Prius Gen2 now getting ~30mpg - help!?!

Did you check the 12v battery voltage yourself or just going by what the dealer tells you?

Go check the 12v battery voltage using a digital volt meter after the car has sat for 8hrs or so.

If the 12v battery is failing then the engine will stay on when it should be off because the HV battery and inverter is constantly trying to keep the 12v battery charged. It is common to lose MPG due to a weak 12v battery and the dealer often says it is fine even when it is not. Sometimes they will see the float charge and assume the 12v battery is fine but after the float charge has dissipated after sitting overnight you will find the battery is putting out less than 12.6v.

If you are seeing less than 12.4v with the car completely off and after it has sat for 8hrs then you should replace the 12v battery. This will also eliminate one potential reason for your mpg loss.

 

If you have the original 12v battery in there - replace it right now with an Optima Yellow Top. I think F8L has it pegged ^^ - you ARE only running on the gas engine to get that bad of mpg.

 

Optima now makes a yellow top for the Prius that is a drop in replacement. It should last better than the oem battery. 12V failure is common at 4-5 years. Once that's done, the car either behaves normally or it doesn't. IOW, the engine should shut down in glides once a certain coolant temperature is reached (see warmup stages thread). You ought to get a scanguage II to watch all the variables. Your gen II has a built-in battery test procedure for the 12V that went away in the gen III, so you don't even need the digital volt meter F8L suggested to get some data there.

As you know, winter tires, winter weather, low tire pressure, wind, mountains, etc. all lower your results. But you should be able to test on a clear, mild day and get EPA results as well as you ever did, subject to those factors.

 

No. It has been shown over the years that the dealers have not diagnosed the 12v battery correctly.

It is a wear and tear type item and is not likely covered but you can ask. Some batteries can fail VERY quickly if you run them down a few few times by leaving lights or the stereo on while not in Ready Mode.

You can do it yourself rather easily. It is a 10-30min job if you get the new pencil post battery and a bit longer if you use the standard D51 battery and use the conversion kit of elearnaid.com.

Here are the instructions:

[FONT=arial, helvetica]Prius battery test procedure for 2004 to 2009 Prius Cars (just remember what I said about the float charge. Ideally you want the car to have sat for at least 6hrs to dissipate the float charge)


Here is a battery test you can perform on a Prius battery that is easy to do. It will not always detect a weak battery that does not allow you to park you car for long before it is drained down by the security system.

However, this test requires no tools, is easy to do and will give you some idea of the state of your battery. You can also perform this test after the car has been parked for a while to see how well the battery holds up under the drain from the security system.

• Do this test after you have driven the car for at least 30 minutes. This will give the battery a chance to be fully charged.
• Put the transmission in Park.
• Engage the parking brake
• Push in the power button. (if not already on)
• Press and hold the "Info" button on the MFD while turning the headlights on and off 4 times.
• Push on Menu in upper right of display screen.
• Push on Display Check
• Push on Vehicle Signal Check
• You will see the 12 volt voltage displayed. In accessory mode the voltage should be 12.6
• With no electrical accessories on including lights the voltage should be fluctuate between 12.6 and 11.9. The lower the voltage, the less of a charge your battery currently has. If the voltage is low do not proceed as the load test might totally discharge your battery. (If you see a voltage around 13.8 you are seeing voltage from the high voltage battery being converted to around 13.8 in an attempt to recharge your battery. This normally does not occur till after you start the engine but might occur earlier if the battery is very drained.)
• You can also test the battery by turning on the headlights, rear window heater and the heater fan. For a new battery the voltage would be around 11.3. If the voltage drops below 10.2 it should definitely be replaced. For voltages in between the lower the voltage, the lower current charge of your battery.
• Push on the break pedal and push start. You should see Ready in the Speed display. The voltage should now read between 13.8 to 14.2 and higher in winter. This is the charging voltage!
• Note: If in doubt disconnect the 12 volt battery completely (start with the negative post to avoid shorting the battery) and hook it up to a battery tester.

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I assume Oregon has all 4 seasons which means right now you are in Winter. If you are seeing 40 degrees or below, chances are your "poor" fuel economy comes from one of or a combination of the following:

1) winter gas
2) increased warm up time (more run time on the ICE) (biggest factor)

My prius gets 42+mpg during summer and now has dropped to 35-38mpg even with upper grill blocking

but I still prefer 35mpg over my other car's 17mpg

 

My pleasure.

Generally speaking no BUT the MAF sensor wires could be dirty and that could cause a loss in mpg and combined with a dirty air filter could make matters worse. If all of these items (12v battery, MAF, air filter, low tire pressure) need to be replaced/cleaned/adjusted then you could definitely see a 5-10mpg decrease! The fuel system should be fine with your mileage so no additional cleaners are really needed.

You can clean the MAF yourself but you must be careful not to damage the fine wires.

http://priuschat.com/forums/gen-ii-...leaning-mass-airflow-sensor-instructions.html

http://priuschat.com/forums/gen-ii-...roubleshooting/57617-cleaning-maf-sensor.html

 

Please report back with your findings. This especially important if you find the cure. It will really help other owners and may save them a lot of money.

Thanks!
Justin

 

Ohh and driving like a maniac can drastically drop your mpg. I decided to see how bad my mpg could get so I slapped the 17" wheels back on and have been driving aggressively with no mind towards fuel economy as well as running the heater on high. My result? I'm sitting at about 36mpg with 150mile on this tankful. I could normally get 46mpg in the current weather (30-50degrees and dry) conditions or even 48mpg if I tried hard. With the 15" wheels I could get 52+ in these conditions.

 

if you are getting 30mpg come summer, something is definitely wrong and at 55k, you aren't ready for a tune up just yet...

bad gas? have you tried running a bottle of Chevron Techron fuel injector concentrate?

 

As you read yourself and others have stated already, there's more to blame than "winter gas". See http://priuschat.com/forums/gen-iii...mileage-gets-worse-in-winter.html#post1411488.

Besides checking the 12 volt via the procedure already mentioned, I think we need more info. Please answer the questions at http://priuschat.com/forums/gen-ii-...-answer-these-questions-esp-if-youre-new.html.

 

+1 and FWIW:

Earlier today, I was at my Toyota service dept. for my 5000 mile
service at 65K miles; oil, filter, air filter check, tire rotation, front
wheel balance.
(No, never, not ever a dealer xxK "service package." :nono

I've been thinking about a throttle body/MAF cleaning "just because."
Before I do it myself, I asked what it would cost for them to do it.
Well, all they would offer is the "induction system servicing," for a
mere $160... What the hey? Apparently they take off the
entire induction system from intake to the intake headers (or
whatever) and clean it with unicorn urine or other magic elixir.

"No thank you," says I, thinking I can do both the throttle body cleaning
and MAF cleaning if needed for $25 or less.

Instead, I had the "transmission fluid" changed, which I've been
planning to do between 60 and 75K miles.

It's cold and rainy now, so I'll lift the air filter when things dry out and
see how the throttle plate and MAF look.

 

:grin:

 

Absolutely, DIY. It is incredibly simple. Can of Maf spray was about $3.00, one can actually wipe the walls of the air throat down manually also with a clean cloth. Gives you a good idea of condition. If you take the MAF sensor out, handle carefully. Lubricate spring or throttle mechanism. These are all things that the dealer probably never does on regular services. For the most part the services that are "checked" are visual and are to use a term from my working years are "pencil whipped". :cheer2:

 

I used expensive gasoline most of the time and it reflect on the mpg. Typically a Shell station is more expensive than a Conoco or a Murphy or a 7-Eleven. The differences are even higher when you use premium for your SUV.
Toyota synthetic oil is better than most aftermarket oil. Possibly you used cheap oil & gasoline that result in build up of gunk in the gasoline engine?
If your car battery level on the display is always low, then you have a problem with your regenerative power, possibly your battery and re-gen. brake system.
Then lastly you could have purchase cheap tires that result as more friction and more tear. It is like buying a shoe. You got to stick with performance.

 

Disregard this post. It is confusing and worse is not very accurate.

 

Yep, some tires that are intended for "performance" applications aren't necessarily great when it comes to low rolling resistance.