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Too Lean code, misfire 1/3

Discussion in 'Generation 1 Prius Discussion' started by mohater, Sep 23, 2022.

  1. mohater

    mohater Junior Member

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    Hey Folks,

    Thanks again for all the previous help. Got another donated Prius trying to resolve some problems on.

    2001 Prius, 218k on the odo. Has persistent Too Lean Code (P0171) upon receipt, can reset it and it will finish all monitors for smog, but the code will eventually come back. Occasionally the vehicle will misfire

    More recent update after putting some work into it:
    New air filter, new spark plugs, car misfires more now...

    Couldn't find any vacuum leaks, O2 sensor seems to be working fine (heated and bank 2). MAF sensor from OBDII reader looks find. Mechanic suggested a throttle body from a junkyard (sticky plate issue), he's cleaning and installing.

    What else should I check?

    Thanks!
     
  2. bisco

    bisco cookie crumbler

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    Were those oem plugs?
     
  3. mohater

    mohater Junior Member

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    Yes, the shop only uses OEM parts for cars.
     
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  4. Josey

    Josey Active Member

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    "O2 sensor seems to be working fine (heated and bank 2)"

    More clarity would be good there. There is only one bank, and it's bank 1. And there is only one upstream sensor which is in question here (b/c it's reporting the lean condition). It is a heated sensor - no separate heated sensor. Same sensor - with a heating element for when it's cold.

    What - exactly - makes anyone say it "seems to be working fine"? Tech-y details are good. If that sensor is old, I'd just toss a new one in. Even if it's not the issue, those things don't last forever and do just decline over time - and are crucial for air/fuel mix and thus how the car runs and it's fuel economy. It's not a bad maintenance item.

    Misfires can also give you a "fake" lean code - seems backwards like it should be rich. But O2 sensors only read the O2. If the mix doesn't blow up it's technically rich, but all the O2 sensor reads is the unconsumed oxygen.
     
  5. mohater

    mohater Junior Member

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    Thanks for that. Working fine meaning the voltage is moving as one would 'expect'. The sensor is probably the original. Will talk to the mechanic and get one installed.

    Another recent finding: the car misfires when it is warmed up, not on cold start.
     
  6. Josey

    Josey Active Member

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    Are there actually misfire codes, and of so which one/s? Just a P0300 (random / multiple)? Or specific cylinder/s? I'd also go ahead and clean the MAF. (It's quick & easy and can't hurt if you're careful not to touch the sensing wires with anything but MAF sensor cleaner). I'd also have a look at the readings out of the coolant temp sensor. Longshot, but it could be reading wrong, but not so out of spec that the computer would necessarily see it as an "error." I'm pretty sure the OEM thermostats are 185F. So you could compare coolant temp reading at cold start - should be approx ambient air temp, and then after it's fully warmed up it should be around 185ish - and, of course, the temp climb should be reasonably smooth and steady.

    Before going too nuts, though, I'd throw in the O2 sensor and see what happens. I don't know the specific details for the Prius, but it's typical that the car starts in open loop, not using the O2 sensor info right away. Then once all warmed up, goes to closed loop using the info from the sensor for fuel mix. That's probably not too long though, just because they're heated O2 sensors so should be up to operating temp pretty quickly.

    Throttle body cleaning is also never a bad idea...
     
  7. mohater

    mohater Junior Member

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    Cleaned throttle body, new denso o2 sensor, problem persists. Cleaned MAF as well.

    Codes are specific: too lean, misfire cylinder 1, misfire cylinder 3

    Anything else to try?
     
  8. dolj

    dolj Senior Member

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    The plug coils and then the injectors. I'm surprised your mechanic has not already done this. It's pretty basic for P0171, P03xx.

    What do the fuel trims look like?
     
  9. Josey

    Josey Active Member

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    I'd swap the 1 & 3 coils w/ 2 & 4 and see what happens.

    How hard is it to get to these injectors? I never looked. If it's easy and the misfires don't move w/ the coils then I'd follow with the injectors.

    (Actually, if it was easy I'd already have said to check fuel pressure and compression. But there's no test port for the fuel system - dumb! And compression? IDK? Disable ignition & fuel and keep turning the key? I'm still just used to "normal" starting systems).
     
  10. ChapmanF

    ChapmanF Senior Member

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    There's a feature built into the HV ECU to do exactly that. You activate a "compression test" in Techstream, and it keeps fuel and ignition disabled, and uses MG1 to spin the engine at a low RPM similar to a conventional car's starter cranking, so a conventional compression gauge gives you the right numbers.

    If you don't have Techstream or that sounds like too much fuss, a leakdown tester allows you to get similarly informative results without cranking the engine (an air compressor is needed, though).
     
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  11. Josey

    Josey Active Member

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    Excellent info on all counts (though I tried once and failed to get a Techstream version up and running. One of these days...)

    As for the leakdown, the cylinder being checked needs to be at TDC on the compression stroke. Can this engine be turned by hand without causing any problems? I'd assume so, but I don't like to assume...
     
  12. ChapmanF

    ChapmanF Senior Member

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    Sure can. In fact, it's small enough and low enough in friction that, with the plugs removed, you can turn it literally by hand.

    I would always want to turn it in the normal direction (same as the forward direction for the wheels).

    Double and triple check that the car is not in READY before grabbing it, of course.
     
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  13. mohater

    mohater Junior Member

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    Thanks all for the tips and feedback.

    Gonna try swapping the coils, followed by the other advice in the order of time/difficulty.

    Will follow up soon.
     
  14. mohater

    mohater Junior Member

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    Hi Folks,

    Closing this one out. The following items were done to resolve the too lean code:

    New plugs (didn't resolve the code)
    New coil packs (didn't resolve, returned)
    New upstream O2 (A/F) sensor (didn't resolve)
    Try a different used MAF (cleaned and installed)
    Try a different used VVT solenoid
    Try a different used throttle body (after cleaning before replacing)

    Too lean code still present.

    Bought the el cheapo fuel injector cleaner tool from Amazon


    Replaced all o rings and cleaned all fuel injectors, few hundred miles and no more too lean code. The too lean code used to show up on the third drive cycle before.

    Did have one instance where the car misfired on cylinder 1 (before with persistent too lean, misfire was on 1 and 3). It went away after less than a minute and has not recurred.

    I'm pretty certain the fuel injector is the culprit. Will order some and replace, but pretty sure the mystery is solved.

    Thanks again to all for the replies.
     
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