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Help. My car went from 35mpg to 25 mpg in 1 tank!

anon70anon70 Member Posts: 82
edited March 2014 in Chevrolet
I have a 2002 Chevy Prizm w/15k miles. A couple of weeks ago, I noticed that my gas milage on highway went from 35mpg to 25mpg in 1 tank. I changed the oil and put in a new air filter. Same problem.

Then I used STP super-concentrated fuel injector cleaner. I went back up to 35mpg. Then I filled up again, and I'm back down to 25mpg. I'm using Shell gasoline.



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    Mr_ShiftrightMr_Shiftright Member Posts: 64,481
    You need to measure fuel mileage after a month's worth of fillups. It's very hard to fill up each time at the same car-level, and to the same amount. Also, headwinds, amount of traffic, how you are driving that day, sporadic speedometer error, all affect mileage.

    So you need to average out 4-5 fillups, all from the same spot in the same gas station if possible.

    Of course 10 mpg is a fairly large calculation error but on my car I noticed when I figure fuel mileage that it varies 3-6 mpg tankful to tankful with regularity but the average always comes out about the same.

    Anyway, do this first before throwing money at the car. Your mechanic will appreciate your thoroughness as well as it might tip him off to something.
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    kinleykinley Member Posts: 854
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    anon70anon70 Member Posts: 82
    Took my car in to the dealership. they did a diagnostics. found nothing wrong. o2 and temp sensors ok. spark plugs ok. they have no idea why it dropped 30% and couldnt offer any suggestions.

    sigh..i've changed gasolines (Exxon), but still the same. This is tank #4 that i've been at 25mpg. i've added some more super concentrated fuel injector cleaner. lets see what happens
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    zueslewiszueslewis Member Posts: 2,353
    your fuel filter?? It's a good idea to change it every 15-20K miles.
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    newcar31newcar31 Member Posts: 3,711
    Where do you live? Was it really cold when you recorded the bad mileage? I had a couple of horrible tanks in my Protege here in MN when it got really cold.
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    anon70anon70 Member Posts: 82
    temps were between 30-60 degrees Farenheight in the 5 weeks that i've had this problem, but mostly in 30-40 range.

    but on the warm 50-60 days, milage didnt improve. i still got lousy milage when i filled up.
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    Mr_ShiftrightMr_Shiftright Member Posts: 64,481
    How is tire pressure?

    Is this a stickshift or automatic?
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    Boris2Boris2 Member Posts: 177
    no, no, no... You don't want to change a fuel filter on a prizm. I had '98 prizm (should be pretty much the same as 2002) and was trying to do it ones. The owners manual shows fuel filter replacement as part of the recommended services. Chevy (or Toyota) managed to locate it somewhere inside the fuel tank. To change the filter you need to take off the fuel tank and whole bunch of parts next to it. It costs about $400+ to do this!!! I went through 3 dealers and 5 independend shops and eventually got a copy of manufacturer's memo to the dealers that it doesn't need to be changed... ever. Doesn't make much sense to me, but that's what it said.
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    anon70anon70 Member Posts: 82
    I posted a long list of stuff in the Prizm forum. Here's a cut just to the milage section

    2) i have a 2002 prizm. it's 9 months old w/18k miles.

    i went from 35mpg to 25mpg on highway in 1 tank in january. i've changed the oil, fuel filter, and used different gas stations. tire pressure at manufacturer specs. nothing worked. Dealer says diagostics checked out fine and didnt have any explanation. :(

    it was like that for 2 months (till this week), where i'm getting 32mpg now. the only thing i can think of is that the weather has been constantly warm this week (hi 50's).

    a) i dont remember the temp in dec/jan, but i'm assuming that it was cold before and after my milage drop. if so, WHY DID IT DROP? And why did it drop SO SUDDENLY?

    b) if the week that my milage dropped was because it was colder and stayed colder till this week, WHY DOES MILAGE DROP IN COLD TEMPS? and anybody have a continuous 30% drop?

    3) What does a hood deflector do? it is like those bug shields on pickups? Anybody have a pic of a hood deflector mentioned in #615? THX
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    lrlprizmlrlprizm Member Posts: 15
    ANON70: Been reading your msgs. You've had your prizm for only 8 mos and sounds like you really want to care for it. A drop of 30% in gas mileage isn't unusual in cold weather--especially in January. Not sure where you live, but here in the mid-atlantic, we've been freezing all winter, and yes, your car will eat up gas exponentially faster as temps drop below the freezing mark. I won't get into the specifics, but a cold engine needs more gas to warm it up. Notice how it remains at high rpms longer during the cold start-up? It's also no coincidence you mileage improves tremendously once temps go over 50 degrees. Bottom line: I doubt there's anything wrong with your gas mileage. BTW, you mentioned you changed your fuel filter (do u mean oil filter?). The fuel filter is inside the gas tank and does not require replacement under normal use--especially since your car is still relatively new. Good luck with your prizm. My '95 Prizm is at 147k and going strong.
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