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Thread: Vibrating/ knocking

  
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    Vibrating/ knocking

    Afternoon folks,

    I've posted about this problem previously but didn't resolve it so i thought i'd try again

    Basically after driving the car (2001 Astra SRi 2.2), for say 10 minutes or so, when it is at a standstill the exhaust will start to vibrate/ knock on the underside of the car. Its as though there is pressure or something from the engine thats making it vibrate, if i hold the revs at around 1500rpm it stops however when revving the engine it feels as though the car is going to stall, kind of splutters when i press the accelerator. If i turn the ignition off then fire her back up again the car splutters into life and there is a loud banging from the exhaust as if either a lot of backpressure has been forced out from the engine or something has ignited. This happens every time i drive the car, i can tell when its about to start doing it as the exhaust note changes to a very very deep burble when accelerating and when changing gear i can hear the exhaust vibrate/ knock on the underside of the car.

    After letting her idle for a while this will all stop and will be fine on start up but comes back once she's been driven for ten mins or so.

    I've had it into my local garage and they couldn't fathom out what was wrong with it, to be honest i did tell them to take her out for a good drive which they didn't do so this is probably why! They did mention that it couldn't be any form of 'backpressure' and could be a sticking valve

    I haven't got a clue what it could be and i doubt it could be a sticking valve.

    Has anyone got any ideas what's causing this or failing that does anyone know of a shit hot garage/ mechanic in the North West (preferably local to Warrington) that i could take it to. My car is quite heavily modified so i'd prefer not to take it Vaux.

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    Pete 2's Avatar
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    Are you sure this is the exhaust knocking, & not the gearbox.
    It could be a gearbox/engine mount shot, which will cause the exhaust to move too.

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    Yeah i'm certain its the exhaust, its like its coming from the engine and making the exhaust vibrate. If it was the gear box that wouldn't explain the kind of explosion i get when i start her up. The whole thing is pretty hard to explain to be honest but thats the best i can do. Its as though the car is going to cut out. I really want this sorting because its starting to really annoy me now!

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    Blocked Cat not letting the pressure escape?
    that would explain the vibrations and the engine running rough.
    Quick Test.
    Start the car up, go around to the back of the car and touch the tail pipe.
    Now drive the car until it happens again, jump out and then feel the temperature of the tail pipe(BE VERY CEARFUL THE PIPE MIGHT BE RED HOT).If it is you've found the problem.

    There is another way you can test the efficiency of the Cat, but you will need a Lazer Temperature Tester, some garages may have one but as you said if they don't test drive the car 1st they will get the wrong diagnosis.

    Dave.

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    I'll give that a whirl and let you know, sounds like it could be something like that. How or why would the CAT get blocked though? My exhaust used to blow just after the CAT, everytime i had it re-sealed it would go again not long after so in the end i had it welded instead. This vibrating etc always used to happen around the times when my exhaust was blowing so i presumed it was something to do that. Was hopeful that having it welded would sort the problem...but not long after having it done it came back and hasn't stopped since!

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    Drove the six miles to the gym this morning and did as you said, the exhaust was red hot! Now, i've had the pre-cat removed and a sport cat fitted so is it not normal for it to get so hot so quick? Also as i said before the exhaust note changes once the car has warmed up, sounds shit to be honest. Do you still think it may be the Cat thats knackered?

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    Yep,
    as you said the Exhaust was red hot they do get hot but not that hot at the Tail Pipe, unless there's a problem.

    Was the old Cat that you took off knackered?

    Have you still got the old Cat?

    Can you not refit the old Cat to see if that sorts the problem?

    Is the Sports Cat the correct one for the car?

    Is the Sports Cat blocked or damaged in anyway?

    Did you get it remapped when you fitted the Sports Cat?

    Dave.

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    Quote Originally Posted by mfivebmw View Post
    Yep,
    as you said the Exhaust was red hot they do get hot but not that hot at the Tail Pipe, unless there's a problem.

    Was the old Cat that you took off knackered?

    Have you still got the old Cat?

    Can you not refit the old Cat to see if that sorts the problem?

    Is the Sports Cat the correct one for the car?

    Is the Sports Cat blocked or damaged in anyway?

    Did you get it remapped when you fitted the Sports Cat?

    Dave.
    I bought the car off Steve who used to post on here, used to be a mod i think. He had already had the pre-cat removed and a sport cat fitted. I presume its the right one for the car. Its also had a remap and has the DBilas inlet manifold fitted and full stainless powerflow exhaust. Haven't got a clue if the sports cat is damaged or blocked, is there anyway i can find out?

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    Take it to a different Gge mate,
    tell them the faults (Rough running and exhaust vibrations) and tell them that the Exhaust Tail pipe gets red hot,

    but TBA all they're gunna do is order/replace the Cat and charge you loads of money.

    If it was my car i would fabricate a De Cat pipe or borrow a mates just to test it.

    Dave

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    ok mate, cheers for the advice. I'll try and source another Cat and see if that solves the problem. Failing that i'll have to take it somewhere else! Thanks again!

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