I have a 2004 XG350 with the V6 engine. The car has about 60k miles, has recently seen a significant drop in fuel mileage. Previously I was getting on average 24 mpg on highway. But over the past 9 months it has dropped to 16 mpg highway. It's been to the dealership and they've re-flashed the computer 3 times but with no improvement.

Theories or suggestions?

Thanks

Desiree AKA Crapmileage

 

All I have are questions. Was the change in mileage abrupt or a gradual decline over time? Has the check engine light come on during this 9 month period? Has the timing belt on the engine been replaced during this period? Have you switched gasoline brands or sources? Any other symptoms, engine noise, hard starting? I'm sure your mechanic has asked the same thing, but a drop in mileage with no other overt symptoms is a tough one to figure out. It may be the engine simply needs a good tune up.

 

Ken,

Yes, I imagine you would have questions. Let's see, the mileage took a drastic down turn after the first flashing. It dropped from appox 320 miles per tank to 251 miles per tank. I took it in a second time, they re-flashed no difference. The third time I took it in I asked them to reset to original settings. No difference in fuel mileage. No egine light, no timing belt change. I did have new barings put on and put on a new set of really good tires though. The mechanic at Hyundai told me he can't figure it out, but the computer isn't adjusting to my driving and won't keep the settings. Could this possibly be something to do with that? If I'd never had it flashed, I would be fine.

Thanks

 

May I ask why the computer was flashed to begin with?

 

Sounds like the mechanic owes you a new computer.

 

Ken,

Well, it's silly really. I moved from MA to SD and noticed a 30 mile drop in fuel mileage after I relocated. I took it in for a flash in June of 08, the first time. After that the mileage dropped like a brick. With a loss of 80 or so miles per tank I figured there was a problem. So I took it back a second time, had them download the lastest codes and re-flash. No change. MPG was still crap. So in a last ditch effort, I took it in a 3rd time, asked them to reset everything to their original settings and re-flash one last time. No change. Mileage still crap. I just got off the phone with the guy at the shop who told me that a drop of approx 15-20% for winter mileage is the norm. He estimates miles per tank in winter at about 260. Funny, I've had the car for 3 years and never got 260 per tank even in the coldest winters on the East Coast. Why would the mid west be any different? They are blaming it on maintance issues. Funny again because I've had the car for it's services and change oil every 3000 miles. I know there is something wrong, I can feel it, I just know it but the service dept doesn't want to deal with it. Maybe it's time to trade it. too bad because I love my car.

 

Part of the problem is altitude, the other is the extreme cold of SD. It could be there is nothing wrong at all. Wait until spring and see what happens.

 

Altitude can impact mileage and western South Dakota certainly does have some elevated terrain. Eastern South Dakota is higher than MA, but most of that portion of the state is still below 1,000 feet. The initial drop of 30 miles to the tank (before the first flash job) you experienced is most likely summed up in one word, ethanol. With it's corn acreage, particularly in the eastern half of the state, South Dakota has become a major producer of the stuff. The Federal Renewable Fuels Act required the production of almost 9 billion gallons in 2008, calls for over 10 billion gallons in 2009, somewhere around 12 billion in 2010. All that ethanol has to be used up somewhere and in states that have plentiful supplies, it use is strongly encouraged. I doubt you can buy any gasoline in SD that doesn't have some ethanol in it. Federal law only requires blenders and retailers to report ethanol levels of 10 percent (e.g. E10) or more. So even if you have been buying regular or premium to avoid ethanol, I would bet dollars to donut holes those blends currently contain between 5 and 10 percent of the stuff.

It appears the "flash jobs" are related to most of your mileage drop. I think asking your mechanic to flash the EPROM with newer versions is not helping and may have been the wrong approach. Newer may not have been better for your 2004 model car. If anything, I thing the computer should be reflash to the original specification, if your mechanic has hopefully documented which version that was.

 

Ken,

Funny you should mention the flash to the original settings. I have asked them to do that but they don't have any of the codes amymore. Nice huh?

Any thoughts on the throttle body assemby? Could that be the cause? Also, bare in mind that this problem started in June after the first flash. So the mileage shouldn't have been afftected by the weather until winter.

 

You might consider taking the car to a Kia dealership. Hyundai discontinued the XG350 after the 2005 model year, but Kia has continued with the Amanti which is an XG350 clone. I also think Kia has utilized the 3.5 liter Delta V6 in more models than Hyundai (which largely switched to a different V6 design when they discontinued the XG). Kia may have more experience in dealing with this issue.

Well...yes and no. I think the Electronic Throttle System on this engine is a "fly by wire" setup, which it under computer control and with reprograming of the computer may be responding differently but so other systems like injectors and timing. If it was a persistent malfunction of the throttle, the check engine light would be lit.

Sorry, but I am not much help. Perhaps some other forum members can offer better advice.