thought y'all might be interested to know that you can retrofit a 4-wire
sensor onto an older 1-wire civic without upgrading to the obd1 ecu.
did this recently on my 89, and have been delighted to find my urban
fuel economy has improved significantly - it's jumped from 29-30mpg
crawling around san francisco, up to ~35mpg. no noticeable difference
on freeway driving, but the overall average for mixed use improves.
i have the heater wired into the yellow/black wire that is the same
circuit at the fuel pump supply and the fuel injectors, the sensor
ground and heater ground common-connected to the engine block, and the
sensor output wired onto the old sensor input on the existing harness.
note: this is not california legal for smog. even though it improves
emissions because the ecu goes into closed loop much quicker and stays
closed during prolonged idle, smog checks require the original 1-wire
sensor to be present and wired as per factory. so, keep your old
sensor, use it for smog inspection, then switch back to this 4-wire
solution for daily driving. i used an old 4-wire socket from a junkyard
civic and bullet connectors to make this easy, and so there is no
permanent modification to the stock civic harness.