My accord has suddenly gone from 26 mpg to 17. Does anybody have any ideas as to what could be the problem? the check engine light does not give any codes, and I recently tried changing the fuel filter. I just have no idea what could cause such a drastic change.

 

check engine light lit up and not giving codes? ... thats odd ... well have you already tried doing a routine maintenance ... when is the last time you changed your o2 sensor? also check your tire pressure ...

 

o2 sensor

 

I would guess o2 sensor also, I had a van that went from 17 mpg to single digits almost overnight as the result of a bad o2 sensor.

 

That's gonna take some thought, what with the ECM refusing to co-operate.
EGR system failure?
O2 system failure?
Ignition timing change?
Catalytic converter failure (on an OBD1 vehicle, this can happen and not set a code)

All can contribute, but most would set a code (which you say didn't)
have you noticed any degradation in the performance?

If none of this makes sense, maybe we should take a page from "The Obama"; check your tire pressure. .... but a 9 mpg drop? I doubt it.

P

 

On a previous car (Alero) I had a drop of 28 to 15mpg once and found a faulty thermostat that failed open, as soon as it got cold outside, the engine never warmed up enough to get out of cold start mode...but I'm not sure if the cold-start on the CB7's is triggered by a set temperature in the computer as it was on the car I was driving at the time.

 

Well, after a long time I finally took it to the dealer and they told me everything that was wrong with the car. They said I needed a bunch of stuff including a new ECU. All in all they wanted to charge me 2000 dollars. So I said no and Honda didn't even charge me for the diagnostic. I bought a used ECU on ebay for 50 bucks and installed it and it fixed the gas mileage and the RPM problem.

 

I am having this same issue on my 92 Accord LX. I just put a new low-milage motor in (F22A6 IIRC) and am getting ~22 at best w/ mostly freeway driving. It was 30-32 on the old motor before the head blew. No codes or anything.

1. Is there a way to check if you ever get out of cold-start mode? I could see this affecting it for sure, I've seen it lots on other cars.

2. Can the o2 sensor be bad and not give any flags?

Tire pressure checked and it's awesome all around. Brand new cat + full exhaust. Brand new t-stat (OEM Honda one..) as well. Everything else on the car is fine, purrs like kitten and no hesitation or anything on acceleration. Any more ideas?

I can go make my own thread if need be .