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I changed my car cat converter as it was giving PO4020. After driving for 2 weeks I noticed soot mark as shown in the image above. There is no audible leak sound (tapping) at idle and when revving. When driving there is a slightly "hollow" sound at high revs. I do smell some raw exhaust intermittently. However I think some carbon may have sealed up the leak.

I remember that I may have slightly overtightened bolt #2 as my torque wrench was too long to fit in the tight space so I manually adjusted with a normal ratchet with the same "feel".

To fix this, should I loosen #2 and retorque it to spec, or should I just tighten bolt #4? Also should I loosen bolt #4 and retighten or just retighten it right away.

I've used the upper end of the torque range 46Nm so I'm afraid the bolts would break if I tighten too much.

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Firstly.. If there is a gasket it should be replaced.

If the studs etc are in very good condition then you could try undoing & re-torquing them. However due to the materials and heat cycles these parts go through the studs are often very tight or seized up.

To repair this you'll have to be prepared to remove and clean up all the threads, or just buy new studs and replace them along with the nuts. This is because quite often these nuts/studs etc just jam-up against each other, meaning that they will torque up OK, but they are only tightening against themselves without actually putting any extra pressure on the manifold to seal it. New studs & nuts overcome this problem.

Please be aware though that if seized into the cylinder head you could have a real job on your hands here should any of the studs etc shear off, or have damaged/stripped threads in the cylinder head.

  • Yes the gasket was replaced. Do I need to replace it again? – Andrew Tan Dec 18 '17 at 9:36
  • That's up to you.. . But yes, you should really replace it again as the fitted gasket will have now been compressed. It is possible to reuse the gasket if you must by coating it with a thin layer of sealant on both sides. But it would be a real shame to do the job only to have it leak again due to a used gasket. – Orb Dec 18 '17 at 9:46
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    If you are taking the manifold off again I would check the manifold face to check it's not distorted. – Bonzo Dec 18 '17 at 12:43

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