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MOT Failed - LAMBDA sensor and Catalyst?

Discussion in 'Car Repairs / Maintenance' started by buckaroo, 16 Aug 2011.

  1. GeoffCarne

    GeoffCarne

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    Like a lot of these sensors a simple clean up with WD40 will correct the problem otherwise europarts are a cheap option for these parts!
     
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  3. Agile

    Agile

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    Many cars have a Lambda ( O2 ) sensor before and after the CAT.

    But does anyone know exactly what the 0.97 and 1.03 figures actually represent?

    With gas boilers we measure the % of oxygen in flue gases.

    I thought that MOTs plugged into the OBD socket on the car now?

    Tony
     
  4. Avocet

    Avocet

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    Lambda is just a convenient name given to the "actual" air-fuel ratio, divided by the "ideal" air fuel ratio.

    The ideal ratio is 14.7 parts of air to 1 part of fuel (for a petrol engine at least). A Lambda reading of 1 would be where the actual air/fuel ratio was 14.7 : 1 (so 14.7 / 14.7 = 1).

    Suppose the car is running slightly weaker than ideal (say 15 parts of air to 1 part of fuel. The Lambda would then be:

    15 / 14.7 = 1.02

    If it was running rich (say 14.5 to 1) the lambda would be 14.5 / 14.7 = 0.99

    For the lambda to be 1.6, we'd be looking at nearly 24 to 1 air : fuel ratio (which is horrendously weak).
     
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  5. Avocet

    Avocet

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    Sorry, forgot last bit. The MOT emissions test is still done using an independent gas analyser rather than trusting the car's onboard sensors (some of which could be dodgy anyway).
     
  6. Agile

    Agile

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    Thanks for that!

    When I asked the MOT guy he just looked a little blank and just said all they do is to measure it!

    So it is actually a pretty sensitive measurement.

    So will a slightly blocked air filter make a big difference or does the EMU adjust to compensate for that?

    Tony
     
  7. Avocet

    Avocet

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    No, a slightly blocked air filter would make it run rich, if anything, (because it would restrict the amount of air going into the engine) but the MOT emissions are only measured with the throttle closed (or virtually closed). That being the case, the air filter is only having to flow a tiny amount of air, so if it was blocked enough to affect your emissions at idle, the car would feel very noticeably down on power at full throttle.

    Does the car actually feel OK to drive?
     
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  9. Agile

    Agile

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    I am only asking for my education.

    With three MOTs to be done in next two months its useful to know!

    Thanks

    Tony
     
  10. mfarrow

    mfarrow

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    The risk of a blocked air filter affecting emissions of a modern car is virtually nil thanks to the closed loop system of sensors and other components which keep the engine running at near stoichiometric air/fuel ratio. Manifold pressure, airflow, or both are used to help determine what the mix should be, and the feedback from the lambda sensors confirm that it is correct and that the catalyst is performing correctly.

    Holes in the system before/after/between sensors allows air to get in/out where it shouldn't and leads to the ECU compensating by injecting more/less fuel. As an example, taking the lambda sensor out the exhaust and just hovering it in air with the engine running will cause the ECU to significantly overfuel the engine as it thinks it's running incredibly lean. 'Clever' ECUs may even report a fault with the fuelling system in this case as it assumes a blocked/faulty injector.

    Stoichiometry is important for catalysts as they will deteriorate rapidly if the fuel was too rich/lean. "Lean burn" technologies as developed by Ford in the late 1980s had to be abandoned by 1992 and (ironically) fuel consumption and compression ratios suffered as a result for a few years.

    As emissions have got tighter so the number of sensors has increased. Early emissions legislation was satisfied by only a passive 'open loop' system with the injection system (or carburettor for Rover A-series) set for stoichiometric-ish combustion. As we went through Euro II, III, IV and V a lambda sensor was added before the catalyst, then another after it, then more sensors, fly-by-wire throttles, etc.
     
  11. Agile

    Agile

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    Noting your obvious knowledge on the subject and location in Warwickshire, do you ( or did you ) work in a manufacturer's design team?

    Tony
     
  12. mfarrow

    mfarrow

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    Not guilty unfortunately, though I think I know the place you mean!
     
  13. Agile

    Agile

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    Oh well the Castle Balti then!
     
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