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My "spare" Mk3 LS400 has a blowing exhaust. I thought it was all at the flanges, which I've gun-gummed up for the time being; but I've still got a leak, and it's somewhere between the engine & the cat (maybe in the cat). I can't feel the gas blowing anywhere, but it sounds as noisy as ever.

I decided to take a look under the heat shielding, but of the 4 bolts I've had to remove (3 on a plastic bit, to get to the heatshield), all 4 have sheared. This is obviously going to be the story on every other bolt I tackle on this car :( So, a couple of questions:

1) How easy is it to get the manifold downpipes off? I'm assuming it's 1 big lump either side - but do I need to pull the engine to do this?

2) How the hell can I get the bolts out without shearing them? I can squirt WD40 all over, but I can't see it doing much good, it'll just drip off instead of getting into the threads.

3) Any ideas on how I might more accurately locate the leak? I can't get my hand in there to find the blowing, before it all gets too hot...

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It could be the EGR pipe fractured,mine started blowing at the rear cat joints but it was due to the nuts/bolts having completely failed so 4 nuts/bolts and 2 new gaskets later all was well.

If it is the front cat to manifold joints I have some spare gaskets for those but there are 3 studs fitted here which may shear when trying to remove the nuts.

If you're having difficulty finding the hole get some one to hold some rags against the tailpipes to increase the back pressure which should help locate the problem.

It's very tight in the engine bay so I would think it's an engine out job for the manifolds.

Heating up the nuts may help along with a good approved socket on an impact driver.

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