Diesel Cat CRACKED!

[Mr Butter]

Hi,

Anyone have experience repairing or replacing the downpipe / catalytic convertor?

Our downpipe (on a 1998 1.9D) has cracked just after the cat - looks weldable but I assume replacement is better - expensive though ..... I checked around and the best I can do is online £85 or £120 (caty-4-less and online automotive) and I have no idea if this is for a quality replacement or something made of recycled Polo-mint wrappers.

I tried Kwikfit for a laugh, they wouldnt even quote without me taking the car down and I'm not driving it as it is (blowing hard, centre section looks about to drop out). Basically I want to fix it myself on general principle....

Ideally I would like to chop the cat out as they are not required for MOT on a diesel , but this is not straightforward due to the differences in pipe bores.

I just want to know the best option, as the car is the missus' run about and there has been a run of repairs recently.... she is now talking about a NEW car - and likes FIATS !!!

[Tom_vRS]

Get it welded up if you can find someone to do it

[herbie64]

looked at doing same to my pickup,quoted by local exhaust manufacturing centre £30, all they need to do is cut cat out at front & take apart at rear, i think the pipe is 50mm,& they usualy stretch it to fit over header ( wher it as cut off) or weld on a larger bore sleeve. If you have welding kit should be an easy job,if you can get under the car.

[KenONeill]

No actual problem with welding near a cat as long as the part has cracked rather than disintegrated due to rusting.
As Herbie says though, a cat replacement pipe is just a matter of being able to get the right bore pipe or a bore stretcher.

[AwesomeSarah]

Mine split on the flexi part
They are expensive , I found Butts to be the cheapest , but still expensive
You could try a bespoke exhaust company to see if they can help
Pipewerx may possibly help 01704 897778
Sarah

[Decron]

Get rid of it!

You don't need it.

You can get custom pipes made and/or get the crack welded for about £50. It's quite a job as they are usually Stainless and need a TIG but £50 in the right hands should be reasonable

[Mr Butter]

Thanks for the advice... . I favour chopping the cat and rplacing with a straight through section, but reckon welding could be an option - either way it means pulling the pipe! My neighbour recommended brazing as an option as this is more flexible, but we shall see what happens. I needed the car this weekend so ended up doing a hideous 'gun gum' wrap repair reinforced with additional clamps and strips of metal . Hope it holds on the 100 mile round trip!!! Next week the proper fix.....

[nickdrive]

Mine has had a patch weled to it,done by previous owner at sone time.Fairly sure Butts price is £250(down pipe & cat).Guess I will have to replace mine,but its ok for now.

[KenONeill]

Never done it as close to a box as you described, but I've done an exhaust bandage, bean tin (literally) and Jubilee clip repair a couple of times. I got the idea from my Dad, who once got 3 years (Morris Minor MM, but even so) out of one of those jobs.

[Decron]

Originally Posted by KenONeill

Never done it as close to a box as you described, but I've done an exhaust bandage, bean tin (literally) and Jubilee clip repair a couple of times. I got the idea from my Dad, who once got 3 years (Morris Minor MM, but even so) out of one of those jobs.

They last about a week. On a modern exaust I doubt they would last a day as the higher flow and pressure would be problem. If the crack is on the part where the can joins the pipe it needs welding.

BTW the Moggy has a tiny zorst compared to the Felly/Fabia!

[KenONeill]

Originally Posted by Decron

They last about a week. On a modern exaust I doubt they would last a day as the higher flow and pressure would be problem. If the crack is on the part where the can joins the pipe it needs welding.

BTW the Moggy has a tiny zorst compared to the Felly/Fabia!

You think I don't know most of that?

Where do you think the higher pressure is coming from though? Remember that area, and hence flow volume per unit time rises as the square of diameter.
The extra heat might be a problem, but I've not needed to do one in 10 years or so, and the last one was done to get the car to run well enough to get it 70 miles to an exhaust centre.

[Decron]

Originally Posted by KenONeill

You think I don't know most of that?

Where do you think the higher pressure is coming from though? Remember that area, and hence flow volume per unit time rises as the square of diameter.
The extra heat might be a problem, but I've not needed to do one in 10 years or so, and the last one was done to get the car to run well enough to get it 70 miles to an exhaust centre.

I appreciate that. I used to run Minis and Metros that would shake their exhausts to bits because of duff engine mountings meaning the required almost weekly patching until I could get the mounts done.

There is going to be a significantly higher flow of hotter gas than there would ever be coming out of a 27hp side valve engine as fitted to the MM. Diesels in general have a very high gas flow and even though the pipe is bigger (Therefore less pressure) I would bet that there was a higher pressure due to the volume of the gas.

If it's on the seam it will be a pain to get any repair like thus to stick to it.

I'd still give it a go though if I was stuck

[KenONeill]

I suspect you'd be right of a petrol turbo, but I know diesel turbo units run cooler than petrol ones do. And I was only saying that you might be surprised how long the repair would last, not claiming that it necessarily would last that long.

[Decron]

Originally Posted by KenONeill

I suspect you'd be right of a petrol turbo, but I know diesel turbo units run cooler than petrol ones do. And I was only saying that you might be surprised how long the repair would last, not claiming that it necessarily would last that long.

Actually it's the cat that you need to worry about. They have very high operating temparatures sometime in excess of 500 degrees C. Diesel cats might not be that high but would still be quite warm!

[KenONeill]

Originally Posted by Decron

Actually it's the cat that you need to worry about. They have very high operating temparatures sometime in excess of 500 degrees C. Diesel cats might not be that high but would still be quite warm!

Diesel cats are usually just oxidising units rather than 3-ways, aren't they? I'd have thought that would bring the temperature down a bit.

[rigsby]

i welded the pipe back onto the back of the cat and braced it with a couple of bits of flat bar. three years ,two owners and a backbox later ,and its still going strong!

[Mr Butter]

OK ..... in the end I bodged it twice - Gun Gum embarassment that lasted about 6 weeks round town and a 500 mile motorway blast! I had to bodge it again when the cat exit pipe snapped clean off. then decided to remove the downpipe to get it weled - centre section was seized solid and would not budge. Loads of grief later the car was off the road for two weeks and I end up with a new centre section and a custom decat stainless pipe (£60 plus vat Longlife exhausts - Battersea - 1 hour while I waited on the way to work!!)

On the plus side I swear the car is a bit livelier - no decernible change in MPG - certainly no worse. Havent ahd to MOT it yet so hopefully no emission issues then . My advice, dont mess about - get it CUT and save yourself the grief - I spent hours pissing about with this.... but at least have a reasonable solution!

[KenONeill]

I don't know the legalities of it, but AIUI a diesel can pass an MoT without a cat, and I would expect it to be free-er revving if not more powerful, as a result.

[Mr Butter]

My neighbour is an MOT tester and reassured me it wouldn't be a problem (at least at his station...) . There is definately might be a performance increase, mid-range anyway - all relative I suppose as my commute vehicle is a 100hp 160kg bike....