I have a 2012 Lariat with the 5.0 with 52K miles. This is my first winter of ownership.

Since winter finally hit us with snow (and cold temps), I've been running in 4-Auto instead of 2-Hi.

I've noticed my MPG has dropped form about 15 to 12, and certainly I can attribute some of that to winter fuel, colder air temps, longer warm up, etc.

But a current thread discussing 11.4 MPG, seemed to suggest that its possible to be locked in a 4WD mode due to a vacuum issue ? My truck runs great, displays '4x4 Auto' with the LCD screen ...... not sure that totally guarantees which mode I'm operating in, though. Is there a way to confirm ?

My real question though is, Should MPG in 4-Auto be the same as in 2-Hi, if road conditions do not put me into 4WD mode ?

 

I notice no difference when running in the auto setting, I've actually got my best MPG in that setting. It will be slightly lower then in 2wd but not by much if any.

When you are in auto the front hubs are engaged, if it senses slippage the transfer case will send power to the front axle.

Winter time will cause a drop in MPG all by itself.

 

My understanding is that the hubs are held free by vacuum. Which would mean if the vacuum lines or system failed the hub(s) would lock and stay locked until it was repaired. Seems like a poor design to me.

 

As best I know, you are correct.


The alternative, though, is that the hubs engage with vacuum. In this case, if you lose the vacuum signal you also lose 4wd. So it makes more sense (to me) to have them fail "safe" and keep your 4wd.


to the OP:
I think you can contribute nearly all of your MPG drop to the winter fuel, colder temps, and longer warm-up time (idling). While there will be a drop in fuel mileage due to having the front driveline turning, it is almost theoretical in nature -- that is to say, with all the other variables involved, it isn't really measurable at the pump.


my expedition usually drops from 15.xmpg to 12.xmpg in the winter. I run A4WD most of the winter, but I think the 10 minutes of idling every morning is far more detrimental to fuel mileage than the front driveline could ever be.

 

actually it's a great design, as Mike said it's a fail safe so you don't totally loose 4x4. Problem is typically the issues people see is not a total lack of vacuum but partial which then half engages the IWE's and causes damage.

In auto mode the hubs are locked but the transfer case does not provide power until it detects the rear wheels slipping.

 

OK, so now I'm brainstorming. Does that mean that if the vacuum fails and the hubs lock then the front driveshaft spins, even though it's disconnected from the t-case? If so then it would basically be like Ram, Nissan, etc. trucks are 100% of the time. Those systems seem to have little to no failures. I wonder if anybody has considered allowing their Ford IWE hubs to remain "failed" on purpose?

 

If I had a vehicle that did not have the A4WD function, just 2Hi, 4Hi, 4Lo, I would strongly consider disconnecting the IWE vacuum source so that the front driveline is spinning all the time during the winter.


What I like the best about the A4WD function is the ability to quickly shift to 4Hi when coming up to a stop without the driveline shock of going from Zero to wheel speed when you flip the switch.


I would probably reconnect the IWE's during the summer, but during the winter I go from A4WD to 4Hi a LOT so it is nice to have the front driveline already spinning.

 

I can't agree more, my excursion I run with the front hubs locked from October to April and slip the dash switch to activate the transfer case as needed. Makes it a much smoother transition and I know it's going to engage!

The late 90s Explorers and Rangers went through this, along with the 99-02 Expeditions. Ford removed the axle disconnect, they found the MPG hit to be next to nothing but less parts to fail. That just left the transfer case to shift and less moving parts.

 

Just curious to know what the perceived benefit of this would be. To my knowledge the T-case uses an electromagnetic clutch to spin up the front driveline before the locking collar engages. What would the benefit be to disabling this?

I sort of like that little jerk that I feel when it's engaging. Lets me know it's working!

 

I always understood it the clutch system was only used on the Control Trac type transfer cases (AWD/Auto4x4/A4wd) but the normal BW 2h/4h/4l cases just used a shift motor in place of a rod to physically engage the case, but now i'm not sure that's the case?

My opinion is one less weak link to break, as we all know the IWE's do have a decent number of issues.

 

why would I need a device to spin the driveline up to speed if it is already spinning?


I don't know what it does, but I know it makes a heluva noise when you go from 2Hi to A4WD or 4WD at 65mph (which is allowed in the manual). I don't like making mechanical things react quickly.


Doing the math, the driveshaft is spinning at about 2600RPM @ 65mph. Where's the benefit in making the front driveline go from Zero to 2600RPM in a fraction of a second? Electromagnetic do-dads or not, do that often enough and I think you are going to find the weak link. especially if it is really cold and the gear oil in the front diff is stone cold ...


I could be wrong, but I don't see a benefit to making it spin up that fast.