2011 Prius poor fuel economy in AZ

I also have trouble with performance and fuel economy in the recent AZ summers. My 2010 has 82k miles. When battery temps reach above 122°F, things go downhill in a hurry.

Are you tracking fuel economy (FE) with an app/website like Fuelly? I've tracked mine since the beginning of ownership, and the last 2-3 summers have been brutal on FE. Click through on the logo in my sig and find the lifetime graph. Summers take a huge dip for me.


There's a few anomalous drops due to higher speeds or hill climbs, but you can see definite downward trends during the July-Sept periods each year starting in 2012, and more or less worsening every summer. This summer has been particularly brutal w/ the last 3 tanks below 40mpg at the pump.


MPG by Odometer

 

Shade is a luxury often not found here in the desert.

I've edited my post above to show trends that started in 2012, and have worsened every summer.

Inlet isn't blocked, and I monitor the BT2 sensor (per ScanGauge spreadsheet), 45-46°C is when hybrid mode reduces, and anything much above 50-51 means a very sluggish time.

I've seen as high as 58°C parked in the sun 1-2 hours after driving due to heat soak and chemical activity.

@Tideland Prius mentioned to me in another forum that he recently saw this behavior too for the first time.

 

I start in recirc (which is never full recirc), then once the cabin has cooled slightly, switch to fresh if I don't hear the battery fan running on its own. The battery fan sounds speed sensitive, as it runs faster/louder once I'm up to speed, and gets nearly silent when stopped at a light. The engine doesn't stop idling.

I do have a solar roof. I don't suspect that forcing hot cabin air through the battery vent (it is the quickest way out with the battery exhaust port exiting the rear of the car) when the blower is on mid-day helps either.

Also worth noting, when the battery is above 50°C, A/C compressor will be shut off momentarily during acceleration to shed load (verified by ScanGauge w/ AC watts dropping to zero).

I don't imagine that adding any sort of humidity to the battery would help w/ long-term corrosion. It wouldn't be my first choice of approach.

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On my MPG graphs above, you can see two seasonal dips. One slight dip for winter when heater is required, and a more severe dip in the summer. Very late fall and early spring are the best times when neither heat, nor A/C, nor battery heat are factors in FE.

 

I'd keep it on recirc all the time. Recirc still gets air from outside, but it is down to 10%. Cooling 80F+ takes less energy than cooling outside 110F.[/quote]

I don't have a need to change from auto most of the time, but switching from recirc to fresh has shown a 1-3°C drop in battery temp over a 5-10 min period even if the battery blower isn't running, hence the change. At that point, I'm more concerned about getting the battery below 45°C so that the car enters Hybrid mode. That's a far better increase in FE than the ½-1kW of energy used to cool fresh vs. cabin air.

I haven't pulled the cover yet, but have forced a leaf blower through the vent to help force dust free, assuming it's not caked on. I did also look at the fan motor itself, and it was completely clean. No pet hair, no layers of dust, etc.

Part of my reluctance to pull any covers is that given the car is under extended warranty (and hybrid warranty), I want Toyota to resolve this. I've reported it at least twice to two different dealers, both of whom could either not reproduce the problem, claim it's normal, or couldn't find any fault codes via TechStream, thus dismissed it.

 

What does that entail? A simple duct from an A/C vent into the battery intake? I'm not open to anything intrusive at this point.

 

Alignment is fine (adjusted to near-zero toe before summer started). Tires are Bridgestone Ecopia 422s at 42/40.

Winter drops due to the use of heat in the AM and PM running the engine more. It does get down to freezing in the AM hours in the desert. Spring and fall are no A/C and no heat, mostly with windows down and moonroof open unless on the highway. Driving style hasn't changed significantly in the 6 years I've had the car - Eco mode, moderately fast starts to speed, then feather to keep it steady.

12V battery is the only remaining part (probably time to order an Optima), but I seriously doubt it causes the car to fall out of hybrid mode when the hybrid battery is at or above 46°C. Could be traction battery age - I need to find a way to check resistance for each cell, but wouldn't Toyota's TechStream check for that too?

My ScanGauge doesn't accept new PIDs anymore (seems its flash doesn't write new settings anymore) or I'd set them to check resistance. BT Dongle and Torque would be my way of checking - just haven't prioritized getting a new dongle.