I have a bone stock '07 Sahara Unlimited with 25k miles on it. About two months ago my check engine light came on. I talked to the dealer as well as a service tech friend of mine and they both told me to check for a loose gas cap. Sure enough, it practically fell into my hand. The check engine light went off a couple days later, and the following week I had it into the dealer and they found no fault codes in the computer. Jeep ran fine while all that was going on and has run fine ever since - I haven't noticed anything with performance or anything out of the ordinary at all.

But I have noticed that my MPG's have dropped. I know the MPG computer isn't known to be perfectly accurate, but whereas before it would show an average of around 18.5, with highway mileage around 19.5 or 20, I'm barely breaking 17mpg ever since that check engine light episode. I calculated this last tank using the manual method and it was 16.22 (which was only about .8 less than the onboard MPG computer read).

I took a look at my air filter and it's very clean. Tires are all inflated properly. Any ideas what this might be? I've gotta get it back into the dealership but I'm wondering if anyone else has experienced this.

 

Have you recently changed where you buy your gas or has the gas station started adding
ethanol. The ethanol addition will decrease your mileage.

 

Winter fuel blend and how cold has it been in your area?? If cooler temps then it's harder and longer to get the JK's optimum running temps and will use more fuel till it hits optimum temps.

 

Ya, my mpg goes down 2-2.5 from summer to winter.

 

Maybe you should check into getting a locking gas cap maybe someone is steal your gas?

 

X2 I agree, m.p.g. always go down in cold weather. Or..........could it be the cheap gasoline we are buying now? Maybe your Jeep likes the $4 per gallon stuff better.

 

I noticed it too. It sucks. I've gone from 19 to 17.5 miles per gallon.

 

My 52K Sahara did the same thing. Interestingly I think two factors affect it which might align with yours. If they disconnected the battery and reset your computer, (which I had done on mine), I felt both power and MPG suffer greatly for a month after that. This was also during our coldest weather, so perhaps a combination of oxygenated fuel, colder temps (colder airmass requires more fuel to maintain correct mixture) and the computer re-learning curve.

Mine has improved in the last tank from a mostly city 14 to about 16. I expect the next highway trip I take may see close to 20 again.

Be patient and watch it.

 

My WJ and my wife's Grand Caravan dropped in MPG a month or so ago, then they both have gradually gotten better, but not as great as they were before. Mine went from 225 miles per tank down to 170 miles per tank.

I am up to about 215 miles per tank. so a 12mpg down to a 9mpg.

Yep, my 99WJ with a 4.7V8, 2" lift, and 31's hates me.

 

One other thing to check would be your air filter. If it's plugged up it could drop your MPG.