average mpg went from 46 to 36

Hello,

Background: I have a Toyota Prius 2008 (Turing edition) with a mileage of ~173k. Usually when I press Trip Info in the computer touch screen, and then Consumption, it would show that the average MPG is 45.6-46.

Following up on my initial thread about battery dies in accessory mode:

prius 08: truestart battery drains when going into accessory mode and warranty eligiblity | PriusChat

The problem was getting worst, and would happen also when I would try to start the car, so I ended up taking the battery to the Toyota dealer, and having it getting replaced at a prorated price as I still had it in it's 7th year warranty. They agreed to replace it, so I assume they found an issue. Took the new true start battery (labeled 03/2020) replacement to my mechanic, and they had it replaced (note: he didn't reset the system afterwards).

Since it got replaced, I haven't drove the car much as I started to work from home given the restrictions. Two weeks ago, after a week the car was sitting idle at the parking, I tried to go inside it to get groceries, and noticed that the car battery died again as I couldn't use the remote key to open it. So I had to manually open it with the key and then get it jump started again. Then, given it had that problem (which surprised me, as I just got a brand new battery a week ago), I decided to take it to my mechanic to check to see why it died. The mechanic checked, and said that it must be that I haven't drove the car enough (for "15 mins or so", but I'm sure I drove more than that since I had it replaced), so the car battery drained, and he doesn't think that there is any problem, but he said he will reset the car system just in case and it should be fine now.

So far so good, I drove back home. On my way back, I stopped at the dealer, as I wanted them to update my service mileage records notes (they had it incorrect), and noticed for some strange reason the remote still wasn't working consistently when I would try to lock or unlock from a distance, so I was concerned the battery still having a problem, but figured it could be a problem with the remote itself and I could replace it's battery. I decided to go back home, and replace the remote battery if needed. When I got to my parking, the remote seemed to work fine locking/unlocking, so I realized there was some connectivity reception issue at the dealer parking.

I thought I was finally behind all the problems. Unfortunately later that day, when I tried to start the car again, the following warning signs appeared:
On the dashboard, I had:
-Master warning light (red triangle with an exclamation point inside it).
-Along with it, a malfunction indicator lamp (yellow color with "check" underneath it)
In the manual it says this could indicate a malfunction in:
*The electronic hybrid system control;
*The electronic throttle control system; or
*The transmission control system.
In the computer touchscreen I had:
-A hybrid system warning light (a car with an exclamation point inside it)
Per manual it indicates a malfunction in the hybrid system.

Also noticed when I would try to unlock the car door to get inside, as soon as I would open the door, it would activate the door open sound beep/icon, which is weird because usually it's only happens when you start the car, and then try to open the door.
Other than that, when I started to drive with these warning signs, I looked in my consumption, and noticed the mpg that's showing there is about 36-38 mpg (nothing close to the 45-46 that I used to have).

The next day, I drove the car to the mechanic. As I was pulling over to his parking wait area, I turned off the car waiting to get in. Then when it was my time, I started it again, and noticed that the warning signs disappeared. The mechanic still wanted to take a look at the car and the car computer. After analyzing, he said that next time when the warning signs show up again, he will need to plug to the car computer to get the codes, so he knows that to replace, but he said one of the two parts in the hybrid system might need to be replaced:

1. heat bow (or valve? not sure I got the part name right from him)
or
2. inverter

Then he reset my car system again. I also reset the consumption average myself. And drove away. The average mpg appeared to be promising as I saw 48 mpg, but as I further drover it went down to 43. I was hopeful it will continue to improve and stay this way or more. So it's been a week since I drove it, and today I had to run some errands, and noticed that the average mpg went down to 36-38.

Now some friends told me that it could be that I'm not commuting to work (50 miles a day), the average mpg is not reflecting what I used to have. But is it true? I thought Prius 2008 has a better city mpg than highway mpg, so shouldn't it be around 45 like I used to have? Because I had the car for 5 years now, and I always saw 45+ in the average consumption.

So what could have caused this problem and how do I get it back to the mpg I used to have?
Could it be that something got damaged in the hybrid system, when the 12v battery drained? Could it be the jump starts that I had to do damaged something? Could it be the new 12v battery causing problems? Could it be the car system reset the mechanic did caused a problem? Could it be that the actual problem for the warning signs didn't disappear with the signs, and it's still there? What should be my next step? Anyone has experience with this kind of problem?

Please help!

Thanks.

 

Sorry, I didn’t get the codes from the mechanic, but he said that next time when the warning signs show up again, he will need to plug to the car computer to get the codes, so he knows what to replace, but he said one of the two parts in the hybrid system probably will need to be replaced:

1. heat bow (or valve? not sure I got the part name right from him)
or
2. inverter

 

Yes, I already replied that I didn't get that from my mechanic here: (But unfortunately I'm still in the "new member moderation phase" lol)
average mpg went from 46 to 36 | PriusChat

Yes, I doubt it was a reversed jump, as I put the red plug on the red plus screw, and black on the side screw, so I think it was ok.

I believe he reset with a tablet he connected while he sat inside the car, so probably the code scanner way.

Yes, I think the battery is ok, it shows 14v in the diagnostics, and today (one week later) when I tried to start the car everything seems to be ok except the average mpg.

I think the remote is fine too.

Yes, the mechanic said those were only pending codes.

Are there other people with gen2 prius in this forum who usually commute to work, but now sheltered in place, who noticed that their cars average mpg went down by ~10 miles from 46 to 36-38. I would think if this was the case, there would be more threads in the forum about this, or am the only one who is freaking out about it? lol

 

Again, I think the 12v is ok, as I just replaced it with a brand new TrueStart original battery.

Regarding the mpg, I asked the Toyota dealer about it, and they told me a similar thing, that with short trips in the city the average mpg would go down, but if I drive it more as usually it should come back to what it was (fingers crossed). I also asked my mechanic, and he mentioned one thing I could do, is to clean the hybrid battery fan, but that unlikely would improve the mpg, so I don't think I need this for now. I'll just follow up again, once I commute back to work.

Thanks!

 

Update/Solved: I'm still working from home, so I didn't really get a chance to find out if I get back to the highway, my mpg would restore, however I found that if I change my style of driving, using techniques such as pulse and glide, and others, I was able to get the average mpg back to 44+, and still improving... I guess the city driving exposed a flaw in the way I would drive, where I wouldn't be efficient with my driving, and I probably relied too much on my highway miles to make up for it, but now I learned how to take full advantage of the hybrid.

Some useful videos I used to learn:

Hope someone finds it useful.

Note: This is probably unrelated, but I also ended up replacing the part number 1 I mentioned above: The heat valve as I had again the exclamation point showing (it did disappear by itself after), and that's what the system code was for (mechanic said it's recommended to replace although it wasn't mandatory).