bad gas mileage

[bluelude01]

hi everyone i just recently bought my 2001 manual prelude, and i seem to be getting not very good gas mileage.when i first bought the car the engine check light was on but a week later it turned of itself and has not come back on since.the guy i bought it from said it was the 02 sensor but now im not sure. I have minor mods exhaust, intake.anyone got any ideas? also will 18inch tenzo wheels, tire size 215/40/18 rub when you turn with a prelude.

[plum]

i had a bad gas mileage problem too but it went away im not sure what i replaced but one of the things i did replace was the o2 sensor and also my spark plugs and wires. So between those things i think its was most likely the o2 sensor that did the trick, however my check engine light was on for the o2 sensor up until i put a new one in, never went off like yours so i dunno.

[del]

the exhaust will always set off a CEL. have you reset the ecu since? try that. you may have a bad or dirty O2 sensor, try removing them and cleaning them up. that sometimes works.

[rmckim]

Originally Posted by bluelude01

hi everyone i just recently bought my 2001 manual prelude, and i seem to be getting not very good gas mileage.when i first bought the car the engine check light was on but a week later it turned of itself and has not come back on since.the guy i bought it from said it was the 02 sensor but now im not sure. I have minor mods exhaust, intake.anyone got any ideas? also will 18inch tenzo wheels, tire size 215/40/18 rub when you turn with a prelude.

i never experienced any mileage increases from any new O2 sensor in two ludes and a teg... i once disconnected the battery in my teg for an hour or so to reset the ECU because of bad mileage, but it didn't seem to help...

then a few weeks later i was changing batteries and left it out overnight before i installed the new one...

several days later the mileage started to increase and seemed to take a few weeks to climb back up to the hi 20s again from the mid teens... but it did.

don't know why the overnight removal had more effect than the hour removal - maybe it was the original hour removal and it just took longer than i thought it should have.

anyway, im back to the high 20s again. maybe ECUs "relearn" after a true reset and we don't know all the facts about them? that doesn't seem likely, but i can't explain it any other way.

[pheurton-skeurto]

ecu's are funny like that...they make you want to: laugh, cry, dance, sing.

[Airborn82]

If it's like almost all the other componenets in Honda's then there is a memory for a few hours in the event that the battery is disconnected. From what I understand, when the ECU is truly reset it will use a new map according to how much air flow's available, temperature, atmospheric pressure, etc... and reset your fuel map accordingly. This is probably the explanation I believe.
So I guess moral of the story could be this...if you modify the air/fuel system it's a good idea to reset your ECU, since it will undoubtedly change your fuel needs...this will give you the best setting for gas mileage.
Just one more thing...I'm unsure how far the ECU can go as far as "air availability" if you know what I mean? For example you go from the restrictive air filter and exhaust to a CAI with a 3" exhaust. Will the factory ECU have the best fuel map for your air availability and exhaust takeaway? I don't think so. Please excuse the lack of terminology here...