Unexplained engine racing

Normal due to battery overcharge prevention

 

If you're coming down a long/steep downhill grade, say anything over 2 kms, it's good to shift to B near the top, to help keep your speed down. You may or may not still need to brake, depending on how steep it is. If the grade flattens out enough that you need to give gas, then I would shift back to D, at least for the duration of the flat zone.

B provides engine braking, and reduces the hybrid battery charging rate. Both these measures can delay the full charging of the battery, to the point that you're relying entirely on friction brakes.

If the fully charged status occurs when you've reached the bottom of the hill it doesn't really matter, but if you've still got kms of steep downhill ahead of you, you might end up with overheated friction brakes.

We occasionally need to pick up grand kids from a school that the #$%! planners located atop the steepest hill in Port Moody. Coming back down is close to 3 kms, and a drop of maybe 200 meters. For that I'll use B; it's about the lower limit.

A more clear-cut case for B: coming down Mount Seymour...

 

Couple of things:
1. maybe your vent hose popped off the intake manifold.
2. need a new purge valve - it's under $100.

this engine racing for no reason thing just happened to my 2013 in the last few weeks and I haven't proved it yet but 100% for sure that hose popped off. It is easy to find if you take off the air filter box.

 

Engine behaving strangely and popping hoses off intake manifold | PriusChat

 

There's a hose connecting to the intake manifold just below the throttle body, bring gas tank fumes if I'm not mistaken.

Is this the "vent hose" you're referring to?

There's also the lower PCV hose. And upper PCV hose, plugs into snorkel just upstream of throttle body.

 

If a hose popped off the intake manifold, you'd notice the engine operating extremely strangely, you'll have a lack of power, and you'll soon be forced to a stop as you're put into limp mode with the engine completely disabled (motor only until you run out of battery), "CHECK HYBRID SYSTEM" on the dash amongst other lights and a P3190 code (lack of engine power). Oh and you'd notice a whirring sound of air during engine braking.

Since that didn't happen to you, then it's strange that there was a sound of "sudden release of air"... could it be that a hose popped off but was still aligned and got pulled back on by vacuum? But if that engine revving was caused by a vacuum leak, it should show fuel consumption on the MPG indicator... since it didn't, then it must be the system purging excess battery charge by spinning the engine with the motor, which is normal.

Maybe you should check for any loose hoses on the intake manifold and EVAP line just in case. They're under the air filter box.