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  1. #1

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    Exhaust Issues (PITA)

    I have been lurking here for a while trying to read up on my issues, and I cannot seem to find anything pertaining to it.
    I recently installed a 1991 header and downpipe assembly on my 1995 Sl2, and had a new flex pipe welded on. I am currently running the stock cat, bolted up to my Thermal R&D catback. On the first flex pipe, it made a horrible whistle noise and it sounded obnoxious. I was able to get under the car while I had it running with an occupant giving it gas to search for exhaust leaks. No leaks, so I assuming it was the new flex pipe. I cut off the flex pipe, and installed a new 4" flex pipe, and once again it makes a similar but different noise. It currently sounds like a hissing noise, a blatant exhaust leak. But once again, no leaks from the exhaust, just a sh*t sounding noise. I put a straight pipe in place of the cat to see if the cat was the suspect, but it made the same noise from the same exact area, the flex pipe.
    My question is, could I replace the flex pipe with a spring flange setup? Would that potentially eliminate the noise from the flex pipe? Anybody have experience with this?
    Thanks,
    Chris
    Last edited by cgg17; 06-16-2019 at 12:00 PM. Reason: Added car's year and model.

  2. #2
    6S Moderator S.Bretz's Avatar
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    I have never heard a flex pipe causing a whistle.
    Did you check the header to down pipe flange for any leaks?
    Some of those stock down pipes had a crappy merge at the two-to-one pipe merge. It might be possible that the exhaust flow over the merge might be causing it.
    I have seen some resonators make a slight hissing noise from the hole drilled in the bottom of them...the hole is meant for letting moisture to drip out when humid air condenses in the pipe.
    -6S Resident Mechanical Forensics member #001.
    1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
    1998 F-250 5.4L triton...stock.

  3. #3

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    Quote Originally Posted by S.Bretz View Post
    I have never heard a flex pipe causing a whistle.
    Did you check the header to down pipe flange for any leaks?
    Some of those stock down pipes had a crappy merge at the two-to-one pipe merge. It might be possible that the exhaust flow over the merge might be causing it.
    I have seen some resonators make a slight hissing noise from the hole drilled in the bottom of them...the hole is meant for letting moisture to drip out when humid air condenses in the pipe.
    The catback was fine before I switched down pipe and manifold setups, and so was the cat itself. Maybe it is originating inside of the split, since I changed the flex pipe already and they have a similar sound. I will look into that and see what I can fine.
    Oh, and I did look at the mating points of the manifold and downpipe, there were no leaks as I gave it some throttle by hand and had my other hand cupped around the flange to feel for any leaks.

  4. #4

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    Today I got my stainless pipe in the mail, and was able to get the entire exhaust straightened out. I pulled the downpipe and went over it real well, and all of the welds look good and there was no obvious leaks. I did however see one small spot of soot from where the flex pipe was clamped over the extension I had welded to the downpipe (I clamped the flex pipe and cat on to keep it serviceable). So, I figured that maybe the cat was restricting the flow too much and the gases were trying to escape wherever they could, as the inlet was 2.25" but the outlet was 1.75". I talked to an older Gm tech about it and he said it is a common practice to increase the back-pressure which helps to keep the cat hot even though it becomes restrictive. Well, I decided to get rid of the cat and now I have a full stainless setup from the flex pipe back. No more crappy sounding noises, and the car has a noticeably different amount of power. It is not major, but I do think getting rid of the cat and having the header as well as a 2.25" free flowing exhaust helps a lot. I also got rid of power steering and kept AC, I will have to upload pictures of the belt routing for that one.

    For anybody that skips the novel, I found a small leak that I believe was caused by too much backpressure and restriction from the cat, so I got rid of the cat for now and opened up the exhaust.

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