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.