Symptoms:
-Low gas mileage : ~12mpg in town
-Once temp needle raises off the C, engine starts to misfire on under load. But never does it while it is cold, runs just fine cold.
- air/fuel gauge reads lean all the time, but still shows changes but never comes close to stoich

this is a jdm b16a
obd0
no check engine light or codes

What could be wrong here. just let me know if u need any more info, Thanks