display shows MPG decrease while braking

even tho you were braking, i suppose the ice could have been warming up or recharging the battery.

 

battery at full green? really?

 

how long had the ice been running from cold, mpg's are pretty low in the warm up cycle. why was your battery full, had you just come down a large hill and how many miles on her?

 

Even with a full battery, if the ICE is warming up, your MPG will be low. Nothing to worry about.

 

In normal regen after warmup, the ICE will shut off and the display should show 99+ mpg. But during warmup, the ICE should be burning fuel at a nearly constant rate. That warm-up fuel is counted in the calculation, so as you slow down, so does the computed mpg.

 

OK, so 99 .. 79 means we're not exactly talking low MPGs here right. This is very simply the engine idling. Could be due to being cold and/or cabin heat requirements, or could simply be "stage 3a" (See link).

In any case the drop in MPG when slowing is precisely what you would expect for an engine that is idling. Think about it, fuel consumption of an idling engine is constant (independent of speed), so the slower you go then the more fuel is consumed to travel any given distance. With L/100km (metric) fuel consumption units I see this literally every day.

Link: http://www.techno-fandom.org/~hobbit/cars/five-stages.txt

Edit: fuzzy beat me to the punch. What he said.

 

During the warm up period, the car tries not to use the engine for power, so there are no arrows from the engine on the MFD even though it is turning over and burning fuel. I see the behavior you are describing every trip I make. The initial phase begins with a long downhill section, all braking, engine warming up, and some variable MPG on the display depending on how fast I am going.

 

it could be the sign of a bad cell or two in the battery mains.

 

No, if it's not supplying power to the wheels then it doesn't register. Like nh7o, I see this every day. It's like you were telling me that your wheels are turning when you drive, it's literally that ho-hum.

 

Yeah, or it could be that they accidentally put deuterium oxide in the radiator instead of water and it's just about to go critical. *Or* it could be something very mundane that many observant Prius owners see almost every day.

 

see, the deuterium oxide problem isn't as prevalent over here. but anything's possible, i guess.

 

That is why I don't pay much attention to the instantaneous MPG on the power distribution screen. The long-term average tells me what I need to know. And it says 48.2 MPG today, which is just peachy.

 

LOL bisco. I'm just making the point that there's no need to go for worrying or extravagant explanations when there's a very simple and logical explanation already at hand.

 

agreed, my idea of gallows humour.

 

BTW. There's a minimum speed at which you can detect the engine idling by watching the instantaneous MPG. Fully warmed the engine only uses about 0.2 GPH (gallon per hour), but it can be considerably more when cold.

The MPG due to the idle GPH can be calculated as MPG = MPH / GPH. So for example at 15 MPH the MPG at fully warm idle (stage 3a for example) would be about 15/0.2 = 75 MPG. Above 20 MPH the MPG due to idle GPH is likely the get pegged at 99.9, which is why you cant always notice that the engine is idling just by watching the MPG.

When the engine is warming the idle GPH is likely to be somewhat higher, so you'll probably notice the MPG drop if you travel below about 30 MPH.

On Prius that use the metric L/100km unit of fuel consumption it is much easier to detect the engine idle fuel consumption, as the L/100km increases as the speed decreases.
With this unit you get L/100km = 100 * (L/hr) / (km/hr). So if for example the prius was using 0.8 L/hr at 20 km/hr the idle fuel consumption would display as 100 * 0.8 / 20 = 4L/100km. Again this figure could be higher at cold idle.