I've had my 01 Tib for over a year now. I keep track of every tank fillup with a mileage app. From the beginning I used to get around 29mpg average. Then I put a cheap ebay air intake on it, with the filter down under the battery tray. After that I would get about 31mpg for the next couple of months. It's been slowly dwindling and has been around 27-28mpg average. I added chevron fuel cleaner in the tank (first time I've done this) and now my last fillup was 26mpg (due to the fuel injector cleaner???). I just changed the oil and cleaned the air filter, nothing changed. Spark plugs are only a year old. Tire pressure is all good at 32psi (only 1 year old tires). Car drives just fine, like it always has.



Keep in mind I drive the exact same way on every tank. I only drive this car Mon-Fri to commute 20 miles to work each way. 90% highway driving at 65-70mph. I fill up at the same 2 gas stations every other week.



What should I be checking into to get my mileage back up? I'm thinking a bad O2 sensor or catalytic converter? Or maybe it needs a new fuel filter (where is that located)?

 

The Cold air intake wont actually give you better mileage. Infact, should be the direct opposite. The reason is in the name itself. Cold air is more dense and requires more fuel to burn. You OEM closed loop will compensate. The jump you saw when you first installed it was probably from the ECU not compensating entirely. Now that its been a while, it has compensated. Pull the negative off your bat and allow the ECU to reset, this should help.



In the winter, if you want better mileage, put your old air intake system back in. Its OEM design is to help with warming the air so you get better mileage, especially in the colder months.



Remember, you cant have the best of both worlds when modding the engine combustion. You want more power, well, that requires more fuel. If you want unlock lost power AND get better fuel economy, look into weight savings on the car: lightened flywheel, light weight rims, light weight body panels, etc....

 

dropping from 29-26 isn't the end of the world. Spark plugs, wires, gas, driving habits, terrain, so many variables come into play. You now have extra air coming into the engine that requires the car to adjust fuel to keep the AFR stable. I don't know that I ever saw 26mpg in my Tiburon.





edit: what he said.

 

plus some places switch to a higher percentage of ethanol in the fuel around the end of august

 

I lost a good 5-8mph while my brakes were dragging. Try jacking it up and spinning each wheel

 

Hm, I guess I can pull the CAI off and see if it improves. Just wierd that it's been a year to degrade down to this point. Maybe I'll just try the battery/ECU reset first to see if mileage jumps back up. I'll check the spark plugs to see if they're still good too.



My driving style/terrain hasn't changed at all. The car is literally driven the exact same route at the same speeds/driving style every single day. We take the SUV if we ever go anywhere.



Would a bad catalytic converter or exhaust leak cause a loss in MPG? Just curious cause I know I have an exhaust leak. And it's the original cat's on the car. Thanks.

 

Yes, exhaust leak can cause a loss of mileage. If its close to the first O2 sensor, it could be reading lean. The ECU will compensate and attempt to richen things up.



Depends on where the exhaust leak is. If its way south of the first O2, then maybe not. If its relatively close, then get that fixed as well.

 

I have a leak in two places. First is right where the oem header bolts to the engine block (the known oem cracked header issue). Second is where the header joins with the downpipe...that connection has a slight leak.