Declining MPG (With charts)

Your hybrid battery may be dying. As the hybrid battery ages, it grows weaker and requires the engine to spend more time charging the battery, which reduces fuel economy.

There may be other issues that more experienced Prius owners may know, but that seems to be the most probable cause.

 

I think we experienced poorer mileage as our 12-volt battery was failing after 5 years—what seemed to happen is that the system voltage was staying in the “charging” range of 14.7+/-0.1 volt and that helped keep the ICE more often “on”. After replacing the 12-volt, the system reverted to cycling between the 14.7-volt and the lower 13.6+/-0.1 volt state that indicates the 12-volt battery is fully charged and the overall draw of the 12-volt accessories is low, and our mpg returned to normal levels.
(I Should add that on older vehicles the simple wear and tear of increased mielage would result in worn rings, pistons, and cylinder walls, resulting in lower compression that could ultimately reduce the overall efficiency of the ICE, so lower mpg over a car’s lifespan was expected.)

 

Agree with post number two, could be a weakening hybrid battery.
Have you done any engine maintenance besides plugs and fluids?

 

The two primary causes are:
It is 10 years old and has travelled 162,000 miles.
Do you REALLY expect it to still perform like it is new ??

There is a LOT of information about this scattered all over this forum in hundreds of theads.
A little searching would likely yield all that you need to know.

 

FTFY

It's been slowing dying since the day it was made but as experience has shown us once they get past the 10-12 year time frame death can come at any time and decline becomes more noticeable in several ways: lower mpg's, less power, more frequent ICE start ups, SOC fluctuations, etc.

 

MAF sensor cleaning and run a few bottles of fuel injector cleaner is always good too.

 

throttle body, injectors, test the 12v to be sure, tires pressure, oil level, maybe think about checking inside battery case for corrosion

 

The system is most likely charging more often as the weakest cell is at a lower level charge and reaching the recharge threshold quicker than the rest of the cells. That's what the computer is responding to. Conditioning and balancing cells should restore some (not 100%) of the efficiency and help the hybrid battery last longer. See if someone in your area has a Prolong system or is very comfortable with cell balancing using hobby chargers and can help you out.

 

Yes, as long as proper maintenance has been performed.

 

Good luck with that.
It's not realistic......with any vehicle.

 

Every vehicle I've driven has been able to easily exceed it's EPA rating regardless of age and mileage..

 

Which is quite different than saying that they all run exactly like they did when new.

Some hold up better than others.

 

Many older Prius perform very close to new.
Another suspect is brake drag

 

Fuel economy is only one point, yes, and if you remember, that was the symptom being falsely diagnosed as normal aging of a car.

You own a Toyota. Not sure why you disagree considering both Toyota and Honda have remarkable track records for reliability..

 

I disagree with the impression that you seem to be saying that an internal combustion engine should never wear out.
And that it should run exactly like new right up to the time it falls apart.
Neither of those things is true, no matter what the make and model.

Some things are (reasonably) fixable and some are not.
Most things that are "diagnosed" in places like this are just wild guesses.
Some turn out to be right and some not.

 

I agree...left to its own devices no ICE will last forever, or run like you. But if you properly maintain it and replace things as they wear out, they will last for quite a long time, and run almost like new. Maintenance says more about an engine's health than it's age.

 

My father had a 1st generation Chrysler mini-van, a Dodge Caravan, that he put over 385,000 miles on. He got a letter from Lee Iacocca congratulating him for running it so well. I did most all the maintenance on that vehicle. It ran just as well on it's last days as it did on it's first.


Maintenance is key.