GUTTING THE CAT...the truth

[big x]

GUTTING THE CAT...the truth ?

The honeycombe in my C-180 1997 92k cat started to break up recently, it was the original Ebberspacher one that is made for Mercedes.The internal rattling started to get worse so I bought a replacement from GSF
for £250 plus VAT after asking for a discount.First inspection looked good,all stainless steel construction made in the UK by Eurocats.It fitted straight on no problem and makes the same noise at idle and constant trottle openings,however on the gas it makes a loudish high pitch rasping noise that is annoying.If it was a low pitch rumble I could live with it but it makes the car feel somewhat un-mercedes like.Today I used a metal rod to poke out the rear wire mesh of the old cat and smashed up the honeycomb and shock the bits out and removed the glass wool that it's wrapped in.The comb was not blocked but a very clean gray colour but does have a limited life because it's quite brittle.There are no other metal parts or washers inside the cat.Any noise is chunks of comb rattling against the case,perhaps made worse by the glass wool blowing out over time. On the inlet side of the case there is a 4" long tube with holes in to I presume disperse the gas flow.My guess is the original cat has aluminium coating steel pipework and a stainless steel body which do seem to have a much longer life than the internals. My question is,
1)I have a recent MOT could I put the old gutted cat back on for a year?The back pressure of the cat cannot be that much greater than the centre and rear boxes as the cross sectional area of the cat is much larger than a straight through pipe. Blowing air through a block of honeycombe I estimate a 50% cut in flowrate.If the ECU is running a closed loop it should compensate for changes in pressure and flow rates?
2)Why is the replacement so noisy surely these things are simple to make.(I've checked that there are no leaks)I'm wondering if it's the front stainless pipe being thinner than the original and whether it has the internal perforated tube inside.
3)Mercedes want £600 for a new cat even though most cars use similar designs that sell for £250 or so.I bet Mercedes suppliers charge around £200
wholesale.
4)Has anyone fitted an aftermarket cat that matches the original in noise performance ?

thanx

adam

 

[jimmy]

Unfortuneately like any other product there are good ones and bad ones. If you are not happy then complain to whomever supplied you the CAT. From my experience products like this should be covered by at least a 2 year warranty if not 3.

I know people who simply bolt a CAT on for the MOT and then replace it with a straight through pipe straight away until the next MOT. This is on cars like Evo's, Civic Type R's etc that have been tinkered with a fair bit. One chap I know with the Evo always comments how much quieter the car is with the CAT on the vehicle.

You are right, in most cases noisy CATs are caused by the padding breaking down, they usually use a sort of silicone sealant type glue to hold the honeycomb in place. The most common causes of CAT failures are faulty sensors (Lambda/Oxygen sensors) that cause the CAT to overheat and basically melt, then followed by physical damage (hitting something) and then thermal shock (driving through a flood).

 

[fuzzer]

I know people who simply bolt a CAT on for the MOT and then replace it with a straight through pipe straight away until the next MOT. This is on cars like Evo's, Civic Type R's etc that have been tinkered with a fair bit. One chap I know with the Evo always comments how much quieter the car is with the CAT on the vehicle.

hmm sounds like a plan .. but how do you fool the lambda sensor?

 

[Benzowner]

I know people who simply bolt a CAT on for the MOT and then replace it with a straight through pipe straight away until the next MOT. This is on cars like Evo's, Civic Type R's etc that have been tinkered with a fair bit. One chap I know with the Evo always comments how much quieter the car is with the CAT on the vehicle.

Interesting, my sons got a Suburau Imprezza STi which apparantley has two cats on the exhaust, and if you remove the front only, it will still get through the MOT. I am not technical so I don't know how, but it is apparently not difficult to fool the lambda sensor

 

[Pietre]

My bike should have a cat but I have a Remus system which fools the Lambda probe ok, still does highly excessive speeds and is road legal.

 

[Dieselman]

The Lamda sensor may well be in the pipe in front of the Cat anyway so removal wont be noticed.

Remember the Lamda sensor is upstream of the Cat and is there to adjust fuel mixture and has no real bearing on downstream emissions, Cat or no Cat.

 

[big x]

OK..here's the result

I took a db reading with my mobile Nokia phone on the same stretch of road and the gutted cat is the same as it was originally.If anything the exhaust sounds a bit deeper in tone which is a good thing.The car if anything revs a bit cleaner too. The CO2 probe is on the downpipe on these cars before the cat so it's not affected.By the way the original middle box is a straight through design,the pipe inside has holes drilled into with baffling on the outside.Taking the exhaust off and refitting is easy as the parts are well engineered.With the car jacked the car up on axle stands put the spare wheel under the centre box and undo the 2 nuts holding the front down pipe to the front cat pipe.Remove the 2 captured nuts attaching the rear cat pipe to the transmission and then slide the whole exhaust forward and it slides out of the 3 rubber doughnuts at the rear of the car.I was worried about the weight but in fact the whole system is not that heavy for one person to do it alone.(tip..always have your mobile phone beside you if working under a car in a garage alone.)
Looking at American Merc forums it seems to me over there they are much more DIY minded than here in the UK.There are few DIY threads on this forum which is a shame as Mercs are on the whole good to work on with good access compared to say some French cars I've seen.

adam

 

[Shude]

There are few DIY threads on this forum which is a shame as Mercs are on the whole good to work on with good access compared to say some French cars I've seen.

That's because hardly anyone has an empty double-garage to work on their cars in the UK! I have to do my mods on the street!

 

[grasmere]

The Lamda sensor may well be in the pipe in front of the Cat anyway so removal wont be noticed.

So have I got this right?
If the front 1/3 of the exhaust (96 C220D) with the CAT is replaced, as mine will be on Tuesday (£250 kwikfit) where exactly is the lamda sensor? Is it integral with the exhaust pipe that will be replaced - or in the downpipe before the replaced part? Is it a wire connector that the kwikfit guy will disconnect and reconnect or something else?

Should I mention it or do they know what they are doing