CLK MOT pass with 2 cats removed

[LeighW]

I thought this may be of interest to someone. My CLK320 has four cats; two 'pre-cats' just after the manifolds, two further cats roughly half way along the car. A while ago, the rear cats started to rattle, indicating they had broken up (a common problem). I'd read on here and elsewhere that you 'may' be ok on emissions with only the front cats in place, but couldn't find anything conclusive. After discovering the VERY high cost of the cats on a CLK (£800 approx from MB), I decided to see if the emissions were indeed ok with only two cats. A friend of mine removed the exhaust, made neat cuts in the top of the cats, emptied them, and welded them back up. I instantly noticed how much smoother the engine now performed, even the gearchanges are smoother too.

However, today was MOT day, so time to test the theory!

Anyhow, good news, it not only passed the emissions test, it absolutely breezed through!

Results per test sheet:

Fast idle - CO:- Limit 0.20%, result 0.00%
HC:- Limit 200ppm, result 5ppm
Lambda limit 0.97 - 1.03, result 1.005

Natural idle - CO:- Limit 0.30%, result 0.01%

I hope this is of some help, and maybe saves someone a little cash!

As an aside, I had managed to get hold of some brand new cats (rear) for a pre-facelift CLK320 (this earlier model doesn't have front cats, but the rear section is basically the same), but now I don't have to butcher these, they are up for sale if anyone's interested. I'll put them in the classified section when I get chance, they are a brand new genuine MB part. PM if interested, I'm not after much for them.

 

[GrahamC230K]

If anything like Audi, the precats in the downpipes are normally very small (in comparison to the primary cats) and are for cold start emissions (not MOT tested in the UK).

Potentially you car might have passed the emissions tests without any cats at all. Reality is though tecnically it should have failed as the primary cats have been removed (gutted in your case). I did wonder myself about your technicality - ie saying well it does still have two cats. Don't think it would work though. As you have gutted the cats rather than removed them, I am pretty sure no MOT tester will ever know, so good work.

I'm doing the opposite so soon as I can get booked in (got the parts). I am loosing the precats in the downpipes as for me these are just after my turbos and are a restriction and an unwanted heat source). Removing them is proven to reduce EGT. I will be then having sports cats fitted in replacement of the primary cats.

 

[Baschurchbill]

Hi. The MOT test does not require any cats to be fitted - it just requires the tailpipe emissions to pass. It is a common misconception and called "the cat test". CATs have evolved and been fitted purely as the only way manufacturers have been able to meet the later lower tailpipe emissions. On a side note vehicles running on LPG only have to comply with the emmission standards of "pre CAT" days - provided it is presented for MOT test on LPG.

 

[GrahamC230K]

Hi. The MOT test does not require any cats to be fitted - it just requires the tailpipe emissions to pass. It is a common misconception and called "the cat test".

I believe that to be wrong. There are a lot of people that would like it to be right.

Got anything definitive to prove this?

Anyone else?

 

[panason1c]

I believe that to be wrong. There are a lot of people that would like it to be right.

Got anything definitive to prove this?

Anyone else?

Baschurchbill is correct, the actual presence of a cat is not obligatory for a mot test as long as the car still passes the emmisions test required for a cat equiped vehicle (which is unlikely if the 'primary' cat is removed)



Edit....Just found this....

http://www.bobmckay.co.uk/emission.html

..........."One important issue must be cleared up straight away. There is no requirement for a catalytic converter to be fitted to any vehicle, regardless of its age. It is however the most popular way of ensuring engines meet the emission legislation. The MOT test is testing the emissions from the tailpipe and if those emissions meet the standard then the vehicle passes. The term 'cat' test is inappropriate, the correct term is 'advanced emission test'. Its not the 'cat' that's being tested. There are a number of vehicles which are subject to the 'advanced emission test' but are capable of meeting the requirements without the use of a catalytic converter"...................

 

[Baschurchbill]

Yes - I have an MOT test station and as I say it is a common misconception.

 

[Dieselman]

Baschurchbill
I am aware of the test being only for emissions and there being no requirement for cats to be fitted, but have you tried to get a car through the test with no cats on?
Wouldn't the Co reading be too high if the Lambda was at 1?

 

[television]

On the same engine I too removed the lower cats that rattled, opened them with an angle grinder and closed up again, there was no problem in getting through the MOT, and it passed with a good figure

 

[Baschurchbill]

Ok. When checking the emissions on an MOT, with the vehicle hot two quick tests are carried out and at 2500 rpm the CO must be below 0.2% HC less than 200 ppm and Lambda between 0.97 & 1.03. The second test at natural idle (between 450 & 1500 rpm) requires the CO to be below 0.3%. If these criteria are met it passes.
If they are not met then the specific vehicle details are entered and a new set of parameters are set, these are generally slightly more lenient and again if it meets the criteria it passes.
In simple terms CO Carbon Monoxide is 1 atom of Carbon & 1 atom of oxygen. With complete combustion CO2 is produced, 1 atom of carbon to 2 atoms of oxygen. HC, hydrocarbon is unburnt petrol and Lambda is a theoretical measurment with 1 being perfect combustion - less than 1 rich mixture more than 1 weak mixture.
The very fact that all new cars have CATs is because this is the only way manufacturers have been able to keep tailpipe emissions low enough to pass the new vehicle emission tests - which are more stringent than the MOT, hence some efficient engines can achieve the MOT standard without CATs.
Sorry for the long post and hope it helps.

 

[GrahamC230K]

Have we got a more official source than someone's personal website?

Getting interested now. I don't mind the emissions being close to the mark, but I thought it was illeagl toi remove the cats entirley regardless..

 

[Dieselman]

Graham, it's definately not illegal to run without cats as long as you can get the emissions within spec.

I am concerned that the Co reading couldn't be brought in because the setting for the engine will be approx 1% so would fall outside the spec if there were no cats to convert the gasses.

This is really a question for baschurchbill to answer... What percentage of upstream Co would one see at a lambda of 1?

 

[television]

This is from Hot Rod magazine

Before removing the catalytic converter, keep in mind that the law says that you cannot change the converter unless it is on a vehicle that is over five years old or has more than 50,000 miles on it. If the vehicle has less than that, the original vehicle manufacturer must replace the converter when it goes bad.
-----------------------------------------------------------------------
this one is USA
http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=wi&vol=wisctapp2%5C2004%5C03-3536&invol=1

This is uk
http://www.cfit.gov.uk/mf/reports/laq/index.htm

 

[Dieselman]

The only Hot Rod magazine I could find in the UK didn't have anything to do with catalyitc converters...

 

[GrahamC230K]

Good info guys, thanks.

 

[television]

Good info guys, thanks.

Would you like to borrow my angle grinder

 

[Baschurchbill]

Here are a few facts from a Bosch technical book.
For its operation, the spark ignition engine requires a specific air-fuel ratio for complete combustion. The ideal, theoretical ratio is 14.7:1 and is refered to as stoichiometric air-fuel ratio.
For contemoray engines, the minimum fuel consumption occurs at an air-fuel ratio of 15..18kg air to 1kg fuel. Excess-air factor (Lambda) has been chosen to indicate how far the actual air-fuel mixture deviates from the mixture theoretically necessary for complete combustion (14.7:1). less than I, air deficiency, mixture rich. Engines develop maximum pawer at Lamba 0.85 - 0.95. More than 1, excess air, lean mixture with Lambda more than 1.3 the mixture will no londer ignite.

One of my Mechs has a Volvo V70 turbo without cats and has not been able to quite get it low enough - down to 0.4% CO

 

[Alfie]

This is from Hot Rod magazine

Before removing the catalytic converter, keep in mind that the law says that you cannot change the converter unless it is on a vehicle that is over five years old or has more than 50,000 miles on it. If the vehicle has less than that, the original vehicle manufacturer must replace the converter when it goes bad.
-----------------------------------------------------------------------
this one is USA
http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=wi&vol=wisctapp2%5C2004%5C03-3536&invol=1

This is uk
http://www.cfit.gov.uk/mf/reports/laq/index.htm

What are you doing reading that old rag?????

You are a Mercedes driver!

 

[Merc-C180K]

What impact did the removal of the cats have on the exhaust note...?

 

[television]

What impact did the removal of the cats have on the exhaust note...?

The exhaust on my V6 stayed with the same noise as always, it did feel more responsive

 

[television]

What are you doing reading that old rag?????

You are a Mercedes driver!

I searched through 15 sides on google not easy to find this info