Ok, I think this will require a little longer testing as just a trip to work is too short of a time frame and distance to get an accurate avg mpg, but I did get 14.8 on the way which seems to be a big improvement. I turned off HSA, but it still felt like the vehicle was being held back all the way up to and through 40mph. I also tried the tip with the transfer case, but that didn't seem to make a difference in the being held back feeling. I don't think that it is a brake issue because if I put the car in neutral, it coasts just fine down a hill, but if it is in gear, I get held back. I am leaning toward an overdrive issue. When I am trying to coast on a down grade at 35-40 mph, and I hit the OD off button, I don't feel a difference in how much I am being held back, so I am wondering if I am never getting to the OD, and staying in 3rd. With all that being said, it did seem like I got better mileage this morning after everything I tried, but I think it is too early to tell. I will take the next couple of days to get a good average and post back.

 

A few things come to mind...

My first thought was that a brake is/was dragging.

I wonder what you'd find if you jacked up each corner of the Jeep independently and tried spinning each wheel by hand. IE: Is something binding or making a wheel or pair of wheels hard to turn.

Also cant help but wonder how the motor runs.. Is it wheezing, or does it rev and run pretty freely?

I'd check the airbox for any obstructions, and probably pull the plugs and check each one. This weekend I was just poking around under the hood and found one with a cracked insulator. Replaced it, and got 1.5mpg back. I had no idea there was ever an issue.

Lastly, what does the JK do when you put in neutral when rolling down a long hill on the road -- does it still decelerate, or does it feel 'ok' and lightly accelerate as you'd expect?

 

Oops, didn't read your last post before posting a reply!

I think you're on the right track with the tranny issue... I'm pretty distrusting of autos. Had a few with strange issues.

Are you familiar with tranny fluid? It shouldn't smell 'good', but it shouldn't smell 'burnt' either. And if dropped on a white cloth, should not have much of any discoloration to the red tint.

 

Turning OD off does very little to slow a JK down. Just the nature of the gearing and transmission programming I guess. Unless you have a long commute with some open highway, OD wouldn't make much MPG difference anyway.

Keep looking at the brakes until they can be eliminated as the problem. Do the "coast to a stop" test and check the rotor temps. A rotor with dragging pads will burn you; a rotor that only pulled the Jeep to a stop from 15 MPH won't. Jacking the Jeep and rotating the tires won't tell you if the computer is applying the brakes when it shouldn't while going down the road. I've identified which brake was dragging many times with this method.

Also, has the Jeep been checked for codes? Its possible to read codes on the dashboard without a reader:

Reading DTC Codes
To check your Jeep JK Wrangler’s DTC codes, simply insert your key into the ignition, turn it to the ‘ON’ position but do not start the engine. Then, push the key into the ignition cylinder and while holding it in, turn it to the ‘OFF’ position and then back to the ‘ON’ position (3) times straight making sure to end with it in the ‘ON’ position. It is important to keep pushing in on the key to prevent it from catching at the different positions along the way.
2. If you do step #1 correctly, all your dash lights will illuminate and your odometer will show up as a series of (6) dashes. If your Jeep JK Wrangler has logged any DTC’s, they will be displayed soon after the dashes appear. Write down the DTC(s).
3. If your Jeep JK Wrangler has not logged any DTC’s, the dashes on your odometer will be followed by the message ‘-done-’. Turn your key to the ‘OFF’ position and you will in fact be done.

 

Something that just came to mind...

40 mph is typically the speed at which a the ECU will lockup the converter. Not sure what speed 4th gear gets applied, but that also could be something to make note of.

The 40 mph thing is pretty consistent for the converter lockup though - across many makes and models.

My wild guess is that somethings slipping or binding in the tranny. Either the converter isn't working correctly, or there is a problem with the 1-3 gears in the tranny.

 

But an unlocked TC would let the rig roll easier on a trailing throttle. Still, a transmission problem could totally be the cause.

If there is a transmission problem, it could be the ECU or a mechanical failure. If its in the ECU, there should be a code. Mechanical stuff probably requires a transmission tech.

I think it is easier to eliminate any possible brake issues before diving into the transmission however. Also check for any engine breathing problems as posted earlier. I've cleaned squirrel/chipmunk nests out the air box on every rig I've owned until I remember to install hardware cloth across the opening.

 

Hey that my thread! Lol but yeah right now i got my Rubicon tires and and a dynomax muffler so im getting about 14.3

 

Sorry to get of the OP's original question, but I think what you (Mr_Showtime) and some others are feeling is the fuel injectors cutting off the gas when you let off the accelerator.
Unlike most computer controled fuel injected vehicles that kind of curve or slowly cut of the gas going into the engine when you take your foot off the pedal, the JK cuts it off imediately giving you the feeling that the brakes are being appiled.
It's a normal feeling in JK's.

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Not true with an auto, maybe because the torque converter slips. When I get off the gas in my 12 the rpm drops quickly but I maintain speed very well. I can get off the gas at the bottom of an off-ramp and coast up hill and only drop a couple mph. The auto coast very well when you get off the throttle and I feel non of the breaking like effect that the OP has described.

 

On the 3.8's, autos and manuals, at speeds below a cetain rpm (like in town driving) they have that braking feeling when you take your foot off the throttle. I have driven several JK's, Unlimiteds, sticks and autos and they have that odd braking feeling.
Maybe the new 3.6 is different. It does have a different auto transmission too.

I've never noticed it on the highway now that you mention it. I'll go try it out and see.

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