10 mpg loss --->SOLVED!

Toyota says 5W-30 is fine at least from -35'F to 110'F. Do you drive outside that range? Recall that after a few minutes of driving, inside the engine it's about 180'F in any case.

Rather than take a chance on some watery oil for low temps I'd get a block heater. That would somewhat improve winter fuel economy as well.

If you worry about how well the oil ages use a synthetic.

 

My dealer put in the 0w30 Mobil 1 without question and said it was great oil for Florida.
0w20 is another issue// if you live in the great white North then go for it. I found that the 0w20 in my Honda hybrid was too thin even though it was the recommended oil. Piston slap on some of the Honda engines is an issue. Be careful.

 

Just remember that the a XwY oil, the first number X relates to how thin the oil is at cold temperatures and the second number Y relates to how thin the oil is at warm temperatures.

You always want the lowest number possible for X (to reduce friction during cold starts), and then just stick with 30 unless the manufacturer recommends something else.

In general, oils with a wider spread between the two numbers tend to be higher quality oils. So a 0w30 oil is normally superior to a 5w30 oil.

 

I've experienced a similar problem in loss of MGP. Was running around 48 to 50 since I bought the car and this fall right after installing new tires mileage dropped to around 44. I took it to the dealer and they ran all the computer tests and said everything was fine. Mechanic suggested that the problem was related to change in gas ingredients for the winter. A while after this the mileage dropped even further to around 39. Back to the dealer, same response but the mechanic added that with the cold weather the gas tank bladder may have lost some of its resiliency.

Specifically asked if the problem could be related to the battery and the mechanic said NO, it tested out fine.

Puzzled, can anybody help with this mystery?

 

No mystery. You installed new tires which are proven to drop mileage at least for a 1000 miles or so and you now have cooler weather and likely winter formula gas. Do some searches on the subject. There is nothing wrong with you car.

 

I'm new to this site and have been experiencing a very similar issue as the original post. Over the past 6+ months my mpg has dropped ~10 since I purchased new in Jan 08 and was pretty consistent until then. After doing some research and discovering this post I did the battery test and found the battery was reporting 11.4v unloaded, 11.2v with a load, and 14.1v when charging. Does this seem to indicate a problem with the battery itself or with something else. I have about 17.5K miles on the car. Thanks for any suggestions.

 

When you say "unloaded", do you mean disconnected from the car? Even when off, a Prius puts a small load on the 12V battery.

11.4v is low for a good charged 12v battery. Normally you should see something in the high 12s, not low 11s. The charging voltage is about right: 13.8v is nominal for a 12v system, but I recall that the Prius runs a tad high.

Tom

 

Thanks for the response. By unloaded, I mean I pressed the power button without a foot on the break so I don't believe the engine is running. Thanks!

 

Not a very old battery, I'd suggest touching it up with a battery minder before giving up on it. I'm hoping to stretch mine another year or so.

- D

 

Is there any way it's a problem with the car and not just a battery that needs to be replaced? Should I take it to the dealer for what I would expect to be something under warranty for free or just purchase a new battery? Thanks.

 

The DC to DC converter that supplies the 12V bus could be dead, but that is highly unlikely. DTC codes would be logged if this were the case, and you wouldn't get very far.

Tom

 

11.4V during the unloaded part of the built-in battery test means a dying battery. If I recall correctly the 12V battery is under the 36 month/36k miles warranty, in which case you might get a new one for free. Was this rather youngish battery ever drained by a "D'ohh!!" event?

 

To the delight of many and mockery of many friends, I ran the car out of gas a few months ago, so I'm not sure if I ran until the draining the battery as well, so maybe.

I need to go to the dealer for a yearly state inspection anyway, so I'll try and get a new battery as well. Thanks for your help.

 

Well, I am replacing my battery tomorrow (it actually died on me; I jumped, and then charged overnight, but it is barely holding a charge/maintaining voltage); my MPG had taken a hit, too, so I am very curious to see if this brings it back. I have a backup to keep the settings, but now I wonder if I should just disconnect and let the ECU reset. I'd LOVE better mileage than I have been getting, well, the last year, but it really dropped the last month or two.

John

 

FYI in my neck of the woods, Toyota 12v replacement battery (the bigger one, for the Smart key) $170 + tax. Will be buying and installing it later today, thanks to all the great info on PriusChat!

John

 

Battery replaced; easy enough. I decided not to use a battery to keep my settings and just let everything clear. Hoping that will help the mileage as well. I also noticed that on my latest oil change they overfilled, so I also will have to take some of that oil out as well. Oh joy. But perhaps this all will get my mileage up to where it should be.

John

 

Overfilled oil can reduce MPGs. One way to avoid that is to buy *three* quarts of oil yourself and give them to whomever to use. Much easier than getting any excess drained out and cleaning the MAF sensor. In any case always check the level yourself before driving away from every oil change.

 

I'm a new Prius owner and somewhat under whelmed by the performance of my Prius. No where near the anecdotal reports from others here on these boards. I've been chalking it up to our Arctic-like weather in recent weeks.

I was excited that this weekend would be my first long trip in the car and the weather was in the low to mid 50's for the trip; I hoped for some mpg returns in the mid 40's but got only around 40mpg. (65mph/rolling terrain)

Long story short, I was still not happy and stumbled on this thread looking for answers. When I checked my battery voltage via the on-board diagnostic it showed 11.4V after sitting overnight. I called Toyota and inquired if a voltage drop to this level after 8 hours was "ok". The Prius tech said anything above 9.5V was fine.

So, what's the scoop?

 

Well, even my brand new battery drops to 11.8-11.9 under load before starting. Unless you are seeing other issues (exclamation point on starting, slow windows, etc. I wouldn't automatically presume it's the battery, but it might be a factor (esp. if you notice your engine not shutting down at most stops). Do you have a gen II or GenIII Prius? Mileage? Proper tire inflation & proper oil level make a difference, too, but on a long trip you really should be getting close to 50, though the rolling terrain won't help. Winter gas will also knock up to 5 mpg off, too, if they reformulate the gas with more alcohol there like they do here in the winter. If your oil is a litttle under full, your tires are at 40, and your gas isn't winter formulated, maybe the battery is drawing a lot. But don't expect Toyota to be of much help in my experience.

 

FYI, the new battery brought up the mpg at least 3-4; need to drain some oil (and find out about the sensor I have to clean?), check my tires, and block my radiator and see whatI do then.

John