I have a B16A with no mods except a short intake and cat-back exhaust.... I'm getting about 20 miles to the gallon and my car doesn't even run different, it stills pulls like it did when I was getting 30+ mpg. The idle is fine, it really doesn't drive any differently which is throwing me off big time. I'm new to the site did some research but can't really put 2 and 2 together. I'm in my first few weeks of automotive school and was wondering if anyone could give me their input on how they would go about diagnosing a problem like this. I'm comfortable taking stuff apart and putting it back together but I'm new to the whole diagnosing aspect of vehicles. I appreciate any pointers.

 

Is it running rich?

 

Besides engine, check for brakes dragging and tire pressure. Both of these can affect MPG by a considerable amount.

On a mostly-stock setup, the two things that affect F.E. the most are the primary oxygen sensor and catalytic converter.

Do you have any check engine lights?

 

As the above post, does it smell like it is running rich? Have you looked at simple things like the fuel filter, injectors and such yet?

 

it's always run a little rich up top but i haven't checked anything yet, like i said before i don't even know where to start but the fuel filter sounds like a good start since i can't recall it being replaced... The CEL isn't on, tire pressure is roughly 34psi. The cat has about 10000 on it and I'm not sure about the O2 sensor. The worst part about my car is scan tools can't usually communicate with my computer anyway since its different software... Thanks for the help, I appreciate it. I'm waiting for my car to get shipped out here so i can start figuring out the problem.

 

If you don't have any CELs and you're not dragging a caliper, then start by checking the plugs.

 

where are you located ? , If the temperature dipped alot, you will notice a fuel comsumption difference.

Northern states, or just maybe the temperature dropped alot.

 

first, it sounds like it hasn't had a tune up in a while. When diagnosing a fuel economy problem the first thing to do is a full tune up unless its been done recently. change:
plugs (ngk bkr6e or 7e), no bosch or platinum ****
wires (ngk or oem, don't cheap out, but you dont need more than oem size)
rotor+cap rotor
fuel filter
pcv valve
air filter (if you have a cheap paper one, they last 4 months, if you have a K&N, put it in the dishwasher)
Clean your o2 sensor

I'm sure if you are running rich you don't have the power you should have.
Should cost you about 150 with quality parts. It cost me 180 with oil and filter.

 

Don't forget in some places they use winter blend fuel with anti-freeze additives.