Low MPG suddenly

[gkarthikngl]

Hey folks, I purchased a 2012 Genesis 4.6L few months back. I am the second owner and the car has around 90k miles on it right now. Few days ago I have been noticing that my MPG has dropped a lot!! Had an average of 21mpg in city and RN it dropped all the way down to 13-16mpg. I didn't notice the MPG prior to my oil service that was done recently. I had semi-synthetic oil service change done and I am starting to feel worried that it may be the cause of low MPG.

What do you guys think would be the best way to self diagnose the issue with the car other than spending couple hundred dollars just for diagnosis.

Thanks in advance!!

 

[Gunkk]

21mpg is not realistic city mileage for a 4.6. 13-16 is realistic in the city, 25 on the highway.

I average 21 in mixed suburban + highway driving (32 miles/day), averaging 35mph over ~50mins each weekday morning. And that's with a lot of very chill acceleration between stoplights and stopsigns (ECO light on while accelerating) and a lot of time spent at 42-45mph where the car is in 8th gear at 1200rpm on level Florida roads.

I have found that grade and quality of gas can make a 1-2mpg difference, and tires can make a 2-3mpg difference. So use top-tier gas and check your tire pressure now that it's getting colder. Also, start tracking your tank-to-tank mileage (miles driven per fill divided by gallons per fill).

Suggestion from your humble servant, a long time three-time 4.6 owner: Next time, put full synthetic oil in the car, not semi. Semi will form sludge much more readily (especially with short winter trips), and you don't want sludge in your timing system.

 

[mikec]

Where you live can make a difference too - some cities/states require gas retailers to use a different fuel blend in winter. Areas prone to high smog will often have "oxygenated" winter blend gas at the pumps once winter approaches. The gas has oxygen added to it which helps it burn more completely, especially in older tech engines (think carburetors and early fuel injected engines)... but that oxygen displaces some actual gasoline. So "1 gallon of gas" at the pump isn't a true gallon of fuel thanks to the additives. So your car engine has to consume more of that diluted gas to make the same energy/horsepower. Also, many fuel stations sell a mix of gas + ethanol, again for emissions reasons. Ethanol has less energy density than pure gasoline so MPG drops. Using lower octane fuel makes the Tau engine retard ignition timing a bit to prevent pinging/knocking; this reduces overall combustion efficiency leading to a MPG drop.

When you are checking MPG are you depending on the dash MPG display or are you actually calculating it yourself based on miles driven divided by gallons from the gas pump? If you are calculating it yourself, a fuel system leak could easily be the issue. In theory the "check engine" light should illuminate and an OBD-II code reader would find an error code corresponding to a leak in the evaporative emissions control system. My dash MPG indicator tends to read a bit high.

Last thing: with the car on a level surface, engine running, transmission in the normal Drive position, will the car move forward on its own with your foot off the throttle pedal? Or do you have to give it some gas to get it to move? If you have to push the pedal to make it move, a brake is dragging or the parking brake assembly is jammed/stuck and dragging. Whatever brake is dragging will get quite hot after just a few miles of driving. Drive, park the car, and put your had NEAR BUT DO NOT TOUCH the wheel rims. If any seems to be significantly hotter than the others that is a sign of brake system issues. Look closely at the disk brake rotors: if you see any blue streaks in the metal that indicates that brake has been dragging for a while and got too hot - the rotor is ruined and the brake pads are fried too. You can't feel the temp of the parking brake assembly as it's underneath the vehicle. But if it'll roll with the transmission in Drive and your foot off the throttle... apply the parking brake and see if that slows the vehicle; then release the brake and see if the vehicle starts slowly picking up speed again. If so, odds are the parking brake isn't screwed up enough to affect MPG.

mike c.