New 'style' MOTs

[Firstandbest]

I use the term 'style' loosely!

The green Certificate has been replaced with a printed A4 white sheet! This year I read it and for the first time the exhaust results.

This states limits to be

CO <4.50% vol

HC <1200 ppm vol


Comparing my results this year against last year

CO 2012 1.411% vol 2011 4.33% vol


HC 2012 98 ppm vol 2011 336 ppm vol

Is such a change/improvement normal. 2,500 miles between readings and same MOT station.

 

[malcolm]

Normally you would expect lots of CO if the car was running lean or lots of hydrocarbons if it were running rich. If you just twiddled the mixture screw you would increase one and lower the other.

Have you had any work done on the engine? Otherwise might have been missing a bit last time or inconsistency in the test.

 

[pepper]

The top ones are the requirements for your particular vehicle to pass an MOT, if you don't have a separate piece of paper printed out from the exhaust analyser then you won't have the specific ones for your vehicle. On searching for a quick link to show you I found an answer from Malcolm on his MIG welding forum! Below is a link.

http://www.mig-welding.co.uk/forum/showthread.php?t=10871


Before 1 August 1975: A visual test for excessive smoke.
1 August 1975 to 1 August 1986: Limits are 4.5% CO and 1200ppm hydrocarbons.
1 August 1986 to 31 July 1992: Limits are 3.5% CO and 1200ppm hydrocarbons.
After 1st Aug 1992 you need a catalyst and my eyes glazed over at that point.

This years results look a lot lot better and look normal, last years results look quite high, perhaps you've sorted a problem with a leaking pipe or somesuch?

 

[Firstandbest]

Thanks

Really helpful.

I've not touched since it was serviced bb Derek at the time of the last MOT,
Only the fast run through France last year an I'm using additive with octane booster (!)

Thanks again. At least readings went the right way! I've never read before!

 

[angel]

Normally you would expect lots of CO if the car was running lean or lots of hydrocarbons if it were running rich.

It's not exactly so, Malcolm. Correct (stoichiometric) mixture: low CO and HC.
Rich mixture: high CO, relatively low HC (but higher than stoichiometric).
Too rich mixture: High CO, high HC.
Too lean mixture: Low CO, high HC.
You will also see high HC when ignition timing is advanced too much.

CO2 should be around 12,5%-14%, if too low you have an exhaust leak and other measurements may be false, too.