As well I would like to take a tour through Montana and Idaho, specifically the road to the sun. Does anyone know if that mountain pass would be too much for the Volt?
Keep in mind that Mountain Mode has been poorly named. It’s not a special mode that provides the Volt with special performance characteristics for driving in hilly terrain that would be unnecessary on flat ground. Perhaps it should be called Hold Mode at Fixed Battery Charge Level.
The 2011/2012 Volts had no Hold mode. When driving with the ICE running in high-power demand environments (for example, when trying to pass at freeway speeds while headed up steep hills, and hence "Mountain" mode), the generator’s output might be insufficient to maintain performance without "borrowing" extra power from the battery, resulting in Reduced Propulsion Mode. GM thus developed Mountain Mode, which would maintain the battery state of charge ~4 bars over the fully depleted level in the Gen 1 Volt. The Gen 2 Volt has a more powerful ICE, and its MM-maintained buffer is ~2 bars (some say it’s not really needed at all).
Choosing MM over Hold (at MM-maintained level or higher) seems to offer no particular advantage for the intended purpose. Just remember to maintain that buffer reserve until you pass over the last summit crest of the trip.
MM has one "feature" that is elsewhere lacking. If your battery is already at or nearing full depletion, switching into MM will run the generator a little harder and use some additional gas to recharge the battery back up to that ~4 bar (Gen 1) or ~2 bar level (Gen 2). This ability to recharge as you drive allows you to avoid time-consuming recharging stops before heading into the mountains. In moderate driving circumstances, this takes ~15 minutes for a Gen 1 Volt (less for Gen 2). This is why the manual says to switch to MM 15-20 minutes before reaching the mountains.
In practice, you can switch into MM even with a full charge. The bottom bars will change from green to gray, indicating the MM buffer, and you continue driving on battery power in Electric Mode until the battery SOC drops to the MM-maintained level.
The Wikipedia page on the Going-To-The-Sun Road indicates the road is so full of hairpin turns that the speed limit is 25 mph (40 km/hr) on the upper roads, and 45 mph (72 km/hr) on the lower level roads. Chances are that at those limited speeds, the Gen 2 Volt wouldn’t need MM at all. Descending in L would surely keep you in control (perhaps even with cruise control on at the lower levels) and provide lots of downhill regen.
The article also mentions the road is one of the most difficult roads in North America to snowplow in the spring. Up to 80 feet (24 m) of snow can lie on top of Logan Pass, and more just east of the pass where the deepest snowfield has long been referred to as the Big Drift... On the east side of the Continental Divide, there are few guardrails due to heavy snows and the resultant late winter avalanches that have repeatedly destroyed every protective barrier ever constructed... hmm... no guard rails, hairpin turns, high up on the side of a mountain... were it me, I’d choose a different road...