engine seems to be operating fine. idles without shaking 700 RPM. accelerates fine, starts easily. but the gas mileage is very, very low. even when idling at low RPM the gas is being consumed too fast. No gas leak visible on ground. No gas smell.
maybe it's intermittent. Car was getting 14 mpg in city. I buy five to nine gallons of 87 octane gas and keep track of how many miles I drive until I buy more gas.
One day I had mistakenly removed a fuse (that I assumed was a spare) from an EFI-2 slot. that was followed (maybe just coincidentally) by drop in MPG to 6. I replaced the fuse (when I realized it was not spare) and mileage returned to 14 MPG. But recently mileage dropped to 6 MPG (even when I let the car idle gas is being consumed too fast). Took the car to a Toyota specialist who can't find a problem. there is a P0147 code (lean fuel mixture). Mechanic doesn't believe there is a fuel injector problem or O2 sensor problem. He is only saying it might help to change the spark plugs or clean the throttle body.
Has anyone experienced such a low gas mileage? what was the cause of the low gas mileage?

 

I had that happen to our 2012 Honda Fit, I ignored until it went into limp mode and had to be towed to the mechanic. New spark plugs and new injection tips (not sure if that’s the name in English) and general tune up solved it (including throttle body clean up). I’m inclined to think that it could’ve been avoided with a preventative seafoam treatment.

 

1st where are you reading the mpg? Regardless, try to do it manually where you fill up the tank full and record mileage or zero out trip meter. Empty the tank (means drive until you know it is almost empty) and record the mileage before you fill up. Then calculate mpg. You may need to do this at least 3 times to see consistency of miles per gallon calculation. Good luck.

 

P0147 ? Odd, car has 2 banks, 1 A/F and 1 O2 sensor per bank, but P0147 means:

P0147 is the OBD-II generic code indicating the Engine Control Module (ECM) has tested the O2 sensor heater circuit during startup and detects problems with the bank 1 sensor 3 O2 sensor heater circuit.