Mysterious Loss of MPGs?

Car in question is a 2009 Hyundai Sonata GLS with 220,000 miles.  That's the 2.4 liter inline-four with a five speed automatic.  It's my wife's commuter and she used to get in the low 30s and up to 35 on the highway.  But the gas mileage has dropped to 25 MPG in mixed driving [even with me driving].  I've maintained it for the last 180,000 miles.  At 185,000 I replaced the plugs and fuel filter as well as air filter and changed the transmission fluid.  It runs fine and makes normal power and, although the tranny shifts a little lazy sometimes, the torque converter locks up as it should.  There is no CEL, no codes and no pending codes.  It doesn't use any oil.  All the wheels rotate freely.  It is rated at 22 MPG city and 32 highway.  Tires are fine.  Everything seems fine, expecially for a car with so many miles.

What should I check?  I've cleaned the MAF sensor but it's been a year or two.  I appreciate the hive-mind thoughts!

It could be a few thousand things, honestly, but try the basics first.

1. Do you have an exhaust leak upstream of the O2 sensors?  The air that gets sucked in might be fooling the sensors into telling the ECM to inject more fuel.  
2. Inspect the intake boot between the MAF and TB.  At that mileage, the rubber often turns to dust and gets cracks in the base of the bellows
3. A small vacuum leak around the intake, vacuum hoses, etc can do the same thing
4. Have you put a scanner on it?  STFT and LTFT will tell you more about the above three
5. Compression test.  At 220k you should still have good compression, but if it's more than 10-15% below factory spec, you might just have wheezy compression that isn't extracting the same BTUs from the fuel as it used to.  The result is progressively more right foot for the same driving style/acceleration.
6. Have you recently changed tires?  It's unlikely that tires would cause a huge change like that, but I've lost/gained significant MPG with tire changes.  I had an F250 powerstroke that I bought with brodozer mud tires on it.  I got 17 on the highway.  After switching to 215/85-16 H/Ts I got a pretty consitent 20+ mpg.  That's a 20% change.  It's not likely if you just switched brands or went one size wider, but it can really change if you went from Michelin X-one LRR to [insert the cheapest imported brand Amazon tire] 

Other things that aren't quite as easy to test might include the catalyst.  The downstream O2 sensor(s) compare data with the upstream O2 sensor(s) to make sure the catalyst is doing its job.  It's possible for it to be partially clogged and still doing it's job, meaning that it isn't tripping a CEL, but might be restricting things.

Aero? Got a loose undertray or something flapping around?

Thanks for your thoughts.  I just changed the oil and the belly pan seems secure.  I can tug on the rear bumper but it's never been disturbed so I'm guessing it's ok.

Curtis, I will poke around under the hood and see if I can check your suggestions.

Anyone else have anything to suggest?

Searching for vacuum leaks can be made much easier with a can of starting fluid, brake cleaner, or an unlit propane torch directed at suspect areas.  Any change in idle (up or down depending on which you use) can help pinpoint vacuum leaks.

Thermostat not closing all the way?  

Check your temperature sensors, coolant and intake air.

If they're reading incorrectly, the pcm will think the engine is cold and add more fuel.

The sensor for the gage will probably not be the same sensor that reports to the pcm.

Could one of the brakes be dragging? It's not always noticeable from the drivers seat. You could check by driving a while then checking rotor temps. Front to back will probably be different but left to right should be roughly the same.

gearheadmb said:

Could one of the brakes be dragging? It's not always noticeable from the drivers seat. You could check by driving a while then checking rotor temps. Front to back will probably be different but left to right should be roughly the same.

Came here to say this. at 220,000 miles I am betting your calipers are rusted and pitted and the rubber lines falling apart internally. When was the last time you flushed your brake fluid and replaced the rubber lines?

Curtis, I plugged in my scanner and it tells me that the STFT ranges back and forth from -15% to 12%.  LTFT is pretty steady at 0.8% to 1.6%.  Is that reasonable or does it indicate a problem?  The MAF plugs into the intake; I don't see any indication of a leak there.  Tires are Goodyear Assurance FuelMax and I monitor pressures so I don't think they're the culprit.

I did check and there's nothing flapping or blowing around.

I had a seized LR caliper so I replaced it and did a full bleed without any issue moving fluid.

This car lived half its' life in Arizona so it's not as rusty or deteriorated as the age and mileage might suggest.

I just got an infrared scanner so I'm also going to check some temperatures and compare it with the scanner data.

Thank you again for your expertise!

The Fuel trims are fine if LTs are close to zero.

Ive seen a bad alignment greatly increase fuel consumption.

If the wheels spin freely I wouldnt be  concerned about brakes, if you want to be sure, hit the brakes and release them and make sure they immediately spin freely again.

Converter lock up could be the issue, seen a few on other cars kinda fly under the radar without a code, but not commonly.

My initial concern being a 2.4 hyundai is oil consumption. Happens all of the sudden and is severe, condemned probably 60 of them

Have the injectors ever been pulled and professionally cleaned? If they are down to a dribble, fuel economy will suffer.

I replace the injectors in my XJ and it made a noticeable difference in fuel economy and power. 

Another thing to consider is the CAT may be done. They have a recommended lifespan of 70k-100K and you are well past that. 

Was this engine in the group of Hyundai engines that went bad whenever they wanted?

In reply to Toyman! :

injectors and cat were my idea as well. When I replaced the cat on the Rio at 118k the center was melted in an unnatural way. Injectors were running a little lean and long term highway/track sustained rpms were enough to make it go bad. But at 200+k miles theres a lot of little things that it could be.

Hell, it could be chain stretch to the cams. 

yupididit said:

Was this engine in the group of Hyundai engines that went bad whenever they wanted?

 

that was the GDI engines in the 2011-up

ShawnG said:

Check your temperature sensors, coolant and intake air.

If they're reading incorrectly, the pcm will think the engine is cold and add more fuel.

The sensor for the gage will probably not be the same sensor that reports to the pcm.

Usually this will throw a P0128, though.

Cold start enrichment usually lasts just long enough for the O2s to warm up and then they run closed loop.  They want that cat working NOW from a cold start!  Usually within 10-15 seconds.

It isn't like the old days of engine control, but a hot engine is still an efficient engine.

bobzilla said:

yupididit said:

Was this engine in the group of Hyundai engines that went bad whenever they wanted?

 

that was the GDI engines in the 2011-up

I got one of those this afternoon.  Engine seized on the Interstate.  When it got to me, it had cooled down enough that the starter could lug the engine over, but after it fired it was sending Morse code signals and the only letter it knows is E.

Kia is replacing the engine under warranty.

My new $20 Kizen infrared pyrometer tells me that the LR hub is 13 degrees hotter than the RR and the rotor is 29 degrees hotter.  The LR corner is where I had to replace the bearing and the leaking brake caliper.  That wheel freewheels with a little more friction, too, and judging by the temperatures it's proabably enough to make a difference.  I'll have to take it back apart and see what I can find.  I don't really look forward to exploratory surgery.

Just for thoroughness, the ambient air measured 80 versus the OBD showing 75, intake at the MAF measured 130 versus 131 OBD.  OBD Cat temps were 964 and 756.  Radiator end cap measured 160, upper hose 177 and lower hose 130 and the OBD said the ETC was 194; I was guessing that was coolant temp which would, I theorize, indicate the thermostat is working properly.  I couldn't get a good reading on the thermostat cap but just for fun the side of the cylinder head was 290.

Any other thoughts are much appreciated!