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'.....
