My MOT pass this morning gave a Lambda reading of 1.15 (pass range 0.95 - 1.30).
The tester commented on the high level and the highest pass range he'd seen.
It got me wondering what exactly is it measuring and what it's level would be without a cat fitted?
Other readings:
Fast idle. CO-0.02%(pass-0.5). HC-44PPM(pass-200).
Natural idle. CO-0.01%(pass-0.5).
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Why worry, it's a 16 year old motor and it's passed its MOT!!
I bet the tester spends most of his time testing 3 year old cars with tighter test criteria.
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Recycling some text I wrote on a recent similar question;
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Petrol engines give their maximum power when running slightly rich, and their maximum economy when running slightly weak when compared to stoichometric.
The value reported during MOT testing is the calculated lambda value, and this represents the excess air ratio. So, if lambda is greater than 1, the mixture is weak. In engine fuelling calculations, there's also a commonly used greek letter, phi, which denotes the excess fuel ratio. Lambda = 1 / phi
This value is not related to the output of the car's lambda sensor which is sometimes called an oxygen sensor. The car's lambda sensor cannot calculate a lmbda value, it switches near the stoichometric point, and simply tells the engine management system rich or weak. This means that the sensor is quite non-linear.
During warm up, the lambda sensor is too cold to switch, and the car relies on its open loop mapping (the open loop mapping is the starting point for all the fuelling calculations). As the exhaust heats up, the lambda sensor switches for the first time - this signal is detected by the engine management computer which then switches into closed loop mode.
When running in closed loop mode, the cat needs to be held close to stoichometric to allow both oxidation and reduction reactions to occur. As the lambda sensor is downstream of the combustion, the signal it reads is delayed. This delay, combined with the non-linear response of the sensor means that the control cannot be smooth, like idle speed control for example, but oscillates between rich and weak. This oscillation is about once per second at tickover, but, becomes faster at higher engine speed as the delay becomes shorter.
This non-linear response is also why the lambda sensor doesn't have full authority in closed loop mode. For example during a transient, like acceleration, the lambda sensor can only "say" weak, it can't give any indication about how weak the mixture is. So, there's no quick way for the ECU to reach the right fuelling point quickly using that approach. Instead, the open loop mappings plus stored adaptive corrections are used to get close to the operating point quickly, and then use the lambda sensor to prod the system back towards stoichometric.
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In this case, the lambda reported by the MOT machine is greater than 1, so, there is excess air, which partly explains the verly low CO and HC values. These values would be low even without a CAT if there is excess air - and the CAT will be oxidising any CO and HC further.
When you're next under the bonnet, check for anything obviously being loose which might cause unmetered air to be drawn in to the manifold.
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Thanks for that NC.
I'll have a look for a leak, unfortunately it's got a not very straightforward carburettor, with a very basic EGR - a pipe from the cat feeds into the dirty side of the air filter box. I thought the HC was a bit high, as my other (FI) car has a much lower HC.
I was wondering (purely hypothetically of course) what power I might release by removing the cat and what the emissions might look like with it removed.
Edited by bathtub tom on 25/02/2009 at 18:57
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The lambda reading may be caused by the smallest of leaks in the exhaust or the probe not being far enough up the tail pipe. A minimal air leak on the analyzer will also cause this. the air leak can be so small that even whilst the analyzer has to pass a leak test each morning a tny air leak will increase lambda.
being a carburetted model I suspect the throttle spindle is leaking causing very low co reading & increased lambda.
I wouldn't worry about the HC for an older vehicle as its still well within limits
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