98 accord coupe V6 sudden excessive gas use

THis car bought new began getting only 10-12 mpg about six months ago.
Dealer did major tune up. No improvement. Dealer hooked up computer
which shows perfect air fuel mixture. Sent the readout to Honda
engineers in California who had no suggestion. No evidence of leaks,
does not smoke, runs without skipping, seems to have same power as
ever. Dealer gave up, won't do more. Independent shop put in a new
oxygen sensor, no help. Any help?

 

some obvious stuff - make sure the brakes are not binding.

other than that, replace the 10-year old thermostat - ensures it's not too
open and therefore the engine too cool. that confuses the computer into
thinking you're in warm-up and thus excess gas mode. you may want to
check the tw sensor too. hook up a computer reader that gives you live tw
readings, then compare that to an i.r. reading of the head temperature.
sometimes they "drift" and read cooler than actual.

 

Hi,
If in warm up mode, idle rpm will be higher than normal.

 

only if the engine is idling! while driving, the symptom for automatics
is non-switching of the lock-up clutch. i can tell you from personal
experience, the idle can be fine, but the computer refuse to signal
lockup. you don't have that indicator on a stick. hence you need to
check the sensor by comparing what the computer thinks it's seeing and
what temp the head really is.

 

Did the dealer use the proper HDS device? HDS gives them extensive
information on all engine parameters. If they can't figure this out, you
need a more diligent dealer.

If the air/fuel mix was "perfect", then you wouldn't be having mileage
problems. Not like 10-12mpg, anyway. Were there no error codes stored at
all?