Help - 14 MPG to 11 MPG. Out of ideas :-/ - Page 3 - Toyota FJ Cruiser Forum
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post #21 of 29 (permalink) Old 12-28-2018, 07:20 AM
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Re: Help - 14 MPG to 11 MPG. Out of ideas :-/

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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post #22 of 29 (permalink) Old 12-28-2018, 10:04 AM
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Re: Help - 14 MPG to 11 MPG. Out of ideas :-/

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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post #23 of 29 (permalink) Old 12-28-2018, 11:47 AM
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Re: Help - 14 MPG to 11 MPG. Out of ideas :-/

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FJtest previously said: View Post
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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post #24 of 29 (permalink) Old 12-28-2018, 06:07 PM
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Re: Help - 14 MPG to 11 MPG. Out of ideas :-/

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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post #25 of 29 (permalink) Old 12-28-2018, 07:46 PM
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Re: Help - 14 MPG to 11 MPG. Out of ideas :-/

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

Last edited by LeeA; 12-29-2018 at 02:05 AM. Reason: not wanting to sound like a jerk
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post #26 of 29 (permalink) Old 12-28-2018, 08:28 PM
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Re: Help - 14 MPG to 11 MPG. Out of ideas :-/

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.

Last edited by FJtest; 12-29-2018 at 07:28 AM.
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post #27 of 29 (permalink) Old 08-18-2019, 11:45 PM
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Re: Help - 14 MPG to 11 MPG. Out of ideas :-/

Do these bad filters throw out any codes that I can check?
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post #28 of 29 (permalink) Old 08-19-2019, 05:28 AM
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Re: Help - 14 MPG to 11 MPG. Out of ideas :-/

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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post #29 of 29 (permalink) Old 08-19-2019, 07:46 AM
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Re: Help - 14 MPG to 11 MPG. Out of ideas :-/

I think I would check all four brake calipers before anything else.
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