On the 26/06/05 (some 4000 miles ago) I had a new cat fitted. Ever since then, the car has made a metallic rasp or buzz on acceleration. In fact, the degree of the 'rasp' depends on the throttle opening more than engine speed. Its the kind of noise you would expect if the was a pinhole in the exhaust near the engine, or if the metal itself was too thin (its hard to describe odd noises!)
Initially, I thought it could be because the exhaust is clean and polished on the inside (with it being new) and would quiet down with the carbon deposits which cover it during use. Similarly, with a small leak in the gasket, this too would be 'plugged' with carbon.
However, this doesn't seem to be the case. I tried listening out for leaks under the car and the gasket joint (on the end of the manifold / beginning of down pipe) but couldn't pinpoint it to any particular area. I'm toying with the idea of putting a bit of exhaust paste around the joint (not removing anything) but I?m weary putting this ahead of the cat. Other than that, I could try replacing the 'offending' gasket, but this will be easier said than done, and I could make it worse
What do you think?
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You would be wasting time and effort putting a bit of exhaust paste around the joint,it wont work,only way is to remove old and replace, unlikely you will make it worse as long as you do it properly
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Steve
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I had a new cat fitted. Ever since then, the car has made a metallic rasp or buzz on acceleration. In fact, the degree of the 'rasp' depends on the throttle opening more than engine speed.
What do you think?
IMO I think the honeycomb inside the cat is loose.
So replace the cat or live with it
I have just recently had my cat replaced because of a similar noise that developed into a diesel type rattle.
I was advised to drill a hole in the box and screw in a self tapping screw. This would, hopefully push the honeycomb to one side inside the box and stop the noise. I was warned that the screw point would eventually make a larger hole and the whole cycle would need repeating.
The car easily passed the MoT with the rattling cat.
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What do you think?
You had the work done less than a year ago. Personally I would take it back to whoever fitted the cat and get them to fix it under the warranty period.
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You can't seal a leaking gasket by stuffing in paste afterwards, but I imagine what you meant was temporarily trying it to see if you could detect a difference that might be worth investigating.
Another way is just to try holding a rag aginst the joint at different places while someone opens the throttle a bit.
A badly welded flange on an exhaust pipe can cause a surprising amount of noise, through gas turbulence inside the pipe.
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Thanks for the help so far.
steve.o: It looks like a straightforward job, getting the 4 bolts out is probably the hardest bit.
henry k: I don't think its the cat itself. The monolith is placed a considerable distance away from the manifold (see: www.autobits.co.uk/store/images/BM90128.jpg ) and when listening for the noise, it sounds as if its coming more from the engine bay that the underside of the car (where the cat is situated).
DD: I had seriously considered this. The garage is an independent (not like a national chain or franchised dealer) but they are quite approachable. I have a feeling they would agree to look at it, but under the instruction "if you leave it here for the day, we'll try and have a look", cos they're quite busy. This would mean being without the car for a day, but that?s no real hardship I suppose. If it can't be traced to something simple like the gasket, I?ll take it back. (because I could replace the gasket myself in a morning or afternoon)
Cliff Pope: Yes, I was hoping this would say one way or the other if its was leaking. I'll try with the rag. I had thought of the old 'soapy water - look for bubbles' approach, but thought that gaskets are probably never that air tight anyway, and if it was leaking, it'll probably too strong to blow bubbles.
Thanks again, much appreciated. Any more ideas?
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Just to add, I have sometimes been disappointed in the noise characteristics of various replacement exhaust systems. Sometimes a brand new section seems inexplicably noisier, a bit like a tiny whispering leak, but with no actual or obvious leaks to explain it. In the end I have put it down to different manufacturing tolerances, slight misalignments of pipes welded into flanges.
In one case the new downpipe was irritatingly louder, and I did experiment with removing and grinding off the welding burrs inside at the manifold end, and it did make a big improvement.
I have also found that OEM exhaust systems are quieter than cheaper, and presumably thinner, replacements.
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"In one case the new downpipe was irritatingly louder...OEM exhaust systems are quieter than cheaper, and presumably thinner, replacements"
Yeah, I too thought I'd been ripped off, being charged full price money for a cheap and nasty part. Closer inspection revealed an OEM Ernst part. The quality of the other Ernst parts I?ve had fitted are excellent, first class, so I wouldn't have any other. Because of this, I?m struggling to accept that it?s the part as opposed to the join that?s causing the noise (as it does sound as if it?s too thin).
If I replace the gasket, I might as well peer down the pipe and see what the quality of welding is like for the end flange to pipe area.
Cheers, Roberson
(Keep your thoughts and opinions coming)
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start the car and put your hand over the tailpipe and pressurise the system you will hear if its blowing.
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Well, better late than never
Yes, its the exhaust gasket. I replaced a little while ago and guess what? Yep, its still doing it :-(
My dad suggested putting a copper gasket in instead. Because its softer, he explained, it might allow for a better seal. Does anyone know of a place that sells or can make copper exhaust gaskets? My dad also said he could fabricate one if he can get a sheet of appropriate copper.
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I've recently got a rasp on acceleration although I've pinned it down to a resonance (rattle) with the CAT heat shield.
Martin
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