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!
MOT Failed - LAMBDA sensor and Catalyst?
- Thread starter buckaroo
- Start date
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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
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
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).
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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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
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
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?
Does the car actually feel OK to drive?
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.
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.
Not guilty unfortunately, though I think I know the place you mean!
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