Exhaust/ Cat Problems. You Guys Think These Are Related?

Discussion in 'Planes, Trains & Automobiles' started by weedandtubes, May 23, 2013.

  1. I have '97 Firebird 3.8L. Over the past 6 months or so, there's been a very occasional knock coming from the exhaust. Up until last month, I'd hear it maybe once per 45 min drive. Last month I started having a misfire and the knock became 20x worse.
    Fixed the misfire, brought it to an exhaust shop finally. Now to me, the front exhaust hangar looks bad, because there's a good inch of play of in the exhaust up and down and the bolt that holds the cat looks like it's hitting the undercarriage and I hear the exact same knock when I pull it down. Can't tell 100 per cent, though, just what it looks like.
    They try and tell me the knock is coming from inside the cat, because the honeycombs are broke and those are apparently knocking around. Has anyone ever heard of this? Then they tried to tell me that I had another misfire code appear while it was with them. Along with an o2 sensor code. Along with a "catalyst efficiency low bank" code. All while they had the car for a hour.
    You guys think that these are all of these related? I was thinking the cat is causing a misfire, or vice versa, the long standing misfire from before ruined the cat. Would the downstream o2 go bad from the cat being clogged? I was thinking there just isn't enough flow through the exhaust and the sensor is reading that, would that trip the code for a bad sensor or would it trip the faulty cat code? Or does it actually have to be a faulty sensor to trip it?

  2. and that, my friend, is why i don't fuck with obd2.
    gut the cat, snip the o2 sensor. my .02 haha.
    its serious time now: what did you do to fix the misfire?
  3. #3 weedandtubes, May 24, 2013
    Last edited by a moderator: May 24, 2013
    Spark plugs and wire change. 2 wires got crossed and that's when the knocking became worse, because I made the mistake of trying to keep up with highway traffic on 4 cylinders and it was at pretty high RPM's for a decent while. Fixed those, but the knock persisted. Checked the resistance on the coil packs with a multimeter and all were good.
     I'd do that, but I have to pass emissions every 2 years in Az and cats are mandatory.
    Edit: I fixed the misfire about a month ago, didn't feel one on the way home, either, even though the code's on.
    If anything, I got a quote for $150 for a replacement cat with install from a shop that's done work with me before. Then I'm in another $70 for an o2 sensor, hopefully the hangar isn't bad too. Don't have money for this shit right now.
  4. jeez. i crossed two wires on my e30 once and it would barely run - you were on the highway? :eek:
    if cats weren't so expensive i'd say throw another on there. is the knock coming from the engine or is it from the exhaust hitting the undercarriage?
    i would take it to someone you trust to run the codes. the shop could just be giving you random things to up the time your car spends in the shop.
  5. #5 weedandtubes, May 24, 2013
    Last edited by a moderator: May 24, 2013
    Yeah, actually they weren't crossed, just weren't fully on and they popped off on the free way and they lost connection, so the 2 cylinders weren't sparking.
    Personally, it sounds like its coming from the bolt right behind the cat to me that attaches one of the exhaust hangars, but there's all of an inch separation between that bolt and the cat, so I can't say 100%. I had the codes ran by someone else and they all popped up. I just think it's odd that they all popped up while with the one shop. They're saying  a bad cat will cause a knock, but the only knock I would imagine a bad cat would cause is from back pressure in the engine, not the actual cat knocking.
    Now I can definitely hear some loose honeycombs in the muffler rattling around. So, new muffler too... Stuck my hand under the exhaust, barely any flow coming through at 3k RPMs, so it's for sure a cat. Just not sure if a new cat will fix the misfire and the o2 sensor code
    Definitely not the engine though.
    I think I'm going to ghetto rig it for a minute tomorrow and pull out the o2 sensor before the cat and see if the misfire code goes away, since the exhaust will be flowing better.
  6. check your timing maybe?
    have you pounded on it lately? could it have jumped a tooth?
  7. #7 weedandtubes, May 24, 2013
    Last edited by a moderator: May 24, 2013
    That could be. I was WOT for a minute yesterday and literally accelerated not a single MPH in a good 10 seconds or so. Went back to normal right after, though. I don't see it going back to normal if the timing was off...
    That woulda been right after I felt a slight pause like 10 min before hand and didn't think anything of it.
    Like I said, it was fine all day today, though. Even went WOT again right after yesterday and picked right up.
    Edit: A shop just quoted me $100 for the cat, $50 for install. They said the honeycombs in the muffler will break down and won't be necessary to replace. So, cat will be in tues and then I'll figure out the misfire again....... or hopefully it goes away. I really don't wanna fuck with the o2 sensor either, cause it looks like the threads are melted...
    Edit Edit: Forgot to say that the knock only picks up when I'm idling and braking.
  8. i wonder if its pieces of the sound wafer or whatever rattling around in the pipe or muffler.
    i would check for contact points (shiny metal) on every hanger and the top of the whole pipe if you haven't already.
    good puzzle for me to ponder today...
  9. Knock only comes up when I pull down on the exhaust, not up or sideways, so no need to check the top. The only possible contact point was that one bolt, which was still pretty grimy from my memory. I tried to look on top of the bar (not sure what the bar is called, just runs sideways right behind the cat) where the bolt seems to be hitting, but I couldn't fit my head to look haha.
    I'm honestly starting to wonder if there's some sort of baffle that goes in cats that could have broken.
  10. is it an automatic or manual?
    'cause the transmission could be on its way out, especially due to the WOT with no forward motion. the only time i've had that happen was when the clutch went in my manual e30 and when the automatic tranny in my ex's galant was drained and not filled back up.
  11. Auto unfortunately.
    And it wasn't no forward motion, it was just no acceleration. I was doing 75, gunned it to pass someone and nothing. Just kept even at 75.
    Ima be pissed if my tranny goes out at 55k miles haha.
    looked at it today again to verify no contact points, bolts still grimy so I dont think it's hitting. Couldn't get the o2 sensor out, threads are melted, or damn near close to it. I'll post back Tuesday with the other codes if they dont turn off with a new cat.
  12. Did your engine rev up when you gave it throttle to pass or did it just stay at current rev? A clogged cat can very well cause a lack of power but cats don't clog for no reason either, are you sure your engine isn't knocking? Get a screwdriver, put it up against your oil pan while the cars on and listen for knocks by putting your ear to the driver. I ask you to do that because bad rings with blowby will clog a cat.
  13. #13 weedandtubes, May 25, 2013
    Last edited by a moderator: May 25, 2013
    I'm 90% sure the cat clogged with the original misfire when I replaced the plugs/ wires. Like I said, it was a real occasional knock before, but I think the first time I bottomed out a year ago or so, I may have hit the cat and knocked some honeycombs loose and it's been slowly clogging, then the misfire made it back up the rest of the way. It took me a while longer than it should have to get around to fixing the misfire, too. No knock from the engine, it's the actual exhaust knocking. I used my stethoscope a while back to give it a listen and am 99% sure the knock was from the exhaust, just not sure which part (inside the cat or just the bolt).
    It went up to 5k RPM or so, just didn't accelerate. Dropped it back down to 2,500, accelerated again and it picked up like normal. I doubt it's a piston ring, because I haven't been blowing any sort of smoke and I have a regular idle. I just think the over revving and cat backed up made the exhaust gases seize the engine for a minute, then when I backed off the gases had a chance to clear and then I could accelerate normally? Not sure how that would work, but it's all I can think of.
  14. An exhaust wont knock unless it's loose and banging on the body or something in the engine bay, tighten it down and replace the cat. But I'd still be on the lookout for something causing the cat to clog. Your misfire could cause it as well, unburnt fuel will clog a cat up. Just be readin your car real good for a while.
    As for your whole rev but no move it's gonna be something driveline related transmission or rear end; may have just been a hiccup or it could be clutches getting worn. I'd suggest changing the trans fluid/filter; you'll be surprised how big a difference fresh fluid can make in your trans shifts.
  15. Yeah, I was planning on doing a trans flush soon. The tailshaft seal is worn, so it leaks just a tiny bit. I've been keeping a close eye on the fluid levels, though. Possibly the tailshaft bearing, since the seal is worn? I'm not sure if that would cause a problem like that. That would be drive line/ trans related, though, only thing springing to mind. I have a spare one, so I'll throw that in when I change the seal.
    The general consensus that I've found is that cats will knock if they're clogged enough. I've never heard of it, but I've asked several people now. I'm going to beef up the mount, too, if the knock doesn't stop with a new cat.
    I wasn't thinking of this, either, but I had to replace the charcoal canister a while back, maybe in Nov (which is why I didn't think of it) or so and that's about when the knock started, if I remember right. Maybe the unburnt gases that weren't getting recycled in the canister caused it to clog, then the misfire finished it off?
    I've never heard of a knocking cat, I've heard of rattling cats from broken combs rattling around in it. If I were you I'd get a mechanics opinion, we're just sorta throwin darts at a board for you trying to diagnose it over the internet.
  17. Cat's fixed- knock is completely gone. Jackasses didn't bother to clear the codes, so I have to wait to scan them. Until then I've come up with this
    I'm actually hoping that either a.) the cat was causing it and it'll clear by itself, but that still leaves me with the 1441 gremlin or b.) it's the egr which is causing both 1441 and 0300.
  18. A clogged cat will make a vehicle run like shot. If you think there is a rod knock, do this start, put on the brakes, then put in drive, give it some gas and if there are bad rods it will knock. Do not put a new cat unless you get a few more estimates. I believe this garage is trying to rip you off. Pm me if you need to. I'm a mechanic.
  19. Read the last post :p
    Cat was replaced last week and it fixed it. Only code I have left is 1441, which I'm not 100% worried about right now. Tried a few simple fixes (purge solenoid, hoses) but there is a ton of shit in the evap system that could cause it.

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