Lambda emissions - 1997 Carina E 1600cc 4A-FE

[JohnD]

car has failed MoT with emissions of 1.074 (should be 0.970 - 1.030)

there is a slight blow on an exhaust joint at the cat, the tester was talking as if fixing or changing it would fix the problem.

What does lambda emissions mean, and would a leaky joint cause it? If it needs a new cat, any money spent would be wasted. Last year I pointed out the noisy joint to the main dealer and they quoted £800 for a new cat, so I didn't have one.

the engine was also running very rough on the day of the test, possibly because I had hosed it down after cleaning off some spilt oil. It is now dry and running smoothly again.

 

[Burnerman]

Its not a million miles off, and I reckon that plugging the hole with some exhaust cement, and then getting the cat hot - and I mean really, really hot - should see you through the test.
If the engine was running rough (misfiring by the sound of it) the car would definitely fail on its emissions.
John

 

[Avocet]

Lambda is the name they give to the number denoting the air : fuel ratio. For a petrol car, the "optimum" ratio is 14.7 parts of air to 1 part petrol so they arbitrarily call that Lambda = 1. If it was running a weaker mixture, the Lambda value would be higher (e.g. if it was running at 15 : 1, the lambda would be 15/14.7 = 1.02). Conversely, if the car is running "rich", the lambda value would be lower than 1.

For a cat-equipped car, this needs controlling much more tightly than could ever be achieved with a carburetor, and the only way it can be reliably done is using "closed-loop feedback" - where there is a Lambda sensor in the exhaust that constantly measures mixture strength (from the burnt gasses passing over it) and feeds the signal back to the engine management system - which then adjusts the mixture based on what it's being told by the lambda sensor. This typically happens once a second or so. In normal use, the Lambda sensor will sample the gas, send the signal to the management system, it will compare that value with one in its "map" and decide whether the mixture needs to be weakened or enriched, and alter its output to the injectors (holding them open for a fraction longer or shorter) depending on what it wants to do.

Most petrol cars seem to work to the "default" MOT limits of 0.97 to 1.03. Outside those limits it will fail. The fact that yours was 1.07 means it was running weaker than permitted, so it failed.

HOWEVER, it's true that an air leak in the exhaust can cause a fail because the MOT gas analyser is shoved up the tailpipe. It will be sampling the exhaust gas (just like the car's own lambda sensor) BUT it will also be sampling the air that gets sucked in through the leak, so it will "think" that the car is running weak, when in fact it's running fine, it's just that the gas in the exhaust has been diluted slightly.

As John says, just plug the hole and take it back. Getting the cat hot is also a good trick, but I suspect not necessary in this case.

 

[JohnD]

thanks very much. I think it is a flanged connection that is leaking. I wonder if there is a high-temp silicone that would squidge in? Anything inflexible would presumably crack.

it started after I had the down-elbow changed for a new one a bit over a year ago, the agents said the joint on the end of the cat was too old to seal correctly.

 

[Belle427]

http://www.ebay.co.uk/itm/HYLOMAR-E...arts_Vehicles_CarParts_SM&hash=item3ccafb5db7

Get some of this, my local factors stock it but not sure where else they sell it.