Is your car blowing white smoke? Is the coolant a milky-brown color? It might be a head gasket going bad.
if you're in california, the smog laws are tough, but the smog tech.
should have told you what is wrong. you may have a bend in the pipe
which could affect the end result exhaust reading
Off-hand I think of calculating how much antifreeze to use to protect a radiator from freezing down to a given temperature.
There's a certain implied knowledge about combustion reactions when mechanics talk about "rich" and "lean" air to fuel ratios.
There are laboratories that analyze used engine oil to diagnose many things about the engine it came out of. It's not the mechanic that does the analyzing, but the mechanic does use the information supplied by the chemists at the lab.
Sounds like a head gasket, with a leak between two cylinders.
It's 13 years old though, so with a lot of miles, might need a total rebuild.
If you could get a used Japanese engine and swap the short block, it may be worth it.
Hand calculate the MPG. See how much gas is put into the tank and how many miles a full tank goes. Then divide the miles by amount the tank holds.
You could try a fuel system cleaner. If anything mechanically is wrong it's with the fuel system. Is it showing any check engine lights? How many miles on your car? It may need a tune up. You could also clean the MAF sensor using a specialized cleaner. Don't use anything else or it would ruin the sensor.
Did you even fill the tank fully up? Did someone else drive your car? Are you packing more weight than usual? Did you get new tires? Has you driving been different?
Sitting for 3 years with a blown head gasket. You're better off getting another motor.