W202 Emmissions "took a long time to test"

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mr. shr

MB Enthusiast
Joined
Apr 22, 2004
Messages
1,879
Hi guys,
My 202 was MOTed this week.
It passed ok without any advisorys but the tester said that the emmisions test took a long time to do, and I probably need to get cats changed before next test.

Now a good few years ago, I was getting a rattling noise from under the car. You good guys suggested that it's probably the cats breaking up.

The advice you guys gave back then was split two ways:

1) Get the cats changed soon otherwise the little bits of honeycomb will break off and end up in your engine,

2) Don't worry about it. It will soon deteriate into nothing and the car will pass MOT no probs.

I went with no. 2 and as many of you suggested, it has indeed passed it's MOT every year since then. (I'm talking 4 or 5 years continuously).

Anyway, question is, assuming I need to change the cats, is this a DIY job?
i.e. can it be done with car on axle stands on the driveway or is this a job that needs to be done on a proper car lift at a garage.

Also, which aftermarket cats are you guys recommending?
Car is worth next to nothing (I mean re-sale wise) so don't want to spend much on it.

Thanks,
shr.
 
Glad your car passed its mot. I fail to see how bits of the cat can end up in the engine as their on the exhaust side, cheers Tony
 
I would change the upstream O2 sensor if your emissions took a while to come into spec.
Have a read here:
testing-Oxygen sensor
I would recommend getting a Bosch one as they make them for MB, they are identical except for the MB number, are half the price and they work, unlike the brand new one from MB that I bought and didn't work :wallbash:.
From my reading it seems they get slower to react as they age and cannot keep up with the changes to tell the ECU what is the correct mixture.
Concensus is to change them at 100,000 miles.
Anyway, cheaper than a new cat and will improve the mpg.

Tony
 
The car really does seem to smell.
Can't really describe it and don't know what a car needing new cats smells like but would the O2 sensor cause the smell?
 
It will stink of ammonia, especially around 2000 rpm, mine did and struggled to pass the emissions test too.
 
A catalytic convertor causes a reaction to take place and takes no part in it.
If you sort the overfuelling problem quickly then the cat will not be permanently affected.
Things like leaded fuel and silicone from sealants will poison the catalyst and destroy it in very short order. Silicone will kill an O2 sensor too.
I am assuming the car struggled with a low lambda reading as that means it is running too rich.
There will be two O2 sensors on the c280, Bosch ones are around £75 each.
I have spent a few weeks sorting out my car since it failed the MOT on low lambda. I found various issues but the most annoying was the fact that a brand new genuine MB sensor was duff, causing me to chase my own tail for a couple of weeks until I replaced it with a Bosch one.

Tony
 
Hi guys,
My 202 was MOTed this week.
It passed ok without any advisorys but the tester said that the emmisions test took a long time to do, and I probably need to get cats changed before next test.

Now a good few years ago, I was getting a rattling noise from under the car. You good guys suggested that it's probably the cats breaking up.

The advice you guys gave back then was split two ways:

1) Get the cats changed soon otherwise the little bits of honeycomb will break off and end up in your engine,

2) Don't worry about it. It will soon deteriate into nothing and the car will pass MOT no probs.

I went with no. 2 and as many of you suggested, it has indeed passed it's MOT every year since then. (I'm talking 4 or 5 years continuously).

Anyway, question is, assuming I need to change the cats, is this a DIY job?
i.e. can it be done with car on axle stands on the driveway or is this a job that needs to be done on a proper car lift at a garage.

Also, which aftermarket cats are you guys recommending?
Car is worth next to nothing (I mean re-sale wise) so don't want to spend much on it.

Thanks,
shr.

Try some of this before spending money Welcome to Cataclean, Home. Euro car parts normally have it in stock
 
Try some of this before spending money Welcome to Cataclean, Home. Euro car parts normally have it in stock

+1 and put decent fuel in it and take it for a high RPM blast.

When the cat breaks up very fine particles of the ceramic (dust) can make there way into the engine when the engine has turned off. Ceramic is very hard and can wear cylinder bore quite quickly
 
I would think my cats are non existent now as they were rattling about 4 years ago. No rattling now so I'm guessing there's nothing left of them?

Would that stuff still have an effect even if the cats have completely disintegrated?
 
I did all of the above, including a Terraclean (money wasted). An Italian tune-up was a short term "fix" to get through the MOT but the problem came back the very next time the car was driven normally.
The MB garage's recommended next option was a new ECU at £1000.
Although the upstream sensor was working according to Star it was just not quick enough in switching to keep the fuelling correct. No way to truly test the sensor without lab equipment.
After many hours of research I took a gamble on £90 for a new O2 sensor and since fitting it the problem has all gone away.
That is my recent experience.
 
What year is your w202? If it's an early model (straight 6) there are 2 barrel cats, if your is a later v6 you will have 4 cats I doubt all the cars have broken up, as said cataclean and a hard blast up the motorway.
 
It's a 1999 year. V6 engine.
Car has done 3.5k miles in last 12 months, all around town journeys. Literally a couple of motorway trips throughout the year, the rest town stuff.
So you guys reckon this stuff will sort it out?
 
I've ordered some Cataclean.
Will give this a go and see if it improves the smell in the garage.
 
Glad your car passed its mot. I fail to see how bits of the cat can end up in the engine as their on the exhaust side, cheers Tony

Modern cars have very close coupled Cats.
Valves operate on an overlap basis and as the cat breaks up it can cause the monolith to slip back blocking the outlet pipe. This then creates a high back pressure so bits of monolith get sent back into the engine.
It does happen.
 
Anyway, question is, assuming I need to change the cats, is this a DIY job?
i.e. can it be done with car on axle stands on the driveway or is this a job that needs to be done on a proper car lift at a garage.

It's an impossible question to answer without getting under the car and having a look at the condition of the exhaust system.

However, the nuts, bolts and joints in an exhaust system are exposed to all sorts of weather and a lot of heat over many years. As a result, they can become very rusty and the older a car becomes the more likely it is that the various components may be extremely difficult to unfasten and separate.

That's why your local exhaust fitter always has his blow-torch at the ready !
 

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