Engine revving, but no acceleration - Solved

A couple of miles into my normal drive this morning the car's engine began to rev, but did NOT accelerate. The car did this about 5-6 times on a road at 35 MPH that is mostly flat, with a slight descent where one would expect the car to glide in electric mode if possible. Battery pack was 4 or 5 bars. Each time I shifted into neutral since I was afraid of acceleration and feathered the pedal to see if it was somehow stuck (ignoring for the moment I was using constant pressure on the accelerator while driving). Also looked down at the first red light to verify the silly floor mats were "locked" into position. No "check engine" type lights ever turned on.

After these events happened the car did NOT stop the engine at the red lights, despite the battery level of 4 to 5 bars. After about 10 minutes of driving the car returned to its normal operation with the engine turning off at the lights and no more revving. After dropping off my passenger I carefully to the dealer who says there is nothing wrong with the car (also said they have never heard of this type of complaint). I only noticed the revving because the engine should have been quiet/off during the stretch of the road and the MPG dropped from where it should be to zero as the engine revved.

My 2012 Prius has almost 45,000 miles and this is the first time it has ever behaved like this. Have I overlooked others with similar symptoms? (Only found the one posting where someone's intake vent was dirty.)

Should I be purchasing some type of OBD device to collect data to prove I am not making this up?

 

No, it was not hot out although those of you outside of Texas may disagree. Below 80 degrees unlike now where it is upper 90s.

The dealer said they ran full diagnostics and there is no indicate of any issue.

 

Well, the issue returned today, but MUCH milder than it was yesterday. I recorded some video and the dealer's chief hybrid tech says the car is behaving normally. The video is very short, but the car was doing this on/off for about 10 minutes as I drove through traffic in less than 80 degree weather mostly flat with a short incline and nice decline... consistently behaving odd. Again, after reaching my destination, which was at the end of a 35 mile zone the engine refused to shutdown. I stepped outside the vehicle, dropped off my passenger and several minutes later the engine still ran for several minutes before returning to normal operation.

The dealer said the 12 volt battery is good and said they cannot test the hybrid pack. They said the older models of the Prius had issues with the 12 volt, but not the 3rd generation. He said they have to wait for the car to tell us there is a problem and this should happen before I notice anything (passed full diagnostics yesterday). Yet, I have noticed something. 8-( He said it might be a cell that is going bad and the car is actually turning the engine off/on to charge even though the hybrid battery is half-full, but then he said he would deny saying this if I repeated it... I have ordered a 12 volt battery at a non-dealer location and OUCH!

I tried and failed to get YouTube to process the small video I have (stops at 95% for hours). This site will not let me post a hyperlink, but if you add http to the start of the text below a video should be available:

skippingpebbles.com/2012Prius-0901.mp4

 

Inverter fluid is about 3/4 of the way between L and H when driven over 30 minutes. I will have to see what happens when the car cools. A dealer has done all maintenance since I purchased the vehicle so one would hope they would notice a problem since they see the car every 5000 miles.

EXPENSIVE 12 volt battery was replaced last night by a non-dealer (the battery was not expected until today). I guess that was not the cause. 8-(

First Issue returned this morning (revving). A couple miles into driving the same 35 MPH road with a slight slope downhill the engine revved and did not accelerate as I approached the red light. Cyclist must have thought I was nuts with a hybrid roaring at a red light. I continued to my destination, dropped off my passenger and the car did not exhibit any other symptoms than the one occurrence so I shutdown the car when I unloaded my passenger.

Second issue (gentle up/down as if tapping on accelerator) returned on the return trip after a hard brake due to a cyclist believing he could just walk into traffic. Issue appeared to return 3 or 4 times while driving under 30 MPH - the issue is so subtle you can miss it over the road noise and when in stop and go traffic you cannot rely solely on the MPG gauge.

Outside temperature under 80 degrees. Hybrid battery pack at 4 to 5 bars. Prior to driving 30 MPH there is one stretch where I reach 45MPH after leaving the neighborhood. So far both issues (or two sides of the same coin) have appeared only in the first 30 minutes of driving each day.

 

After living with this issue for 8 days, on day 9 the car failed in spectacular fashion! On day 9 as I approached red lights on the same route the engine did not rev, but appeared to shake violently. At red light number 3 the "check engine" light, "triangle" light and a message about the hybrid system failing and to pull the car over when safe. About a mile into this condition the car turned off without any warning. Luckily, I waited a few minutes and was able to reboot the car to continue to the dealership.

Hybrid tech at dealership said the code was one related to the engine failing to start. Paperwork shows codes 3191 and 171. They replaced a vacuum valve assembly, part number 25860-37020 at no cost to me and provided me a rental from Enterprise for two days as the part was not expected for 4 days (I assume Toyota America reimburses the dealership). I was verbally told the part is $85 and from when I dropped the car off the tech skimmed the tech article which also mentioned updating some firmware when replacing the part. I have no idea what the repair would have cost was it not part of the car's basic warranty.