17 expedition bucking while slowing

Took my 17 XLT ecoboost up into the mountains for some tubing today. On the way down the mountain I locked out 4-6 so I could engine break down most of the windy twisty road in 3rd gear with little break input. I began to notice the truck was surging or bucking as it rolled down the hill, with no throttle applied. I shifted into 4th, same feeling but at a higher rolling speed. Same thing in 2nd but at slower speed.

Applying throttle made it go away. No corresponding rpm change while it was surging or bucking though. It almost felt like the converter was locking and then immediately unlocking, or driveline slack? I put truck in 4x4 though, no difference. Only way I got it to stop was giving it gas, not a good idea on this narrow windy steep downhill road covered in snow and ice.

Anyone else noticed this? Truck has 43k miles, spark plugs were changed at 35k. No check engine lights, although it did feel like it was lugging very badly at all times and not wanting to downshift without liberal pedal pressure during the normal part of the drive. I'm running a 5 star performance tow tune and it feels like I'm not building boost any longer either (like it's lost that woosh feeling when you gas it, just loud motor). Gonna return to stock tomorrow for grins and see if things change. I would imagine if I were having a turbo problem or would throw a code for underboost ?

With your performance/tow tune, it’s possible that’s what causing it.
I have experienced none of those symptoms, but my transmission tuning is far from stock or what 5-Star provides with their normal tunes. I also never lock out gears, as the truck should downshift and provide engine braking by stabbing the brake pedal a couple of times while going downhill. Grade braking is an automatic feature in our trucks. How much brake input were you providing?

Once you return to stock and see if the symptoms persist or not will help narrow down potential issues, as we will at least know if it related to the tune.

Did you have tow/haul mode or descent mode turned on?

I tried with and without tow haul ,it made no difference. It was a looong winding road off the mountain so I had time to try everything I could think of. I've been running the tune for 7 months without issue and done lots of mountain towing. The dealer just changed the oil yesterday and did the maintence inspection, tire rotation and battery test. No issues found.

Sounds like the same issue as in this thread: https://www.expeditionforum.com/threads/transmission-lurching-in-manual-mode.38143/

Mine does the lurch when using manual mode for down hill engine brake until tranny warms up.

One problem seems to be a couple of you are breaking the car rather than braking. (Darn auto-correction.)

I find Tow/Haul provides engine braking during descents without having to work the shift lever but if wanted I just downshift manually (M) rather than locking out any gears. But we only have baby mountains here in Virginia that rarely exceed 5000' ASL.

-- Chuck

I know this is an old thread, but I have had this same experience. RPM does not affect it, however you can hear the sound or the throttle blade changing position or the turbo bypass opening/closing. It cycles in a pattern as it is a not a random surge. If you have ever done the GDI treatment in the intake and holding the throttle open at a certain point it makes this very odd sucking sound then something closes like a valve and it cycles. I think it is throttle related and nothing to do with the transmission. Manual mode or upshift lock out does not change this symptom. Just wondering if you ever found a resolution to this.

The OP, Coolzzy, was last seen Sep 13, 2020...

I routinely run tow haul in the mountains even when neither towing nor hauling. Automatic engine braking (downshifting) on steep down grades works very well for me. I can't think of a time I ever (deliberately) locked out higher gears but have used into M a few times on down grades. All without surging -- I'll try to note this in the future.

There's also Hill Descent which I used once coming down Grandfather Mountain NC a couple of summers ago. Only useful below 20mph. Average grade on the mountain is 7.3% with a couple short sections roughly 15% (last 1/4 mile on the way up is over 13%). Going up is no sweat. Down takes attention.

-- Chuck

chuck s said:

The OP, Coolzzy, was last seen Sep 13, 2020...

I routinely run tow haul in the mountains even when neither towing nor hauling. Automatic engine braking (downshifting) on steep down grades works very well for me. I can't think of a time I ever (deliberately) locked out higher gears but have used into M a few times on down grades. All without surging -- I'll try to note this in the future.

There's also Hill Descent which I used once coming down Grandfather Mountain NC a couple of summers ago. Only useful below 20mph. Average grade on the mountain is 7.3% with a couple short sections roughly 15% (last 1/4 mile on the way up is over 13%). Going up is no sweat. Down takes attention.

-- Chuck

Click to expand...

Ha! I did not check that. I only have tow haul but that does not change the behavior. Hill decent is not an option on my Expedition. Just a strange situation.

I did some reading in the Owners Manual (radical idea I know) and noted that Hill Descent which you seem to not have not only uses engine braking but the ABS system as well which can cause some ragged feelings. I'm not sure if Tow Haul will use the brakes -- anyone? -- it makes some radical down shifts in the mountains. I'm guessing adaptive cruise control or avoidance will do things with the brakes. Have neither on this Expedition but my wife's car has it.

-- Chuck