MoT Lambda question

[wuvvum]

The Innocenti passed its MoT emissions test with flying colours about a month ago. Earlier in the week it got put on the emissions machine again and, although the CO and HC readings were still fine (0.00% and 23ppm respectively), we couldn't get it within the limits for the Lambda reading - it refused to come down below 1.05.

 

This wasn't entirely surprising - being a carburettored engine I think it's highly unlikely there's a Lambda sensor anywhere as there's not a lot it could do if it were there (short of being connected up to a little electric screwdriver to buzz the mixture screw in and out), and the fact that I did get a reading of 1.02 on the day of the MoT test was more luck than anything, but would the car still fail an MoT on this? I can understand it for EFI'd cars as a low or high Lambda reading is indicative of a faulty Lambda sensor, but on the Innocenti it's not indicative of any fault on the car, and the other two emissions readings are so low as to be almost laughable, so it's obvious that the engine is running pretty clean. I'm just a little worried that this might cause a problem for next year's test.

[M'coli]

Carburettor you say? It should just be tested for visible smoke, CO and HC - or is the Lambda coming into the test for carb cars next year too?

[wuvvum]

I hadn't heard anything about changes to the emissions test for next year - my worry was more to do with whether the system assumes that anything with a cat will be fuel injected and therefore doesn't make any provision for non-Lambda-sensor-equipped vehicles.

[mattblack]

0% CO?

 

Shurley shum mishtake?

[wuvvum]

Nope, that's what the machine said. 'Twas the same at MoT time too - I'm looking at the printout now - although HC was higher at 64ppm - still bloody low for a carb'd engine.

[mattblack]

I'd be richening up a bit, TBH...

[Barry Cade]

You may find you have a tiny pinhole in the eggsorst. That can cause this.

[oldford]

Lambda is a measure of the air to fuel ratio. Lambda 1 is 14.7 parts air to 1 part fuel, a high lambda means a lean mixture. A blowing/leaking exhaust pipe can lead to a high lambda as air gets sucked into it between the combustion chamber and the gas analyser nozzle.

 

How old is the car?, depending on age the emissions limits will be either 4.5% (01/08/1975 to 31/07/1986) or 3.5% (01/08/1986 to 31/07/1992) CO and 1200ppm HC, this will be checked at idle and the lambda reading is only taken into account for determining the test result for a catalyst emissions test.

[wuvvum]

The car is a 1993/L, so would have needed a cat by law when it was new. Emissions limits according to the printout are 3.5% and 1200ppm.

[oldford]

First the tester would have carried out a Basic Emissions Test (BET) where the limits would be 0.2% CO, 200 ppm HC and 0.97 - 1.03 lambda, upon failing this test the machine will prompt for a vehicle specific test. If there is an exact match for your car in the gas analyser database or the emissions book those limits will be used. If there is no exact match then the deafault limits of 3.5% CO and 1200 ppm HC will be used, note that the lambda is not taken into account.

 

So as long as the gas analyser database or the emissions book doesn't get updated with an exact match emissions limit for your car, you should be OK for next years test.

[garethj]

It's not the only car with cat and carb, I think BL did some A series with it, and Lada were the same. Ladas even kept the manual choke