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My initial guess on the MPG issue is with the exhaust system. I had a similar issue with a different vehicle years ago. At 137k miles is the FJ running with the original catalytic converter, muffler, and exhaust? This can also help explain the sluggish throttle or acceleration. I've done some research in this area and the best options I was able to come up with are either the TRD exhaust system or the Borla dual exhaust upgrade.
 

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OTE=Sasman;9168648]My initial guess on the MPG issue is with the exhaust system. I had a similar issue with a different vehicle years ago. At 137k miles is the FJ running with the original catalytic converter, muffler, and exhaust? This can also help explain the sluggish throttle or acceleration. I've done some research in this area and the best options I was able to come up with are either the TRD exhaust system or the Borla dual exhaust upgrade.[/QUOTE]

This is the first time I've heard this theory ... what (specifically) could degrade in the exhaust system that would cause extremely poor fuel economy?

A restricted catalytic converter (or a severely crushed crossover pipe) could certainly affect full-throttle power, but that would also affect intake airflow, which would be sensed by the MAF, and fuel delivery would be reduced correspondingly.

Please explain your theory.
 

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OTE=Sasman;9168648]My initial guess on the MPG issue is with the exhaust system. I had a similar issue with a different vehicle years ago. At 137k miles is the FJ running with the original catalytic converter, muffler, and exhaust? This can also help explain the sluggish throttle or acceleration. I've done some research in this area and the best options I was able to come up with are either the TRD exhaust system or the Borla dual exhaust upgrade.
This is the first time I've heard this theory ... what (specifically) could degrade in the exhaust system that would cause extremely poor fuel economy?

A restricted catalytic converter (or a severely crushed crossover pipe) could certainly affect full-throttle power, but that would also affect intake airflow, which would be sensed by the MAF, and fuel delivery would be reduced correspondingly.

Please explain your theory.[/QUOTE]

The computer can only compensate so much. Exhaust restriction can change the volumetric efficiency (VE) of the engine. The ECU has a map of the engine's VE, and it primarily makes the fueling choices based on the VE. The ECU can only make minor corrections based on the O2 feedback.

When "tuning" an efi system, getting the VE table correct is the most important step. The "self learning" EFI systems (such as FAST EZ 2.0, Holley Sniper, FITech, etc.) have an algorithm that allows their VE tables to change, allowing it to "learn" the VE of the engine at various loads vs. RPM points. OEM ECU's do not have this capability (emissions concerns).

I worked at a GM dealer in the early/mid 1980's, when catalytic converters were still "pellet" designs. When they clogged up (fairly often) the engine ran like crap everywhere, not just at WOT.
 

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In the Toyota fuel injection system, the primary determinant of fuel volume is purely the mass of air entering the engine, as measured by the Mass Airflow Sensor.

The MAF sensor doesn't know if the volume of air is entering the engine is being limited by the throttle butterfly position, a restricted air cleaner, a crushed exhaust pipe, or a restrictive catalytic converter.

The fuel injection system attempts to maintain the 14.7:1 air/fuel ratio regardless of air flowrate, and has a very wide dynamic range, from a few CFM at idle at 600 RPM to many hundreds of CFM at WOT at 5,800 RPM.

So again, I don't see any way for the system to lose control and allow an excessively rich mixture because of reduced volumetric efficiency caused by excessive exhaust backpressure. Because of the reduced total airflow, the engine should "act" like a smaller displacement engine, processing less air and thereby burning less fuel.
 

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Edit for sounding confrontational - I know better than to post when tired :) I'll re-address at a later time.
 

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Perhaps you are confusing the operation of Toyota's fuel feedback system with the strategy used by other manufacturer's?

The "speed-density" system used by Chrysler and GM utilizes inputs from the throttle position sensor, the manifold pressure sensor, the coolant temperature sensor, and engine RPM to estimate airflow through the engine, and derive the fueling requirements from LUTs (look-up tables) stored in the engine ECU.

The Toyota system (once it is operating in closed-loop mode) directly measures airflow through the engine, and derives fueling requirements from the mass airflow sensor and the air-fuel sensor(s), with additional fine adjustments from the coolant temperature sensor, intake air temperature sensor, transmission gear, throttle position rate-of-change, etc.
 

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Do these bad filters throw out any codes that I can check?
 

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My FJ dropped from the 15-17mpg range down to 12-13. Finally I figured out it was a seized aftermarket brake caliper. After replacing them with Toyota parts, and the 6 flex lines for good measure, my mpg is right back up.

YMMV.
 

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I've had my 2007 2WD from new to now. It has 77K on it. I have never changed a part except the dealer doing pads and a new battery. The spark plugs are still original. It passes smog every time. I change the filters except the oil and I've always used Mobil 1 syn. The mileage is somewhere around 16-17 and stays the same. I didn't buy it for gas mileage, I bought it because it is neat looking and has the most head room of any car I've ever seen. It drives like an Impala with Power steering, air and that nice poor man's stick hydro.

These new cars are so complicated and tender, sometimes you can do harm when you think you are doing right. Beyond the smog sensors, I'd just drive it and use it. No offense intended, but you sound like you are on the excitement plan over this.
 

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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.
Listen to what fjtest says.
I changed my o2 sensors and maf sensors at 100k. No Codes, but I could smell my fj running rich. Yes keeping sensors current does make a difference.
 
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