Prius (2004-09) :: P0505 Diagnostic Code, ICE Would Not Shut At All

I had our '04, 149,000 miles in for PA state inspection and emissions test as well as to be looked at because of a P0505 diagnostic code. The symptom of the diagnostic code was that the ICE would not shut at all. The past few months I simply cleared the code (ScanGauge) and things were immediately back to normal until the check engine light came on again.

Anyway, in response to the P0505 today they replaced the air filter (I checked, it is installed correctly), checked for vacuum leaks (all okay) and cleaned the throttle body. My wife picked up the car this afternoon from the repair shop (independent, but mechanic trained by Toyota for Prius). Upon arriving home she said I needed to drive it because it is acting very strange. We drove it this evening and indeed it is acting strange. A number of times the engine revved up to 2800-3000 rpm when my foot was off the gas, e.g. slowing to go around a corner or even when sitting at a traffic light. The SOC was normal (blue bars), not high or low. It was also interesting, because even though the transmission was in D, the car was not pulling to go forward, e.g. at the traffic light-- in D, engine revving, but car just sitting there as though it was idling or ICE off.

The ICE would shut down and go to battery mode appropriately. However I noticed that the mpg would not go about about 110 when backing off the gas going down a long hill at around 50 mph (engine warm, battery normal charge). Normally this would bump it up to around 200.

This latter symptom of mpg not going about 110 is a bit subjective and we only drove around 20 miles this evening, but I know we did not experience the engine revving before the car being serviced today.