Sub-par gas mileage.

PittSpeed Automotive
1

My quest for decent gas mileage in this shitbucket continues. Here is a current list of parts that have been replaced in the last year:

primary and secondary oxygen sensor
pvc valve
exhaust manifold
fuel filter
spark plugs
spark plug wires
air filter

Gumout fuel injector cleaner and Lucas fuel injector cleaner has been ran through the car for the part two fill ups. Only modifications are as follows:

2.5 exhaust
crappy air filter
1.8l ECU
Beta II motor

On the highway, with the air conditioning on, I can barely break 23mpg on all highway driving, and with it off, I can break into the 26mpg. Most of the driving is 50/50 of city and highway, no mashing the throttle, just normal driving. The only thing I can think that can be causing this somehow is the 1.8l ECU which leans the fuel trims out. But that should help mileage. I’m at a lose of what the issue could be. Any thoughts?

2

do you have access to tuning software for that ecu?

3

Is this for the Accent? That is surprisingly low if that is the truth. I can usually get about 31 mpg highway in my Mazda3 hatch which weighs more and has a 2.4 L. Even in the city/burbs I get like 24 or 25 mpg. My first thought would have been bad plugs, wires, or an O2 sensor, but it looks like you have already taken a look at those.

4

clogged cat might be a factor… or real low tire pressure.

more then likely bad cat…

5

ditch the 1.8 ecu and go back to stock ecu. my guess is it is in there for a couple of reasons.

6

Maybe stick a wideband on it, and see what kinds of a/f ratios youre getting with the 1.8 ecu vs the stock ecu?