel being dribbled in by unsophisticated domestic carburetors.

6. When replacement of the thermostat in your poor-running FJ supposedly resulted in a 35% decrease in fuel consumption, did you perform a water-bath/thermometer test of the "bad" thermostat to confirm that it was truly bad?
 

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Hi FJtest,

I did not do any further testing of the thermostat. I had very little time to run tests and just took the advice of some others in an effort to eliminate the easiest things first before digging in deeper. I recall two things I noticed that gave me hope that the thermostat might be my problem. One was that the warm-up time for my FJ seemed to take longer than I would have expected before getting warm air out of the interior vents. It was over a year ago that I dealt with this but if I remember correctly, the temp gauge climbed slowly and it took about 7-8 minutes to get decent warm air from the vents when running at a average of 35-50 mph on the back roads here during the 30 degree winter days.

The other thing I recall seeing was the temperature gauge would rise and fall quite a bit when fully warmed up. It never got terribly high or low, but it fluctuated enough that it was noticeable while driving.

I'm clearly no expert on the subject. I was just sharing my personal experience in hopes it gave him something to try. I did nothing else but replace my thermostat and my problem disappeared completely. Maybe I was just lucky, but it worked none the less.

Happy Holidays everyone and good luck fueledpassion109.
 
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