2010 Prius’s mileage going South

Well, I reset the ecu’s by disconnecting the Negative lead on the battery. And tested the 12volt battery. Then the mileage got a lot better. I noticed that all the EV, ECON and POWER modes were off. I had been driving with the Econ mode on. So, I left them off. Now the mileage is up to lower 50’s.
I’ll just let it go a while to see what happens.

 

Well, you’re right. It seems counter intuitive. I changed the ambiant temperature sensor just to see what is going on with the outside temperature readings. Doesn’t seem to actually (or even approximately) read the correct temperature. When I was back at the battery, I noticed that there is another temperature sensor there. I wonder if that had anything to do with anything.

 

I tested the 12 volt output. Seems ok. And the battery is a Toyota put in 3.5 years ago. Not a lot of noise coming from the running engine. No grinding noises.

 

Further reading told me that the other temp sensor was for the computer to analyze the charging amount of the 12 volt battery. It seems that outside temperatures have something to do with the charging load applied to the battery. Who knew!

 

It was on the Internet (I think it was through DuckDuckGo) as I was searching for info about what that other temperature sensor was for. There some discussion in another forum (I don’t remember which) about how the computer regulates the voltage charge to the auxiliary battery via an algorithm based on the outside temperature.
Something about adjusting load needs. I’ll see if I can find it again. They were talking about a gen 3 Prius though.

 

It was Prius chat. A post by Ken1784.

 

It was February, gen three technical discussion.

 

ken1784SuperMID designer
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I have seen several Gen II owners saying that 14.7 is high. I have seen 14.6 to 14.7 quite often on my Gen III from day one. No problem with my 12V battery to date.

Apparently the Gen III runs higher than the Gen II.
The Gen3 auxiliary battery has temperature sensor and Gen2 one does not.
The Gen3 controls the 12V charge voltage depending on the temperature and charges more efficient than the Gen2.

Ken@Japan

 

A interesting development with regarding mileage happened. I remember that I was having trouble with lower headlight bulbs blowing out. The dealer told me there was a recall on the electrical harness leading to the bulbs. That Toyota designed a fix with wires that had a higher resistance. Since I like to drive with the headlights on I never thought that they would be contributing to losing mileage. But, After the trip to the junkyard and after repeatedly checking the 12 Volt charging system and noting low voltage readings ( 12 and no higher than 13.6). I tried driving the hour back from the yard with the lights off.
And sure enough, the mileage shot up and stayed up. I measured the 12 volt drain effects by testing the readings with a full load ( lights, a/c and brake, radio etc.) And with no load, but it showed little difference.
I surmised that perhaps because of the New increased resistance in the new fix for the Lights, caused The original factory standards of voltage requirements needed for 12 volt system to be efficient was being undermined. And over time the expected voltage was never able to be supplied to the auxiliary battery and so starving the ecu and other components Which may have caused the mileage to go down.
I will continue to monitor the 12 volt system to see if my hypothesis is just smoke or not. Thanks for the trophy .

 

Update about mileage. I’ve been using 0-16 oil and that helps the mileage quite a bit. As to the other mileage posts I made, naw! I can report a mileage average of about 42-46. I’ve also decided that if it eventually throws a 401 code, I’ll deal with it then.

 

Haven’t done anything about the egr cooler yet. I was researching the possibility of changing it without removing the wiper motor. Tight squeeze but I think it’s possible. I’m at 177k and still avg. about 40-44 mpg.