Diagnosing exhaust leak

[bobbe]

A few weeks ago I replaced the pre-cat header with an aftermarket header that I'd successfully used in another car for the last couple of years. The exhaust sounded louder than I remembered and there was a ticky blowy leaky sound, so I assumed I'd got something wrong along the way and had left a leak somewhere. So at the weekend I took off the manifold, cleaned all the mating surfaces and used new Toyota manifold to head gasket and crush rings all properly seated, new nuts at the head and new studs and nuts at the cat, and torqued everything to spec

It still sounds leaky. So, now I need to figure it out

I know you probably can't spot a lot of things by sight but everything around the manifold looks okay, I think:

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You cannot view this attachment.

(I'm 90% that what seems to be a gap between the header and the cat is in fact a shadow from the header's flange being a slightly different shape)

Now, to be fair, the exhaust had sounded a bit boomy at times before I replaced the header, so it's quite possible that changing the header didn't cause the leak but faffing around with the exhaust simply exacerbated it. And I'm concerned about these black marks on the flexi joint:

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You cannot view this attachment.

I've also had my hand in around the manifold's connections to see if I could feel any exhaust coming out and didn't detect anything. Various heat shields are all solidly attached. There are no OBD codes. I'll probably do the soapy water thing at some point, but I can claim no expertise on exhausts at all so I have a couple of questions:

1) Are those flexis done? Those black marks look like soot marks, and one of them seems to be cracked in the metal around the end of the flexi? Again, I have no expertise in this, so maybe this isn't unusual, maybe it is, I don't know

2) Does it matter which way round the manifold to head gasket goes? I know there are three little dimples/tabs on the OEM manifold, but I couldn't for the life of me tell you which way round I put it

Thanks in advance!

[Alex Knight]

Black soot marks on flexis = exhaust gas leak I'm afraid. It doesn't get there by accident.

Very common. You can source some stainless steel ones for circa £30-£40 and ask a mechanic or exhaust shop to weld the new ones in.

[bobbe]

That's what I suspected, but hoped it wasn't. Is it likely to be more economical to buy new flexis and have them welded or to just buy a replacement cat and fit it myself? I suppose I could sell my old cat and/or the pre-cat header to cover some of it either way

[Carolyn]

That's what I suspected, but hoped it wasn't. Is it likely to be more economical to buy new flexis and have them welded or to just buy a replacement cat and fit it myself? I suppose I could sell my old cat and/or the pre-cat header to cover some of it either way

You could get a decent aftermarket cat from Cats2U, but it's around £150.  One thing to bear in mind is your original cat will be near its end of life by now.

[bobbe]

I'm leaning towards getting a new cat, since getting the flexis replaced is £30 for parts + labour and I can offset some of the cost of a new cat by selling the pre-cat manifold and the old cat. It might have to wait a few weeks mind you since last month I paid too much to get the subframe swapped

Does Euro 2 vs Euro 3 matter? And does anyone have any clue what the scrap value of pre-cats + cat would roughly be?

[ucb]

I recently changed my cat mid section for a standard pattern part.
I got offered almost nothing for the old cat pipe and indeed it was largely contingent on buying a refurbished cat from them anyway.
Seems the market has fallen out of cats so far