The IWE on our 2019 Expy (22000 miles) started failing the other day on a road trip. Went from an occasional brief metal grinding noise to a near-continuous grinding over the span of 30-40 minutes, and having to keep dropping speed by 10 mph increments to keep the grinding from being continuous. Kept stopping looking for something dragging or stuck in the front wheels, etc, but nothing in evidence.
Luckily, about the time it escalated into the truly alarming stage, making it sound like it was about to seize up on the freeway, turned out there was a Ford dealer within 5 miles, and the vehicle stumbled into the service area shortly before closing on a Saturday. We were going 20 mph by the time we got there because it sounded so bad to go any faster.
(Also luckily, the service advisor was able to, on very short notice at closing, get a big enough loaner for all of us and our stuff to drive the 200 miles home.)
They diagnosed it yesterday as a faulty IWE check valve not holding vacuum and fixed it under warranty. Said it was lucky we got there as rapidly as we did, that it could have stranded us on the freeway most unfortunately at any time if we'd kept driving it.
Seems like Ford trucks have a history with IWE valve failures. So many posts out there about F-150 and Expeditions experiencing similar failures. There's also a series of TSBs for those vehicles about this issue. For example,
TSB 20-2189 covering 2013-2018 F-150/Expedition/Navigator vehicles. It's interesting it doesn't (yet?) cover 2019 Expys. Maybe they were using the updated valve from the TSB in 2019 production (vs. 2018) and thought it was already resolved? I was told this was the first 2019 Expy they'd seen with a faulty IWE check valve.
Sure hope it doesn't happen again, but out in the middle of nowhere far from a dealer. Seems like a poor design choice, to use a vacuum valve, and for it not to be bi-stable, requiring voltage (or vacuum) only to switch between the two stable states (4WD, 2WD). Failing that, they should've at least made the single stable state be 2WD (hubs disengaged) so that when there's a partial vacuum, you could just flip the switch to 2WD and keep on trucking down the road. As it is, with 4WD being the stable state, flipping it to 4WD isn't really an good option since that'll damage other components while driving on concrete.
