"It's got a funny rattle..."

[SB118]

"It's got a funny rattle.." that's what the guy told me. Worse when he goes over big bumps.

So i take it out for a quick drive (cap, dark glasses and false beard, don't want to be seen in a '51 Corsa!) and the noise seems to be coming from the exhaust.

I jack it up and the back box is absolutely fubar, seams have let go and it sounds like a bag of gravel when i give it a kick.

He's keen to get it sorted as he's got his MOT next week. Hour or so later and the new backbox is on and the noise is gone, everybody is happy.

Until i pick up the old back box and "stuff" falls out.

Rang the motor factors and new cat is more than i'd pay for the whole car!

Isn't there a cheaper option for cleaning the exhaust gases?

Think he's in for an expensive MOT

 

[TaKeN]

"Excuse me sir, there appears to be a wasps nest in your exhaust.. a black one"



Not seen anything like that before! Scrappers for a s/h cat?

 

[Fido McNasty]

The old platinum matrix has clearly collapsed or the bees have really taken up residence! And a proper OEM spec catyclysmic converter costs that much money becasuse the matrix is made in a platinum compund. Platinum is more expensive than gold. http://www.monex.com/monex/controller?pageid=prices

When the matrix collapses it becomes more brittle (they are alreay fairly brittle at manufacture by virtue of the fact that the matrix is so very fine. The platinum contact area being equivilent to the size of a football field or more in that little box). Time takes it's toll along with extreme heat cycles (they need heat to work effectively), vibration and the thing eventually breaks up. Sometimes the pieces can roll backward under exhaust gas pressure into the silencers out pipe effectively blocking it under throttle or even just idling sometimes. Quite a common occurence.

Cheaper catalytic converters tend to be manufactured internally in stainless steel. It sounds good on paper but the downside is that they can often cause lambda sensors to misread and feed that mis-information back to the ecu which results in less than stellar performance, engine management lights coming up, that kind of thing. The car will drive and do all the things it should, but problems are often attributable to the fitment of stainless converters.

If you're budgeting, then yes, a scrap yard is probably your best bet for an oem catalyst for sure and probably for good money. It might not last so long but at least the engine should remain relatively happy and content. The other common misnomer over 'cheap, half price or less' replacement exhausts is that they are single skinned whereas oem are usually double skinned, well in the case of BMW, Mercedes, VAG etc items anyway, hence expensive.

yes that first typo was by design

 

[Lum]

How does a dodgy cat cause a lambda sensor to misread when the lambda sensor is located before the cat?

Well the important lambda sensor is, some cars (usually American) have one after the cat, the purpose of which is to tell you that your cat is shagged and light up the engine management light, a problem that can be solved by replacing said sensor with an appropriate resistor.

 

[Clarkey]

I'd say virtually every car on sale today has pre and post catalyst sensors. The post-cat sensors are obviously important too, just as an example an A3 I was looking at the other day had a 6-wire sensor post-cat

 

[Fido McNasty]

How does a dodgy cat cause a lambda sensor to misread when the lambda sensor is located before the cat?

Some do, some don't. Many late BMW's (say, post 1992) for example e60 525i have before and after cat. e36 M3 has both after the cats. e92 330 has two right in the manifold pre cat. e36 328i has two after cats and so on.

The lambda feedback to ecu issue when it occurs is specific to the scenario with steel cats and isn't in itself a general issue alone. The steel cats comparison to oem is a general running/efficiency issue. Post cat lambda's can compund that issue. Thats what I meant. Apologies for not going into an epic length thread to clarify that for you already Lum.

 

[Scania]

What typo?

 

[Slime101]

Just leave it out - ive had cats disintegrate in the past and left them unrepaired and they passed their MOT's fine

 

[Deleted member 66701]

e36 M3 has both after the cats

No it doesn't - it has two cats. Two sensors are in the collector of the manifold before the cat and both cats have sensors at the rear of them. Four sensors in all - two pre cat, two post cat - this is required to comply with the OBD2 regs - other configurations are not allowed.

From my BMW college service manual:-

The 1995 M3s had only one oxygen sensor (located before the catalytic converter) due to OBD I standards and the 1996-1999 M3s had four oxygen sensors (two before and two located after the catalytic converters) due to more stringent OBD II standards.

Post cat Lambda sensors do nothing except monitor the efficiency of the cat on ALL cars that have them fitted. Pre cat Lambda sensors are the ONLY ones that feed back to the ecu for fueling adjustments.

If fully OBD2 compliant, the post cat sensors will light the MIL (Malfunction Indicator Light) and store a DTC (Diagnostic Trouble Code) in the ecu. If the vehicle isn't fully OBD2 compliant (like my Scorp Cosworth) then there is no MIL and just a DTC is stored.

To answer the original posters question - get the cheapest cat you can - yes it might not last as long, yes it might not be as efficient but it will do the job and get the car through it's mot. It wont make the MIL light come on unless there are other issues with the engine/ecu/sensors.

You don't say what model Corsa it is but check out here - they always seem well priced.

http://www.cats-direct-shop.co.uk/v...pageNum_Recordset1=5&totalRows_Recordset1=145

Most expensive cat I can see is for a 1.8 - comes in at £182 + vat.

 

[Fr0dders]

which Cat though ?

my vectra had 2 cats. A main one and a precat

the lambda sensors are after the precat, but before the main cat. But the main cat has no lambda sensors at all, and was about 3 feet further down the exhaust, so could be safely removed allowing for increased noise, and still pass emissions tests.

 

[Deleted member 66701]

which Cat though ?

my vectra had 2 cats. A main one and a precat

the lambda sensors are after the precat, but before the main cat. But the main cat has no lambda sensors at all, and was about 3 feet further down the exhaust, so could be safely removed allowing for increased noise, and still pass emissions tests.

Pre-cats are only to clean the emissions whilst the car is warming up. Once the engine is up to temp the pre-cats are outside thier normal operating temps and then they do nothing at all - they act like a normal exhaust pipe.

O2 correction doesn't take place at all during warmup, hence no need for a Lambda sensor before the pre cat.

What age was your Vectra - if it has no post cat sensor then it isn't OBD2 compliant. OBD2 compliance wasn't a requirement until 2001 in the UK.

A well tuned engine is entirely capable of passing emmisions tests without a cat. Hell, even my throttle bodied 1987 8v Pinto almost passed the 1994 "cat" test!

 

[Fido McNasty]

No it doesn't - it has two cats. Two sensors are in the collector of the manifold before the cat and both cats have sensors at the rear of them. Four sensors in all - two pre cat, two post cat - this is required to comply with the OBD2 regs - other configurations are not allowed.

From my BMW college service manual:-

Realoem dot com will help you.

College manual... Haha.

 

[CaPtBlaCk]

Realoem dot com will help you.

College manual... Haha.

Your sarcasm would be worthy if you were right. Unfortunately none of the models you list have a lambda sensors post cat only.

In your case, realoem failed you, perhaps you should borrow the college manual!?

 

[pinkaardvark]

are you sure that stuff isn't the back box baffle. Surprised those big lumps could travel up the exhaust pipe which I imagine is quite narrow on a corsa.

 

[Fido McNasty]

Your sarcasm would be worthy if you were right. Unfortunately none of the models you list have a lambda sensors post cat only.

In your case, realoem failed you, perhaps you should borrow the college manual!?

Righto. Whatever you say guv.
I was addressing this btw. You guys come out of nowehere going off on a tangent.

when the lambda sensor is located before the cat?.

Few are singular. Few are located only before the cat. Cherio.

 

[CaPtBlaCk]

Righto. Whatever you say guv.
I was addressing this btw. You guys come out of nowehere going off on a tangent.


Few are singular. Few are located only before the cat. Cherio.

I was responding to this..

Some do, some don't. Many late BMW's (say, post 1992) for example e60 525i have before and after cat. e36 M3 has both after the cats. e92 330 has two right in the manifold pre cat. e36 328i has two after cats and so on.

That's where the tangent came from, reading it literally then it is wrong in part. Still thanks for the clarification, I can sleep now.