Catalytic converter troubles

[MainMan]

The cat on my SLK320 went shortly before it was out of (3 year) warranty at 15k miles. It was replaced under guarantee. The guy said there had been a bad batch. Well it's gone again, this time in under 2 years (about 18 months) and 8k miles. Stealer says its £1200 + VAT and it's out of warranty and they refuse any goodwill payment.

The question is how can this happen except by a defective part. Apparently the cat hasn't broken up - it's just come loose in it's mounting, but it will break up, they say, and has to be replaced. The car doesn't do short journeys. It's driven about 10 miles each way to work and at weekends. Anyone any ideas what the problem is and what I should do. It seems pointless to put another cat on at vast expense if it's just going to break up again in a couple of years. Merc dealers are absolutely useless.

 

[Sp!ke]

The car doesn't do short journeys. It's driven about 10 miles each way to work and at weekends. Anyone any ideas what the problem is and what I should do.

10 miles *is* a short journey. Your CAT is never properly warming up. This, combined with doing only 5k per year is your problem.

You need to drive more IMO.

 

[MainMan]

Hmm. You should join the Merc customer service dept, Spike. No offence.

I thought they put the cat closer to the engine to make it warm up quicker. If it's not warm in a 10 mile blast down the motorway I'd be very surprised indeed. Also, the cat itself hasn't broken up - it's just come loose in the downpipe.

 

[Sp!ke]

Hmm. You should join the Merc customer service dept, Spike. No offence.

I thought they put the cat closer to the engine to make it warm up quicker. If it's not warm in a 10 mile blast down the motorway I'd be very surprised indeed. Also, the cat itself hasn't broken up - it's just come loose in the downpipe.

You expect to get your entire exhaust system warm enough to fully dry out in ten minutes?

Think of all that trapped condensation in your exhaust system day after day, month after month that never burns off fully. Your CAT is acting as an anode.

I know its no consolation but my CAT is 14 years old and still going strong.

 

[stats007]

You expect to get your entire exhaust system warm enough to fully dry out in ten minutes? er yes

Think of all that trapped condensation in your exhaust system day after day, month after month that never burns off fully. Your CAT is acting as an anode.

CATS produce water as a by-product, condensation is distilled water - there are no salts for a reaction?

I know its no consolation but my CAT is 14 years old and still going strong.

If your car is running rich all the time this can dramatically reduce the working life of your CAT - sounds like this time it's vibration induced - if it has actually failed of course.

 

[Doodle]

My mother does similar journeys (4-5 miles a day to work each way, about 5k a year). The cat on her old MX-5 managed to last 8 years of this 'abuse' before it finally gave up the ghost, so 2 years is still an exceptionally short life.

£1200? I know Merc parts are expensive, but that's about 4 times the cost of the Mazda's one, and their parts aren't exactly cheap.

Constant short journeys are never a good thing for a car, it normally falls under the 'arduous conditions' description in the owners manual.

 

[Shude]

£1200? I know Merc parts are expensive, but that's about 4 times the cost of the Mazda's one, and their parts aren't exactly cheap.

The Cat' is about £700 or something, the rest will be labour to fit it (~£100 per hour).

 

[Robbo]

Personally, I don't think its the short trips. I do 5 miles each way to work in mine and usually a longer drive at weekends and its now at 37000 miles with the cat still OK. Our Cavalier has done the same trips to work for over 10 years and the cat is still going strong! IMO, 10 miles is more than enough to warm the cat up.

I would say it's either a faulty part, the engine is running too rich or the cat has been subjected to physical damage.

 

[MainMan]

Thanks for the replies. Spike's got some interesting ideas about how long it takes to do 10 miles in this part of the world - 10 minutes? more like between 20 minutes and 45 minutes. The engines definitely not running too rich - at least the mpg is pretty reasonable at about 28 overall - but if it were I'd doubt if that would cause the cat to break up - more likely to poison it. The things that I know can cause this kind of damage are physical damage, sudden temperature change like going through a deep puddle, faulty fitting or defective part. Since it's not the first two it's either been fitted wrong or it's a faulty part. I suspect the latter.

Mercedes "customer support" say it's a "mechanical part and therefore subject to failure at any time". Thanks. Actually it's stretching the meaning of the word mechanical to apply it to a cat.

There are apparently 2 cats - one costs £604 and the other £358 according to the dealer. All in cost including VAT (and 10% of part, 20% of labour for 3 year old car) £1277. The trouble is that the dealer is completely useless. Firstly, if there are 2 cats, maybe they haven't both gone. Do they know which has gone or if it's both? No. They just replace both. Secondly there's absolutely no point in sticking a new cat on if it's just going to go again in a few thousand miles, so can they tell me why it failed, if, as they claim it's not a defective part? No.

So the only course of action is to source another part and get it fitted at an independent - still going to cost about £600 though. I will never buy a Merc again. It's a second rate product with a third rate dealership.

 

[silverarrow]

yep i agree why buy if they do not support the customers you need to find other owners and ask if they do about 5 k miles.
i would expect that its the second cat ie the one furthest away from the manifold
my reason for this is condensation builds in this part because the first cat is too hot for water to stay put
i am sure others have had cat failures on low miles
i would go to an indy and use a after marke part
check to see if the exhaust system has ben modified if so possible manufacturing fault or and design fault

 

[Robbo]

i would go to an indy and use a after marke part

Ansolutely! If the OEM parts only last 8K miles then there's nothing to lose really.