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Sudden change in gas mileage

Discussion in 'Gen 2 Prius Technical Discussion' started by Syclone, Jul 19, 2006.

  1. Syclone

    Syclone Member

    Joined:
    May 9, 2006
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    Vehicle:
    2006 Prius
    Model:
    Five
    As you can see from my summary, I've had my Prius since the beginning of May.

    I now have 3,500 miles on the car, and I guess it's "broken in".

    On long trips I've been averaging 48 - 49 MPG. I took a trip from Long Island (NY) up the NY Thruway and Northway to Lake George, NY - then back down to Albany - over to Rochester, NY (again NY Thruway). This was a total of 530 miles. I don't try to get maximum MPG - I keep up with and maybe go a little faster than the average driver 73 - 78 MPH. The car got 48.5 MPG when I filled up in Rochester. I love it!

    My problem is short runs - like driving 10 minutes parking, coming back to the car 15 minutes later - driving another 15 - 20 minutes, etc. During runs like this I'm hard pressed to get over 40 MPG. Not only that, but something very strange happens to the cars fuel economy. When I start off ( By the way - during this test outside temps were well over 90) even coasting at 40 MPH can't get the economy over 25 - 30 MPG. Suddenly, between 10 and 15 minutes out, the fuel economy jumps up into the 50 range and continues like this. As soon as I stop and shut down - then return to the car the whole process starts all over.

    When the economy jumps up there is a preceptable feeling in my foot on the accelerator like an automatic transmission shifting into overdrive. I find that I can lift up on the pedal and still maintain the same speed that I was cruising at, but at much higher gas mileage. This happens at speeds below 50 while driving on local roads with lots of stop lights and traffic. It also happens at highway speeds, but is not quite as noticeable.

    Now I know that this is some kind of "warmup function" in one of the ECU's, but how much can the ICE cool down in a 5 - 10 minute stop with Ambient temps of 90+? Is this possibly an ECU function that can be changed?

    I did a search, but was unable to find any question/answer that fit this particular problem.
     
    #1 Syclone, Jul 19, 2006
  2. tomdeimos

    tomdeimos New Member

    Joined:
    Feb 16, 2004
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    Location:
    Lexington, MA
    I have the problem daily with my 10 mile commute.

    Not a lot you can do about it. But if you can alter your route a bit try to start out with a quick warm up going at speed or up hill, and not sitting at a traffic light.

    Your AC load is significant and could easily be worth 10 mpg or more if you aren't driving at highway speeds. I find the car cools off and load drops in around 20 minutes to 1/2 hour. Then the cost of AC drops to maybe 2 mpg if going over 50 mph.

    A short stop should not require the full warm up cycle over again, but the car cools off pretty fast.

    Read here about the stages of warm up. Gettting to stage 4 is critical as soon as possible if you are not driving on a big highway.
    To do it right you need a temperature gauge. I have can view in my 2005 Prius. If I am on back slow routes, I often pull over and park for 10 seconds just to let the car switch to stage 4, but you have to time this when the engine temps are up over 160 deg.

    When weather is over 90 deg and I want to be comfortable with AC I always try to commute on the highway and stay over 50 mph.
    This is one case where faster gives better mpg. 70 mph would be even better probably if you can do it. AC cost in mpg is inversely proportional to speed. You can compromise here by switching to manual control on the AC and running it at lower fan speed.

    Just to give you some perspective, here are some typical mpg's I get going to work and coming home after parking in the sun all day.
    For weather typically 75 in am and over 90 in pm.

    going to work mpg could be 55 to 60 mpg
    coming home typically would be 47 to 52 if there is no traffic jam.
    When traffic gets stuck, mpg can drop to 30 or below. I got 16 mpg once, but that was with a 2 hour traffic jam.

    With milder weather like 70 degrees I can get close to 70 mpg going back routes to work and over 60 mpg returning home.
    Or on the highway I can still average over 55 mpg.

    Now if in the 10 mile commute I stop just once to go to a store for a few minutes, my mpg cost would be a min of 5 mpg of from the above.
     
    #2 tomdeimos, Jul 19, 2006
  3. Syclone

    Syclone Member

    Joined:
    May 9, 2006
    540
    4
    0
    Vehicle:
    2006 Prius
    Model:
    Five
    <div class='quotetop'>QUOTE(tomdeimos @ Jul 19 2006, 05:32 PM) [snapback]288949[/snapback]</div>
    Not good, but at least I know that I'm not crazy. Well - maybe I am crazy, but that's the way the car works.
     
    #3 Syclone, Jul 19, 2006
  4. troe

    troe New Member

    Joined:
    May 11, 2006
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    <div class='quotetop'>QUOTE(syclone @ Jul 19 2006, 05:17 PM) [snapback]288972[/snapback]</div>
    I have had a lot of luck lately getting to the stage 4 mode by getting up to 35 MPH fast and using fast accels after stop lights. You always get bad MPG until the engine gets to the magic temperature, and it will get better the harder you work the engine. I try to pulse and glide as I go and it is obvious when the arrows will go to black that you have got up to temp. About 4 or five minutes if I don't baby the engine. I can get a 70 to 100 MPG bar on my second five minutes using this approach. Which I could pulse and glide during my whole commute.
     
    #4 troe, Jul 20, 2006
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