MOT today. The form has now been dumbed down. Instead of the green hard to forge certificate, it's a simple white page print out, with any advisories on the right hand side.

What's more interesting, and worrying is the Lambda requirement. This measures the fuel/air ratio at the tailpipe, and is between two parameters, which for my Morgan is 0.9 to 1.03. This is a very narrow band. On the screen, this displays as a red band with a narrow green pass band. The measure is shown as a white bar, and is taken at a prescribed 'fast idle' - just under 3000rpm. It was a struggle to get below 1.03. After a couple of attempts the tester managed 1.025. A high reading indicates a leak in the exhaust system, but both tailpipes produced the same effect. There is no obvious leak, but the Lambda shows a weak mixture. So there are a number of things to look at. I can refit the exhaust, I can clean the MAF sensor (more of that later). I could clean the sensors in the cats - the connectors are on the wing stays, so there's a definite neoprene fix there. I could renew the plugs. So more will follow, but for now I've passed.
On checking last years result, it was 1.024, so that was right at the top end. The machine can't be conned, so exceed the limit and you will fail.

The science:
Lambda is a measure for the mass air to fuel ratio present during combustion. When exactly enough fuel is combined with the available free oxygen, the mixture is chemically balanced and is called stoichiometric.

Lambda = 1 —stoichiometric mixture

Lambda < 1 —mixture is rich, excess fuel present

Lambda > 1 —mixture is lean, excess air present

The amount of air that is needed depends on the type of fuel used. In the case of petrol, a stoichiometric mixture consists of an air/fuel ratio of 14.7/1.
Peak power is at Lambda 0.84 to 0.90
Optimum economy at Lambda = 1.05
Optimum emissions at Lambda slightly below 1.0
I wonder if the ECU is set for 'economy'.....

Where do they get the figure for the "pass" band from? The engine manufacturer? Or is it a standard figure for a petrol car of a certain engine size?

Nigel

Just a word of warning - the Morgan Motor Company had a dispensation on some of their models for the lambda reading. I remember my 4.6 Plus 8 would not meet the 1.03 and after querying it with Malvern I was told that it was allowed to go up to 1.08. The MOT testers have a little book with all the settings in, but rarely refer to it because the majority of cars are from 0.9 - 1.03. Just might be worth checking out.

Neil

More news.
I was concerned enough to look further into this.
The emission data is published each year by VOSA and is here:
http://www.dft.gov.uk/vosa/repository/Emissions%2017th%20Edition.pdf

This is the "In service emission standards for road vehicles" - seventeenth edition. Published in February 2012 and available on the link above. ISBN 978-0-9549352-7-6

Morgan is on pages 97 and 98.........
The data shown covers the following:
Normal idle:
Max CO limit (%)vol: All models 0.5

Normal idle speed Min Limit and Max Limit RPM.
Varies as follows:
4/4 1.6EFI CVH 750-850
4/4 1.8EFI MVH 850-950
Roadster 3.0V6EFI 750-900
Plus 4 2.0 T16 850-950
Plus 8 3.5 750-850
Plus 8 3.9 750-900
Plus 8 4.6 750-900
Plus 8 4.0 750-900
4/4 2.0 Duratec 700-900
4/4 1600 Sigma 650-1000
Aero 8, 4.8 Manual/Auto, Max and SS 650-1000

Fast idle
Max CO limit(%vol) 0.3 all models.

Max HC limit all models: 200

Minimum Lambda: All models 0.9, EXCEPT Plus 8 4.0 and ALL Aero's which are 0.8.

Maximum Lambda: All models 1.05. (NOTE, this means that my 1.025 is well within parameters).

Minimum RPM - Fast Max limit RPM
4/4 1.6EFI CVH 2800-3100
4/4 1.8EFI MVH 2800-3100
Roadster 3.0V6EFI 2750-3000
Plus 4 2.0 T16 2500-3000
Plus 8 3.5 2500-3000
Plus 8 3.9 2500-3000
Plus 8 4.6 2500-3000
Plus 8 4.0 2500-3000
4/4 2.0 Duratec 2750-3000
4/4 1600 Sigma 2500-3000
Aero 8, 4.8 Manual/Auto, Max and SS 2500-3000.

So if you have any Lambda troubles on test, refer the tester to this tome.

Sounds like taking a printed copy along on test day might be a good idea for the future Dave...

Well done and good research. I understood that with the new MOT test machines, they download the data directly from VOSA and it should setup the test machine with the right parameters. Perhaps someone was pulling my leg

According to a poster on Mogtalk, they should first run the standard test to 1.03, then invoke an exception test from the list which resets the limits to the appropriate level. What a faf that is, and obviously if the tester can squeeze you through first go, it saves time, and messing about. In an ideal world it should be automatic, but it appears that it starts from the default.
What concerns me is that if it had failed, I may have returned home and started taking things apart to fix something which didn't need fixing...........

I know someone who has just done an MOT on a classic vehicle. The gass analysis seemed to indicate the engine was running marginally too lean (too much O2). This could also be caused by incomplete combustion.

As it was only marginal the tester kindly stuck the tube up the exhaust of another car whilst re-running the test

Naturally I have no idea where this all took place. It is probably just a story and I can't now remember who told it to me.

I will remember that useful phrase.

I took mine to the factory and said kindly sort this out. Back came the reply - you do know that the 1.03 limit does not apply to the Plus 8. Looking a bit surprised, I asked if that was in the handbook. Nope came the reply, so I left very confused as to how the hell was I supposed too know . Strangely enough. even McDonald racing didn't know and they had spent some considerable time and effort trying to get inside the 1.03 after having a new chip put in the ECU.
John McDonald was not too happy when he found out - cant remember his exact words and they are probably unprintable for delicate TM forums