406oc.co.uk

Hi All

Car had MOT last week which it failed then passed after a replacing teh brakes but anyways i have a question back in 2007 when they did the emissions test there were 3 readings and they revved the nuts off the car 3 times the readings were.

Absorption Coefficient :- 2.13 / 1.19 and 0.42

But this year they only revved it once with the reading being 0.47.

Can anyone tell me why they only did it once this year and 3 times last year and what these readings mean.

They did the three readings because the first reading was 2.13, the software that the diesel tester uses calculates the average reading form three readings if they are relatively low, if the first three readings are high however, the software requires five readings and the average calculated from the five.

You only had one reading because your first reading was very low.

I believe the pass figure is 2.5 for Turbo Diesels and 3.0 for non-turbo.

The only reason i can think it being lower compare to last year is the redex i put in in a month before hand i always give it a good run down the A1 is 0.47 high is there anything that causes high emissions

I just checked and the emissions are even lower than the MOT 3 years ago how is that

The state of the emissions depends on a lot of things - the outside temperature, the fuel, how hot the CAT is, the testers equipment, the tester, etc etc.

The HDi is a very clean running machine and we never see anyone on here with excess emissions really. I'd only be concerned if it was belching smoke out!

There are some misconceptions with the emission control on vehicles - to understand this you have to view it in two basic parts to keep it simplified.
The CAT is a device that alone can deliver most of the emission control to pass the tailpipe test during an MOT, however, it is not good enough to do this long term without some help from engine controls to limit the things that cause excesses during the driving cycle long term. These controls are sensors you will recognise as - Knock, Lambda, MAF, MAP, TPS and EGR etc. To put it broadly, any one of these sensors can go faulty and although cause a drivability problem can still pass the tail pipe emission test at the MOT station. This is because the CAT will cover the excesses for a short term. So you can regard the ECU OBD2 controls as an early warning of problems that would ultimately cause the CAT to fail, though in most cases because these controls are essential to the proper running of the engine and they will pose running faults such as poor idling,loss of power etc. Conversely the CAT could be replaced with a straight pipe and the ECU could theoretically produce a clean exhaust - enough to pass the test if everything is working perfectly. So as you can imagine the two together act as a belt and braces because nothing is perfect.

So you may read on this site and others, examples of where some contributors will say " well I replaced the CAT with a through pipe and it passed the test" or the "MAP sensor was left off and the engine light was on" but is still passed the test. This is quite true and valid because here in the UK (for now) we only test the emissions with a final tailpipe test whereas in America they will use the OBD2 fault codes to fail a vehicle and they have dedicated test periods for emissions.
So our MOT test has allowed some scope for the tester to overcome some deterioration and climate conditions at the point of test. As the climate change lobby increases we are likely to see much more effort going into prevention than cure and this is why OBD2 should be taken seriously by us. At the moment we have to buy a scanner to read this data but it will come that this will be fitted as standard to all vehicles everywhere - not just a little warning light that "something is wrong"
Hope this helps

adam03031980 wrote:i always give it a good run down the A1

This is why you get a good reading quicker - you are getting the CAT hot and this makes it more efficient at converting. Do it harder and longer in winter to keep your emissions low for the test - more important with a diesel since they take much more warming up,this is where the problem lies because it's a cooler burn there is more chance of particulate matter i.e. black smoke which is unburned fuel. It is particulates that are the biggest problem with diesels and though a CAT helps a great deal it is not enough and later models are using Traps or a chemical called AdBlue. Diesels, to put it bluntly, run leaner because they use a full charge of air on every compression stroke therefore more oxygen left in the burn. Now the HDi and other common rail systems are so good at what they do together with the electronic controls the emissions have been pulled down low enough to scrape through the MOT in the past principally because the MOT test equipment has not been as sensitive as it should be. This has changed dramatically and future testing will surprise many who have been removing the CAT. Already if you don't give the car a good welly down the road, with a CAT fitted it may not pass easily. The days have gone when you see any trace of black smoke from the exhaust and get away with it. Under normal running ( when everything works) the 406 PSA HDi is state of the art diesel engineering. Oh - some will say the car runs better without a CAT. Any restriction you put on the engine exhaust, no matter how small, prevents the gases escaping quicker and so creates some back pressure that slows the gases down thus leading to a small performance loss on any IC engine - this does not mean we can remove the exhaust silencer and CAT converter to get more performance. When a turbo is fitted, this too restricts the gas flow but is compensated by the increase in manifold pressure that ultimately gives more power, to simply state that removing all or part of the Exhaust system will hinder the Turbo spooling up is like saying - if I cut your head off it will stop your heart beating.