so here is what i am dealing with. i have a 93 accord se that has been converted from an auto to a 5 speed. even before the conversion the fuel mileage slowly went into the toilet. i replaced everything in the ignition system, plugs, wires, distributor, cap, rotor, o2 sensor, and coil. come to find out i wasnt getting o2 signal to the pcm. repaired the signal wire. the fuel mileage went from 17 mpg to 21 mpg. still doesnt seem like its where it should be. when i got the car it was getting 28 to 31 mpg. anyone have any ideas?

 

your car should be around 30 mpg, have you check the egr passages?

 

When you changed the plugs, how did they look? A little rich maybe? Did you change the air filter? Did you check the other usual stuff like proper air in the tires, dragging brakes, etc.? If you haven't checked the fuel pressure, it might be worth your effort. Or at least pull the vacuum line on the fuel pressure regulator and see if there's fuel in it. If so, your regulator is gone. If the engine was running excessively rich, your cat could be plugged. How's the overall performance?

The cause could be anything or a combination of things, some of which are not listed above. A little more information would help. How long ago did mileage start going down? With my older cars (15 years or older), I've lost 10-15% in mileage due to alcohol in the fuel. Alcohol in fuel became permanent in late 2008, if I remember correctly. I noticed a step-change (reduction) in mileage then.

 

i havent checked the egr ports yet. but i also thought that the mileage should be around 30. when i changed the plugs they didnt look like it was rich. my air fuel gauge indicates lean alot of the time. it was lean all the time before i fixed the o2 wire. i have checked the fuel pressure all is within specs with the vac line plugged in and unpluged from the regulator. the mileage started to decline about two or three years ago. the tire pressure is good, air filter is clean and brakes are new( less than 1000 miles).