Ive had this problem every summer the last few years when it gets hot. It sounds like a manual car when u clutch but didnt get off the accelerator fast enough. The revs go higher a little and stay there for a few seconds but the transmission is not engaged. It happens at all different speeds. And it starts at about 90 degrees. I changed the transmission oil and it still does it. It isnt getting worse, but every summer it does it. When I am on the gas, it works normal. No slippage. But when I get off it does it. Any ideas?
The car is programmed to slow a little when you are off the gas, so it feels like other cars. (Without that programming, it would just coast free when you're off the gas, and that would feel weird compared to other cars.) The Prius normally gives you that slowing feel by recovering a little of the forward energy and putting it into the battery. But it can also slow by using up that energy twirling the engine instead. It always does that once the battery is fully charged and has no capacity left to accept more regen. It can also be protective of the battery when the temperature is high, and limit the amount of power it puts into battery charge. It twirls the rest off using the engine. The engine doesn't consume any fuel when the car uses it to twirl away excess energy. It's just being used as a place for the excess energy to go. It has to go somewhere. If the engine revs stay high for a moment even after you have come to a stop, the car may be at work to cool the battery. A weird thing about NiMH battery chemistry (at least according to some things I've read) is that discharge is an endothermic reaction: using a little bit of power out of the battery, to twirl the engine a bit, may actually reduce the battery temperature. (This has to be done right, with a limited amount of power; at higher levels of discharge, the resistive heating effect predominates.)
