Lost 10mpg After Replacing 12V Battery

You are likely suffering from my Low MPG Mechanism resulting from a 12v disconnect.

See this Imgur Screenshot Post with others suffering from this Low MPG Mechanism as well.

Basically what happens is after a 12v disconnect... the Prius computers reset certain parameters that tell the vehicle to use EV mode aggressively and instead it runs the combustion engine to supply power.

There is no specific explanation for what causes this to happen or how to get the Prius to use EV mode aggressively again.

I'm currently making a video about this issue as I believe it to be the #1 cause of Prius Low MPG and somehow no one else has ever discovered it. I am basically the only Prius owner that knows about this.

Posted via the PriusChat mobile app.

 

Try this reset.

 

Interesting. I wonder if thats the cause of my mpg drop as well. Still getting mid 30s pretty much no matter what I do. Every time I think I had it solved its dropped back down.

Edit - so that video above, after doing more research, seems to be more of a general Toyota ecu reset. Not specific to alignment. I may try it... anyone else do this?

 

Ok I got the nerve to try it. It put a diagnostic mode message on the screen, so it did something. Waited 30 seconds and shut everything off, removed the clip and restarted.

We'll see if it did anything. I have a full week coming up of the same commute in the same heat. The car has been very consistent in the last 4 fillups. 34-35mpg average. One road trip mostly highway, 47.5 mpg. Up to doing this reset, this tank I've gone 97 miles, 33.5mpg @22mph ave. 7 bars of gas left. Will update in a week.

 

Week later. Ended with 20 miles of range left. Driving-wise my commute was the same, in the about same heat with the AC on same levels.

Start Trip A: 97 miles, 33.5mpg, 22mph ave
End Trip A: 329 miles, 37.1mpg, 23mph ave
Start Trip B: 0-
End Trip B: 232 miles, 38.9.mpg, 24mph ave

After the reset, it felt like the car had a lot more power off the block. The battery drained quickly. It smoothed out over the next 100 miles, and ended up feeling back like it used to. For example, heading to the gas station to fill up I was 1 bar from top on battery. 3 times stopped from 0 to 40mph the battery level didn't budge.

Also - I was expecting the mpg history to also get reset, which it didn't. Should it have?

So did this work? It's better than before, but not by a lot. Will remain to be seen over the next few weeks.

As a FYI the Prius battery has 48% life left according to Dr Prius. Battery is weak. The % test was done before this reset however.

 

Next tank (~48miles left to empty) in. 39.1MPG @ 20MPH average. Everything was mostly the same, commute/heat/ac with the exception of the car sitting for 15 minutes or so with the ac on while the wife waited for a table at a restaurant.

I'm calling resetting the ECU a win. From a steady 33-34mpg to 38-39. And yes, I didn't mention it above but I did replace the 12v battery last year which may have been when the MPG also dropped. I'd have to check receipts.

 

Now give it some more TLC by replacing all coolants, ATF, and give it a bottle of lucas fuel treatment. You’ll be back in 45-50 mpg in no time.

 

Already did all that, didn't change anything. I think my low mpg at this point is due to the weak hybrid battery. Brakes are all fine, tires are good, EGR assembly is clean.

 

how many miles on her, have you cleaned the egr circuit?

 

138k or so, and yes - hope to never doing that again.

 

i'd be surprised if it were the battery. but who knows?

most people report the same mpg's right up and through the red triangle with battery codes

 

have you done the doc prius stress test for block variations?

i don't have it, but i think there is a way to get readouts under load

 

Went on another road trip, near all highway miles. 53.3 mpg. Before the ECU reset the highway trip was 47.5.

 

Good for you!

 

Holy cow - 2014 Prius 4 - after 400 miles driven at 28-32 MPG after my AUX battery replacement - THIS IS WHAT WORKED - I'm back to 51MPG at 28mph avg.

I replaced spark plugs, air filter and did MAF cleaning. Nothing helped. I did notice that my HV battery never kicked in just like @TheLastMojojomo explained. Drove a few 70+ mile trips on the freeway as well. I went to three dealerships, no one knew what was going on. They all claimed bad wiring or battery packs were done. This reset literally worked instantly, I'm still in shock!

@DirkAshburn - thank you for trying this out and figuring out that it was more of a general reset. This post needs more attention, I am sure others are still suffering!

 

Enjoy your prius again!

 

Crazy how this works! I had problems with my 12 volt battery, so I disconnected it. After they tested it, it passed. I reinstalled the 12 volt battery and that’s when my mpg dipped to around 37 mpg. I was stuck at this average no matter how efficiently I drove the car.

3 months later, driving to work today, think the computer system has finally adjusted to my driving conditions. Driving to work today, I got to 45 mpg. Hope it stays this way. We will see.

 

I've always wondered if the fuel treatments actually work! Is there a specific type for a Prius?

 

Of course there is!
Just like the specific oil and specific gasoline that's specific to the Prius.

The prius engine is like no other gasser engine out there and must be treated with very special, specific care and products.