einzelherz

I went on a ~1100 road trip for Thanksgiving and averaged about 38mpg the whole way which is pretty normal for highway driving in my 97 manual LX. Since I've been back though my city driving, which used to be no worse than about 34-36 has dropped to 28-30.

It has been colder though so I'm sure a lot of it has to do with warmup fuel usage. I replaced the fuel filter before I left. Wires are a bit old. Can anyone give me any other possibilities as to what could explain the sudden drop?

Thanks.

B_Swapped93

cap n rotor being worn out causing it to misfire every now and then, had that happen on my 99dx was getting like 300 KM's to a tank when I could normally do 500ish. worth a look they're cheap to replace.

TomCat39

Also in general gas mileage goes down when it gets cold cold out. Colder air is denser so more oxygen, so at the same throttle positions it pumps more fuel to keep the magic 14.7:1 air fuel mixture with the denser air.

Typically you get more power in winter but reduced fuel economy.

Couple that with the longer warm up times, while the engine is cold it pumps more fuel to emulate choke mode of the old carb systems. So since it takes much longer to warm up it takes much longer for fuel consumption to reduce to normal levels.

Another thing that can adversely affect MPG is O2 sensors. If spring comes and your mileage doesn't go back up to normal, I'd suggest a full tune up and new O2 sensors. You could try the tune up first and monitor, then if it still down, do the O2 sensors.

CX-Adam

A minor correction, cars don't operate at stoichmetric air/fuel ratio because not all the fuel is consumed during combustion. Ratios are typically less than 14.7:1. However, your overall statement is still correct.

Did anyone mention drop in tire pressure?

PV=nRT, bro. Volume (V), number of moles (n), and the gas constant (R) remain unchanged. Therefore if T decreases, P does as well.

TomCat39

Yeah I'm sure it's not perfectly at 14.7:1 but the programming is aiming for that ratio, what actually burns is another ball of wax and I assume would be factored into the fual maps so as to get closer to that magic number. I highly doubt the engineers wouldn't take into consideration the inefficiency of the motor itself when building their fual maps for the ECU and motor.

None the less the ecu is going to compensate for the richer oxygen and try and get as close to that air fuel mixture it can.

Not even sure why I replied, sort of a moot point. What's done is done though.

Ciao