1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

MoT test instruments: how accurate are they?

Discussion in 'General Cars' started by Alec_t, 19 Sep 2020.

  1. Alec_t

    Alec_t

    Joined:
    18 Mar 2017
    Messages:
    478
    Thanks Received:
    69
    Location:
    Wales
    Country:
    United Kingdom
    My car recently passed all emissions tests apart from the lambda reading. λ upper limit for a pass was 1.030 and the reading for my car was apparently 1.031. Only 1 part in over 1000 over the limit !!! Can the test centre's instruments really be that accurate (0.1%)?
     
  2. Sponsored Links
  3. Gasguru

    Gasguru

    Joined:
    9 Sep 2005
    Messages:
    11,360
    Thanks Received:
    2,913
    Location:
    Sussex
    Country:
    United Kingdom
    I suggest no because the formula to calculate lambda uses the measured values from the sensors and when you take a look at typical emission tester specs the sensors are not sufficiently accurate to achieve 3 dp.

    I guess by displaying to 3dp and having a 2 dp fail limit of 1.03 it allows a reading of say 1.028 to pass.

    If the reading was only displayed to only 2 dp would you round up the calculated figure lets say 1.0286234 to 1.03 and fail?

    It's a complicated calculation....

    https://www.bridgeanalyzers.com/air-fuel-ratio/

    Take it to another test station and maybe get a pass this year or bite the bullet and find the problem.
     
  4. Burnerman

    Burnerman

    Joined:
    7 Feb 2008
    Messages:
    21,702
    Thanks Received:
    4,194
    Location:
    Northumberland
    Country:
    United Kingdom
    Probably one for Mottie but get the exhaust melt down hot and try again.
    John :)
     
  5. Alec_t

    Alec_t

    Joined:
    18 Mar 2017
    Messages:
    478
    Thanks Received:
    69
    Location:
    Wales
    Country:
    United Kingdom
    Thanks. It's what I suspected. It seems rather draconian to fail an MoT test on the basis of such a tiny reading excess. I'm surprised there's no specified allowance for instrument tolerances (at least, none shown on the test report sheet).
    In the event the garage did an oil change and the car then passed the emission tests.
     
  6. Mottie

    Mottie

    Joined:
    27 Feb 2017
    Messages:
    9,133
    Thanks Received:
    1,113
    Location:
    Essexshire
    Country:
    United Kingdom
    Correct. Or, test again without the engine speed connected and hold it at whatever revs it passes at. Some testers have been known to stick another car on and take the readings from that. Not me of course......
    Tester has no say in it - the machine fails it and an emission failure certificate is issued which must be kept for 90 days.
     
    • Thanks Thanks x 1
    • Like Like x 1
  7. Keithmac

    Keithmac

    Joined:
    10 Oct 2017
    Messages:
    1,559
    Thanks Received:
    248
    Country:
    United Kingdom
    I believe the exhaust gas analyser is digitally connected to the MOT system now (but could be wrong).

    They can't Fudge readings.

    Take it for an "Italian Tune-up" (good roasting down the bypass).
     
  8. Mottie

    Mottie

    Joined:
    27 Feb 2017
    Messages:
    9,133
    Thanks Received:
    1,113
    Location:
    Essexshire
    Country:
    United Kingdom
    At the moment, only new brake testers from October last year (or secondhand ones installed after last October as well as stations that undergo a change of ownership). Decelerometers from Feb this year. Emission testers will soon follow along with number plate recognition software so no more writing out tickets for friends who keep British registered cars in Spain for example............
     
    • Like Like x 2
  9. Burnerman

    Burnerman

    Joined:
    7 Feb 2008
    Messages:
    21,702
    Thanks Received:
    4,194
    Location:
    Northumberland
    Country:
    United Kingdom
    I used to put a kink in the gas sampling pipe - or tread on it years ago when you had to rev the nuts off diesels 7 times :eek: :mrgreen:
    If you wanted a 'donor car :whistle:' Honda was the make of choice.....
    I don't think that would work with todays gear though.
    John :)
     
    • Like Like x 1
  10. Sponsored Links
  11. EddieM

    EddieM

    Joined:
    15 Feb 2009
    Messages:
    9,606
    Thanks Received:
    572
    Country:
    United Kingdom
    I probably along with many others remember when a car didn't need an engine in it to pass an MOT ...... or I am making that up?
     
  12. Mottie

    Mottie

    Joined:
    27 Feb 2017
    Messages:
    9,133
    Thanks Received:
    1,113
    Location:
    Essexshire
    Country:
    United Kingdom
    I think you've always needed an engine. Even the very first tests needed a brake check and the car had to be driveable to carry that out - no roller brake testers in those days.
     
    • Like Like x 1
  13. Tigercubrider

    Tigercubrider

    Joined:
    22 Jul 2016
    Messages:
    3,516
    Thanks Received:
    550
    Country:
    United Kingdom
    I guess it's a bit like being "a little bit pregnant" or breaking the law "in a certain way"?

    There has to be a cut off. I'd thrash the car for a while and retest
     
  14. Alec_t

    Alec_t

    Joined:
    18 Mar 2017
    Messages:
    478
    Thanks Received:
    69
    Location:
    Wales
    Country:
    United Kingdom
    Maybe the garage did, after the oil change. With Covid restrictions, the garage folk weren't allowing Joe Public to observe the testing.
     
  15. ReganAndCarter

    ReganAndCarter

    Joined:
    13 Sep 2010
    Messages:
    2,108
    Thanks Received:
    235
    Location:
    Devon
    Country:
    United Kingdom
    Did exactly that with my van on Friday. Some years earlier, the van had only just scraped through the emissions and the tester's advice to me was to take it for a run immediately before the MOT and "drive it like you stole it". I now do this every year giving it a good rant in low gears and watch the black soot coming out the back in my mirror. Then a blast down the M5 and after all this, no more black soot under hard acceleration.

    Still seems like sacrilidge though, as I hate thrashing an engine and normally drive like Reggie Molehusband. Suppose once a year won't hurt anything. Anyway, it worked as it's the 13th MOT under my ownership, and 13th first time pass. Amazing for an old Fiat, but I do look after it and strongly believe in preventive maintenance.
     
    Last edited: 19 Sep 2020
    • Like Like x 2
  16. Keithmac

    Keithmac

    Joined:
    10 Oct 2017
    Messages:
    1,559
    Thanks Received:
    248
    Country:
    United Kingdom
    Good going, 13 out of 13 .
     
  17. ReganAndCarter

    ReganAndCarter

    Joined:
    13 Sep 2010
    Messages:
    2,108
    Thanks Received:
    235
    Location:
    Devon
    Country:
    United Kingdom
    Yes, was bracing myself for rear suspension arm bushes as it's started clonking slightly from the rear recently. These Scudos tend to need them at this age and mileage, but no mention so they're obviously not that bad.

    Also, always take my cars for an MOT on a Friday afternoon as i reckon the testers will be in a better frame of mind. Wouldn't ever book it in for first thing on a Monday morning.
     
    • Like Like x 1
Loading...

Share This Page