I'm hoping somebody can help me out. Quick history...

Was getting less than normal MPG, by about 4 MPG
Engine light came on, changed plugs and wires, fixed issue
Still getting crappy MPG
Needed new brakes, changed those
Needed new tires, replaced those
Still getting crappy MPG
Decided to change Fuel Filter...its in tank, so no go on that
Oxygen sensor light came on after fill up of gas
Removed front Oxygen sensor, cleaned it
Light went off after a couple tanks of gas
After a couple tanks of gas, finally got normal MPG
On next fill up, back to same of lousy MPG

Ordering new PCV today.
Going to clean second Oxygen sensor this weekend.

Does anybody have any other ideas as to why I'm not getting the normal 26MPG...I currently get 20-21MPG.

Car runs smooth, no hesitation, stalling, nothing.

 

I would not clean o2 sensors replacement is the only way to fix them. Also try using a decarboning system on the upper cyclinder. 3m make a kit. Sea foam is a good chemical and I have used it. Add it to the tank or by using vacuum line to clean upper cycliinders and intake. Just be in a open well vent area. It will smoke but it does clean up. Also make sure your tire pressure is correct and use synthetic fluids. I am getting 27mpg on the freeway and around 24 city driving.

 

Avoid using snake oil crap. Look at obvious problems first.

How many miles do you have?
Did you change the timing belt?
Fuel filter?
Check engine light?... What were the codes? You must scan for codes.
Do a scan and report back here. Go to Autozone for a free scan, but don't let the diagnose the problem or sell you parts. Write down the codes and report back here.

 

Thanks for the responses. Forgot to put mileage...72,000. I have not changed the timing belt nor the fuel filter since it is in the tank. I have checked for codes and there are not any. I've used Lucas fuel treatment about 4 times so far.

 

You need to replace the timing belt. A stretched out belt could account for you MPG problem. Timing belts get replaced at 60K. If it breaks, it will destroy your motor.

Fuel filter is another biggie everybody neglects. It won't generate a code if clogged. HMA service shows an external filter under the car, probably near the tank.

What year is your XG?

 

I'm hoping its not the timing belt...that would suck...don't need that cost right now. Nice nobody tells you to change the timing belt at 60K. I would have done that while under warranty had I known. I called a local shop I do business with...on a 2005 Hyundai XG350, the only fuel filter is in the tank. I already checked under the body to find it as well.

 

Timing belt is not a warranty item, it's a scheduled maintainence item. It's not optional either, neglect it at your own risk. Broken timing belts usually mean a trashed motor and a new car.

If the filter is in the tank, it's not such a big deal to change. You remove the back seat and there is an access panel and pull out the filter from the tank.

Here's good instructions to change the filter in an Elantra. It's probably very similar, if not the same thing:

http://www.elantraxd.com/DIY/fuel.php