Mpg went from 20-21 to 13-14

[OFC4630]

Starting in September I noticed my fuel mileage has decreased significantly. I was averaging approximately 20 to 21 miles per gallon mostly highway driving but now I'm lucky if I average 14 miles per gallon. In September I was thinking I bought some bad fuel or some very low-quality fuel but my mileage has not gone back up sense. Earlier in the summer I installed new Champion copper spark plugs, new spark plug wires and a new coil. Recently I installed a new MAF sensor and a new Air filter but nothing changed. I know that winter fuel can drop your miles per gallon some but not by 6. I was averaging over 400 miles per Full up but now I'm lucky to get 300. Any ideas on what I could check?

2006 EB 4.0 with 120k.

 

[Flag Gibby]

I have never been able to get that kind of mileage out of my 4.0, but then again I have a true AWD so me getting 18/19 in freeway driving conditions before I put on aggressive all terrain tires sounds about right for comparison. Honestly I have never had good luck with champion spark plugs holding up in the long run and cannot stand them because of that. If it were me I would get some good double platinum plugs and change them out. Also, you are due to replace the PCV valve on the 4.0 since you are at 120k miles.

 

[banditxp]

O2 Sensors normally go bad around 75-100K miles and they can drop your MPG by that much. and if you did get some bad fuel that messed up the o2's then theres your problem. also a bad CAT can do the same. Does it still feel like it has the same power at WOT? if so the cats are good. if not then might be bad cats from bad gas. but i doubt it. I bet o2's

 

[OFC4630]

Thanks for the replays. I still have all my power so the cats are still good. I'm going to replace the Pcv valve and o2 sensors. What brand should I go with for the o2's? Oh and the plugs are getting changed again just in case.

 

[Flag Gibby]

I wouldn't change the O2s unless the readings from them are bad. A good code reader can show the signal from them and how fast they respond to RPM changes. I have never seen O2 sensors just die (like you indicate in your original post) especially on these newer vehicles, but it is possible. See this article for testing the O2 sensor without an OBD reader: http://www.fourwheeler.com/how-to/engine/1403-driveway-diagnostics-bad-oxygen-sensor/

 

[fordbluefamily]

Don't discount the drop in mileage from winter fuel blends.

 

[Flag Gibby]

Yes, but never the amount that the OP is experiencing. Winter blends typically take the ethanol content from 6% to 10% resulting in about a 1.5-2 mpg drop unless you have access to ethanol free year around.

 

[fordbluefamily]

I have seen a larger than normal winter blend hit in all my vehicles this year. Not the typical 2 mpg for any of mine.

 

[MotorCityFats13]

my 98 and 07 would drop 60miles on a tank from september to april .... my 07 has went from high 19's to low 16's both in 2014 and 2015.... going from 420 miles to 365 miles is a huge deal when you drive 45 miles 5/6 days a week.... the butane they add to the fuel among the other things kills the power efficiency.... what also adds to this is if you idle to warm up the engine and if you take lunch breaks in your truck like I do and let it run for an additional 20 minutes each day..... summer I can get 6 trips to and from work on a tank vs 4.5 trips in the winter months

 

[suzook]

That winter blend crap is the worst. It's destroying other vital parts of our cars. Fuel pumps, fuel injectors, any kind of sensor. It's total $hit and should be banned.

 

[BootyDo]

Not to state the obvious, but are the tires properly inflated? Not sure how your Fall season is, but we hit a nosedive for awhile and I lost 2-3 PSI across the board. Even if you're in the same boat or lost more and the tire sensors didn't alert you, I can't imagine that accounting for so much loss.

Did you lose your mileage almost immediately or has it been a slow loss to this stage? I'm on board with an O2 Sensor, Thermostat stuck open, or just bad ignition. Starting with the easiest and cheapest, maybe confirm the tires and throw some Techron in your next tank.

 

[Tech By Trade]

Probably the winter blend, if you run winter tires even worse. When I installed my blizzaks in November and they switched up her I went from 12.5l/100kms to 15l/100kms.

 

[fordbluefamily]

Your vehicle will set a code for a slow thermostat. It knows how long it should take to open up and go closed loop. I doubt it would be a thermostat and not have the code on this vehicle. Also, my tire pressure monitors on both my mounties have no problem with 25psi....once they get below that and obviously look flat is when the lights come on.I find this implementation of TPMS to be useless for me. Definitely don't rely on the system for pressure checks unless you get to n there with forscan and see the psi from each wheel.

 

[MNgopher]

Have a scanguage or other device to measure actual coolant temp? I've had several thermostats essentially get lazy and not maintain proper temp, though not be stuck open enough to actually set a code for the thermostat. My F150 just did this - was sticking around 178-180 in cold weather, no code. Replaced it and it now holds 191 no problem.

Could be a bit of it. Everyone here is blaming the fuel, but also remember that all the other fluids in your car are more viscous to start with in cold weather as well - transmission, differentials, etc... and all the drag starts to add up. If the air hasn't been checked in the tires, its a good bet they are low and affecting mileage too. Lots of little things that add up...