6 captured repliesDiscussion pagination complete · Source accounts, not a confirmed diagnosis
THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCHCatalytic converters or something else?
Leading reported evidence
Replace cats about a month ago
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Leading evidence: 'Replace cats about a month ago' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Check loose header, blown header gasket, leaking exhaust clamp' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Suggestion
Seeing as how you are not getting any responses - WAG: check for a loose header, blown header gasket, leaking exhaust clamp.
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Replace fuel pump, he thought the noise was only coming
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Suggestion · Original poster
When Doc at Rennsport had it in over the summer to replace a bad fuel pump, he thought the noise was only coming from left tailpipe.
Counts describe captured discussions, not unique people or a diagnosis. Author identities are not reliably stored. Exact duplicate evidence is counted once across matching discussions; one discussion counts at most once per action/state. Multiple repairs reported together remain a combination. Suggestions and failed attempts do not establish a fix. Matches cover whole discussions and can include related faults, model variants and incomplete threads.
6 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
Rennlist: 1 discussions
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Generation: 20261026T222516Z · outcome-qualification-6. Deterministic extraction; no AI model inference. How we use sources
I posted this at the end of an old thread a couple of weeks ago, but got no replies. 1999 C4 6 speed aero. Fister D mufflers with Fabspeed headers, sounded great when I first got it. About 4000 miles ago, my car started sounding very loud - like an old Volkswagen. I get a CEL with P1275 code (Aging of Oxygen Sensor Ahead of Catalytic Converter (Cylinders 1 - 3) - Below Lower Limit) from time to time. When I clear code it comes back after 300 or so miles - has been off for the last 500+ miles.
When Doc at Rennsport had it in over the summer to replace a bad fuel pump, he thought the noise was only coming from left tailpipe. He didn't see any exhaust leaks. I've had it up on my lift a couple of times and the exhaust system looks in good shape. I pounded on the cats with a rubber mallet, but haven't heard rattling. I'm sure Doc could fix it easily, but I'd like to try tackling it myself. Car runs fine, just sounds loud, especially before it gets fully warmed up. Thoughts?
Also, although I'd love to get Fabspeed 200 cell cats, but am looking at cheaper alternatives (I just replaced 4 sets of tires last week plus a brake job on the 996). I'll probably get flamed for this
, but has anybody tried the Davico stock-style cats that you see offered on the internet?
Seeing as how you are not getting any responses - WAG: check for a loose header, blown header gasket, leaking exhaust clamp. All would sound loud until it heats up and closes a gap.
I just replaced my bad cats about a month ago. I noticed a loud rattle coming from the engine area, only after the car was warmed up a little. I initially thought the noise went away when I drove, but it was really just that I couldn't hear it over the engine. When I hit the cats with a hammer (as you did), I heard nothing. Turns out this is because the cats need some heat in them to loosen up the substrate before it will rattle. Since your noise seems to diminish as the car warms up, cats may not be the answer.
Thanks very much guys.
Joe: will throw it up on the lift again this weekend and look more closely for loose stuff
mjdavis: Was the actual exhaust note loud and "unrefined" (like an old VW Bug) before you swapped the cats? Did the cat swap change the exhaust note?
be careful of loose stuff in the cats... it can get sucked in the exhaust valves and cause havoc. Ive heard this happened to a gt3rs with bad cats... very expensive repair.
It sounded like a lot of different things - for a while I thought it was a valve problem, then I thought it was a loose bracket somewhere. Of course some guy tole me it was definitely the IMS. You know that "sticky lifter" sound when the car has been sitting for a while? It's kinda like that, but louder and doesn't go away when the car warms up. I'll attach a video file...
996 Engine Rattle - YouTube
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