I took my Z4MC for an MOT last Friday after it had been SORNed for the past 6 months. I've owned the car for 4 years and it's now done 32.5k miles. I know it's essential to present the car for MOT when its fully up to temperature and I gave it a proper warm up on the 15 mile trip to the MOT station. The car failed the Lambda on the Fast Idle test, giving a reading of 1.095 when the pass limits are 0.970 - 1.030. It was well inside the HC and CO limits. This is the first time I've ever had any car fail on emissions.
The MOT Test Station (A) does nothing other than MOTs so it should have no reason to fail a car in order to find work. The station is literally next door to the BMW specialist garage I've been using for over 10 years, so I popped into the garage and they hooked up the car to diagnostics and couldn't find any problem with the O2 sensors or anything else. The Indy then tested my car on their own emissions testing equipment and the car produced a Lambda of 1.02, which would be a Pass if their equipment had been used for the MOT. The HC and CO results were very similar to MOT A station. So the Indy suggested that they take my car to another MOT Station (B) and my car produced a Lambda of 1.025 with similar HC and CO figures. The three different tests were all done within a 2 hour period.
So why should the Emissions testing kit at MOT Station A produce grossly different Lambda figures to the kit at the Indy and MOT Station B?
The Lambda value given by emissions kit is a calculation based on the Air/Fuel Ratio (AFR) where the AFR measured by the probe in the exhaust is divided by the stoichiometric AFR value of 14.7. So, a measured AFR of 16.0965 divided by 14.7, gives a Lambda value of 1.095 as recorded by MOT Station A. I know from datalogging my Z4MC and several other cars, there's no way that the fuelling is ever as lean as 16.0965 at Fast Idle. So why and/or how is the tester at MOT Station A getting such a result, when 2 other tests on different kit produced values which are within the MOT limits?
It would appear that if the car is being fuelled properly at Fast Idle and getting fuel at AFR 14.7, the exhaust leaving the engine is getting diluted with air by the time it reaches the testing probe, which would suggest that there might be a leak somewhere in the exhaust system. However, when the car was inspected on the ramp, the tester didn't find any leaks. The ZMs are notorious for being somewhat difficult to get through emissions tests and if we assume that the kit at MOT Station A is properly calibrated and accurate, then the only thing that I can think of is that the rear silencers, having 2 tail pipes, does not function as a sealed exhaust system, at the lower engine rev range. The pressure of air outside the silencers might be greater than the pressure of exhaust gases at the lower end of the rev scale, so that the ambient pressure forces some air back into the silencers?
After my car came back from MOT Station B, and I had 2 Pass results from 2 different emission testers, I went back to MOT Station A and showed them the printouts, they instantly retested my car whilst I was watching and produced a printout which shows a Lambda of 1.029, which is a Pass for the MOT!
How many owners are sent on fools' errands and considerable expense, needlessly replacing O2 sensors and cats etc, when the MOT test can be manipulated and throw up a Fail on the Lambda test?
The MOT Test Station (A) does nothing other than MOTs so it should have no reason to fail a car in order to find work. The station is literally next door to the BMW specialist garage I've been using for over 10 years, so I popped into the garage and they hooked up the car to diagnostics and couldn't find any problem with the O2 sensors or anything else. The Indy then tested my car on their own emissions testing equipment and the car produced a Lambda of 1.02, which would be a Pass if their equipment had been used for the MOT. The HC and CO results were very similar to MOT A station. So the Indy suggested that they take my car to another MOT Station (B) and my car produced a Lambda of 1.025 with similar HC and CO figures. The three different tests were all done within a 2 hour period.
So why should the Emissions testing kit at MOT Station A produce grossly different Lambda figures to the kit at the Indy and MOT Station B?
The Lambda value given by emissions kit is a calculation based on the Air/Fuel Ratio (AFR) where the AFR measured by the probe in the exhaust is divided by the stoichiometric AFR value of 14.7. So, a measured AFR of 16.0965 divided by 14.7, gives a Lambda value of 1.095 as recorded by MOT Station A. I know from datalogging my Z4MC and several other cars, there's no way that the fuelling is ever as lean as 16.0965 at Fast Idle. So why and/or how is the tester at MOT Station A getting such a result, when 2 other tests on different kit produced values which are within the MOT limits?
It would appear that if the car is being fuelled properly at Fast Idle and getting fuel at AFR 14.7, the exhaust leaving the engine is getting diluted with air by the time it reaches the testing probe, which would suggest that there might be a leak somewhere in the exhaust system. However, when the car was inspected on the ramp, the tester didn't find any leaks. The ZMs are notorious for being somewhat difficult to get through emissions tests and if we assume that the kit at MOT Station A is properly calibrated and accurate, then the only thing that I can think of is that the rear silencers, having 2 tail pipes, does not function as a sealed exhaust system, at the lower engine rev range. The pressure of air outside the silencers might be greater than the pressure of exhaust gases at the lower end of the rev scale, so that the ambient pressure forces some air back into the silencers?
After my car came back from MOT Station B, and I had 2 Pass results from 2 different emission testers, I went back to MOT Station A and showed them the printouts, they instantly retested my car whilst I was watching and produced a printout which shows a Lambda of 1.029, which is a Pass for the MOT!
How many owners are sent on fools' errands and considerable expense, needlessly replacing O2 sensors and cats etc, when the MOT test can be manipulated and throw up a Fail on the Lambda test?