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  1. #1
    Senior Member Cyclologist's Avatar
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    Diagnostic engine codes PO171 and PO174 on flex fuel vehicle

    I've got a 2000 Ford Taurus with approximately 83,000 miles. I purchased it used about a year ago, and as far as I can tell, it is a safe bet that it has run on gasoline its entire life. It is a flexible fuel vehicle, so when a station popped up right down the street from us, offering E85, we jumped on it. I wasn't sure how it would take to E85 so I started out with a few gallons. It ran great. We got horrible mileage before on gas and the car felt sluggish anyway, so this change in fuel in a way added new life to the vehicle. I filled her up and went through a tank, then filled her up again. Just this morning, though, the check engine light came on and after taking her in the Auto Zone to pull the trouble codes, it says PO171 and PO174.

    PO171 - system too lean (Bank 1)
    PO174 - system too lean (Bank 2)

    After pulling the codes, I had them erased, so it will be interesting to see if they reemerge. Any ideas on what trouble I might have caused by switching to E85. Should I wait to see if the check engine light comes on again, because perhaps it was a fluke? Could it have been merely a temporary fluke? Will the engine/computer adapt to the new fuel? Should I switch back to regular gas after this 3/4 of a tank is exhausted?

    I would prefer to continue using E85 since it burns cleaner. We ran about 170 miles on 10 gallons of the stuff, therefore we got about 17 mpg, which is perceivably not any worse than what we got with gas.

  2. #2
    Senior Member Cyclologist's Avatar
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    It didn't take long before the check engine light reemerged, so it was apparently not a fluke. I suppose the question is what long term consequences can emerge as a result of running too lean.

  3. #3
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    Quote Originally Posted by Cyclologist
    It didn't take long before the check engine light reemerged, so it was apparently not a fluke. I suppose the question is what long term consequences can emerge as a result of running too lean.
    Lean engines run hot, so burned exhaust valves are common. spark plugs will detiorate quicker. Anything related to excess heat in the combustion chamber. I'm guessing the car wont pass a smog test. Try mixing half and half.

  4. #4
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    While these codes could mean a problem in a number of places, I agree that it was triggered by the fuel. I know very little about flex fuel vehicles. But the very first thing I would do would be to change the fuel filter. It's cheap, and shouldn't be too much of a job to change. If that does not solve the problem, I'd start looking at the flex fuel sensor that monitors the fuel to adjust the calibration when needed. I have no idea where this sensor is located, but there is one some where. If it's bad or out of range and cannot tell the PCM that a different fuel is being used, the PCM cannot adjust it's strategy for E85, thereby causing a lean condition.

    When it sees this lean condition, the PCM should be reacting by adding fuel. When the PCM does this, it's not going to fix anything, but it should prevent any short term damage. Don't be shocked if it starts throwing a rich code when it starts trying to compensate by adding fuel.

  5. #5
    Non Tribuo Anus Rodentum and off to the next adventure (RIP) Stacey's Avatar
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  6. #6
    That darn Yankee TexasGuy's Avatar
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    Quote Originally Posted by Cyclologist
    It didn't take long before the check engine light reemerged, so it was apparently not a fluke. I suppose the question is what long term consequences can emerge as a result of running too lean.
    Your OP stated that the mileage on the car was bad. Were you guy's tracking the gas mileage and if so what was it?
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  7. #7
    Senior Member Cyclologist's Avatar
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    Quote Originally Posted by TexasGuy
    Your OP stated that the mileage on the car was bad. Were you guy's tracking the gas mileage and if so what was it?
    Yeah, on a 14 gallon fill-up, we'd normally pull about 220 miles tops. That would mean we were getting 15.7 mpg at most.

    Interestingly, during this second tank of E85 when the check engine light came on, the gas meter was dropping rapidly, until at about 3/4 of a tank I filled up the rest of the way with gas (~4 gallons). Now, the gas meter gas been dropping at a more normal rate. I wonder whether with this 3:1 fuel mixture (E85:gas) if it still running lean, even though the check engine light is still on.

  8. #8
    That darn Yankee TexasGuy's Avatar
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    Quote Originally Posted by Cyclologist
    Yeah, on a 14 gallon fill-up, we'd normally pull about 220 miles tops. That would mean we were getting 15.7 mpg at most.

    Interestingly, during this second tank of E85 when the check engine light came on, the gas meter was dropping rapidly, until at about 3/4 of a tank I filled up the rest of the way with gas (~4 gallons). Now, the gas meter gas been dropping at a more normal rate. I wonder whether with this 3:1 fuel mixture (E85:gas) if it still running lean, even though the check engine light is still on.
    Not a car engine expert, and I'm going to assume for the sake of argument that a Ford should be up to par with its GM counterparts, albeit I would never say that. My 99 Chevy Malibu (4 door sedan) gets 21-25mpg still to this day. I'f you're going 15 miles to the gallon I'd guess that something is faulty/old and that this is being amplified when running the eth 85. Maybe somebody else can take a guess as to what component may be faulting but 15mpg for a 2k car is scary.
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  9. #9
    Senior Member Cyclologist's Avatar
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    Your absolutely right in that we get horrible mileage; quite astute in your observation. I know I should have taken the car in to have it looked at, but I never did due to a lack funds to do so. I also feared mechanics would most like try a bunch of costly things with null or slighly better results anyway. However after spending a year wasting costly fuel, I just might take it down to the mechanic after all.

  10. #10
    That darn Yankee TexasGuy's Avatar
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    Quote Originally Posted by Cyclologist
    Your absolutely right in that we get horrible mileage; quite astute in your observation. I know I should have taken the car in to have it looked at, but I never did due to a lack funds to do so. I also feared mechanics would most like try a bunch of costly things with null or slighly better results anyway. However after spending a year wasting costly fuel, I just might take it down to the mechanic after all.
    Yep and maybe somebody here could give a better guess as to what is wrong so that when the mechanic talks to you, you can kind of tell whether he is talking BS or not. Might need a better title though or might have to wait a day or 2
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  11. #11
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