Sudden Significant Drop in MPG!!! 50 to 40 overnight!

There are a lot of things that can keep the car out of EV mode. Some of these are... cabin temperature colder than the temperature set on the climate control, low (2 or 3 bars) EV battery, high electrical drain (e.g. heated seats), engine not up to temperature, heaving braking demand, sudden braking, repeated braking in quick succession, being stopped on an incline, hybrid battery too hot (check the EV battery air filter in the cabin is clean and not blocked) or too cold, too much accelerator demand... I've probably forgotten tons of them.

Anyway... if it genuinely feels different, I'd definitely check the cabin EV battery air filter, given the weather.

 

Them new tires?

 

You are probably already aware of this but I’ll mention it anyway, that wonderful Texas heat and humidity could be playing a part in this, especially if the hybrid battery filter needs cleaning or something is blocking the filter intake grill (restricting air flow into the battery). There is also a air duct underneath the front passenger seat that needs to be clear for the cool air to have a good shot at the filter inlet.
I mention all of this since I just noticed that with the higher temperatures here even in Illinois that my 2022 has a tendency to favor the ICE over the EV mode more than in cooler temperatures, even without the a/c running.
The other thing I noticed is you didn’t mention cleaning the throttle body, which with the non-TopTier gasoline you’ve been running is more likely to need it. FWIW

 

Have you checked your brakes? Could they be dragging slightly (an easy check would be to check the wheel temperatures). I mention this because a few years ago when I lived in your area with a different vehicle, my brake fluid was bad (I obviously didn’t know it at the time) and in the summer heat the brakes started dragging.

 

Yeah raise the rear and see how free the wheels spin (with parking brake off, of course...). There's a link in my signature, how freely they should spin, and why they might not. It's says 3rd gen, but equally applicable to 4th I believe.

 

It sounds like you may be describing my Low MPG Mechanism that I call WEAK and MAX EV States. it's kind of a schizo-post but has lots of useful information that I am in the process of refining.

Basically what happens is after a 12v disconnect or ECU reset that erases certain learned parameters... the Prius significantly reduces EV mode output to about half of what it should... supplementing Combustion Engine Torque in place of what normally would be supplied by the Electric Motors in EV mode. Until such parameters are re-learned by the ECU's... the Prius will stay this way permanently

I still don't know exactly why this happens or how to get the parameters to relearn but I am in the process of making a video about this.

What's strange about your post though is that it doesn't sound like the 12v was disconnected beforehand so maybe it's something different.

If you are in a very hot climate... a similar EV mode limitation will be placed if it the HV Battery is overheating so that could be the cause as well.

Posted via the PriusChat mobile app.

 

papppy?

 

Has your low mpg been occuring since roughly the same time as the 12v Disconnect? This would give good evidence to that it is related to the 12v disconnect.

You can also see this Imgur post with screenshots of others effected by this Low MPG Mechanism.

Posted via the PriusChat mobile app.

 

are the brakes okay?

 

no doubt they are working hard at it. all the best!

 

Are you sure it's not the Texas heat? I also stated in my original reply that it could be the heat.

If you are experiencing temps in the 100's or close to it with direct sunlight all day... the Hybrid Battery can easily overheat.

When the Hybrid Battery overheats EV mode will be greatly throttled back or stopped completely along with regenerative braking.

On my Gen 2 NiMh HV Battery... this starts at 130°F.

Have you used Dr. PRIUS or a similar OBDII app like Torque to monitor HV Battery temps? Just something to rule out.

Posted via the PriusChat mobile app.

 

i wonder if some of the cells have become weak, might be worth a health check

 

I’ve got a gen3 but run scangauges to monitor the battery temps. There is a gauge called BT2 that checks one of 3 battery temp sensors - this one seems to usually be the hottest and triggers the car to do different things. Above 104, the fan kicks on. Above somewhere around 120, the car goes into a protect mode where it won’t charge or use more than about 20 amps at a time. I don’t know what these values are for the gen 4, but I’d be willing to bet that has something to do with it as the last couple weeks I’ve seen as high as 124 even pulling out of the garage and often start my drive in protect mode. I’m also down about 5 mpg at least.

Only thing I can tell you to try is parking in the shade or using sun shades in the car. If you’re already doing this, it’s probably just an unfortunate part of the Texas heat. One thing I have noticed is that the car’s internal temperature in terms of how hot it feels to me doesn’t always correlate with this value, instead it’s how much sunlight the battery compartment in the trunk is getting that seems to matter most.

 

Have you checked the dates and vehicles that were involved in the posts TheLastMojojomo sent you to?
Have you even taken it in to have your theory checked? Perhaps you are right, but then perhaps not. You wanted suggestions as to the cause but don’t seem to have followed up on any of them (such as checking your brakes, which in the Texas heat combined with bad brake fluid are legitimate possibilities, (“brakes good”???? What does that mean? Have you checked them or had them checked since this started?)).

Don’t mean to come across as overly critical but you are accusing Toyota of ignoring a problem that, at this point, you only suspect might exist, (and that suspicion SEEMS to be based only on something you read on the internet) and haven’t even turned to them in checking it out.
Please correct my observations if I am missing something here.

 

OK: I’m confused: When did you camp out in the car? After you changed out the battery, right? Was it before the sudden drop in mpg or after the drop?