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Thread: knock on startup

  1. #1
    Apprentice js5d's Avatar
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    Default knock on startup

    Hi All,

    I only owned my S3 2.25P (3B) for about 18 months, but eversince it has had a distinctive knock on startup, disappearing almost instantly when oil pressure builds. Maybe 2-3 seconds after starting.

    The engine runs fine, very smooth in fact, smooth and no knocking, but I was just wondering if this was a common issue with a quick fix. I checked the valve clearances and made small minor adjustments with no real effect, obviously changed oil and other regular maintenance. Oil pressure looks good.

    Is this just general bearing wear and should I just expect to change bearing shells at some point. Can this be done in situ or just drop in an overhauled short block at a some point?

    Looks like the PO changed the head (black) with temp sticker etc. no rust, Looks fairly new.

    Cheers.

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    Engineer phoenixdave's Avatar
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    Default Re: knock on startup

    Quote Originally Posted by js5d View Post
    Hi All,

    I only owned my S3 2.25P (3B) for about 18 months, but eversince it has had a distinctive knock on startup, disappearing almost instantly when oil pressure builds. Maybe 2-3 seconds after starting.

    The engine runs fine, very smooth in fact, smooth and no knocking, but I was just wondering if this was a common issue with a quick fix. I checked the valve clearances and made small minor adjustments with no real effect, obviously changed oil and other regular maintenance. Oil pressure looks good.

    Is this just general bearing wear and should I just expect to change bearing shells at some point. Can this be done in situ or just drop in an overhauled short block at a some point?

    Looks like the PO changed the head (black) with temp sticker etc. no rust, Looks fairly new.

    Cheers.
    If it disappears as soon as oil pressure builds up it could be big end bearing wear or cam follower wear. Either can be done with the engine in as long as the crank doesn't need removal. Could be worn pistons/ bores too I suppose. If yours has the oil filler in the rocker box you could try pouring a pint or so of oil in and starting the engine before it drains back to the sump; if this temporarily silences the knock then it's a fair bet that it's coming from the valve train somewhere. David
    1982 ex-mil 109 2 1/4 petrol on lpg series III, 'Phoenix'

    1983 2 1/4 diesel 88" series III, 'Willy', currently in bits.

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    Apprentice js5d's Avatar
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    Default Re: knock on startup

    Quote Originally Posted by phoenixdave View Post
    If it disappears as soon as oil pressure builds up it could be big end bearing wear or cam follower wear. Either can be done with the engine in as long as the crank doesn't need removal. Could be worn pistons/ bores too I suppose. If yours has the oil filler in the rocker box you could try pouring a pint or so of oil in and starting the engine before it drains back to the sump; if this temporarily silences the knock then it's a fair bet that it's coming from the valve train somewhere. David
    Thanks, I had not thought of poring in through the filler (fitted to the rockerbox), one challenge might be the timing since the knock stops after a few turns so it be hard to notice the pored in oil making a difference.

    I am thinking it is more likely to be the bearings knocking prior to oil pressure building up. When you say the shells can be changed in situ does this include the big end, middle and front. Surely you would need to drop the crank.

    Since it is not a problem during engine running, I will just save up for an overhaul, maybe this winter.

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    Engineer erubus's Avatar
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    Default Re: knock on startup

    The big end bearings can be changed in situ, but not the mains as too much needs to be pulled apart to get the crank out. I would expect main bearing wear to produce more of a rumble than a knock though.

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    Engineer phoenixdave's Avatar
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    Default Re: knock on startup

    Quote Originally Posted by erubus View Post
    The big end bearings can be changed in situ, but not the mains as too much needs to be pulled apart to get the crank out. I would expect main bearing wear to produce more of a rumble than a knock though.
    You can change the mains without dropping the crank, at least the middle and front ones just by pushing the shells round. Not sure whether you can get the rear bearing cap off with the flywheel casing/rear oil seal in place though or not. It is not ideal to change the big ends and not the mains because the big ends receive oil via drillings from the mains, so if the mains are worn enough to reduce oil pressure, the new big ends might not receive enough oil and therefore seize up. This effect might be reduced enough by replacing just the thrust washers each side of the centre main as they can be a major cause of low oil pressure if worn. David
    1982 ex-mil 109 2 1/4 petrol on lpg series III, 'Phoenix'

    1983 2 1/4 diesel 88" series III, 'Willy', currently in bits.

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    Default Re: knock on startup

    Does the knock coincide with the starter disengaging? And does it do it after starting, turning off and restarting with oil everywhere it should be?
    I replaced all the big ends, centre shims and big ends - all except the one at the flywheel end. Cured a bit of low rumble but not improved my knocking on a diesel. Turned out to be piston slap from a worn bore. Mine was constant knocking though not just on start up. I realised that bottom end knock dissapears when you go uphill - as the pistons are doing more work they hold the crankshaft down more I think.
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    Engineer holly76's Avatar
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    Default Re: knock on startup

    could it be a timing chanin issue?

    bafore plling the engine though, check nothing is catching like the exhaust or a heat shield. maybe trying with the starting handle to eliminate the starter(it could be loose)

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    Disco 1 TDi specialist zollaf's Avatar
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    Default Re: knock on startup

    i had a similar knocking on my audi 80. the engine needed some work anyway so i rebuilt it in situ, big ends, honed, new rings, tappets etc. imagine my suprise when it was exactly the same. turned out to be a tooth missing on the flywheel.

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    Apprentice js5d's Avatar
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    Default Re: knock on startup

    Quote Originally Posted by holly76 View Post
    could it be a timing chanin issue?

    bafore plling the engine though, check nothing is catching like the exhaust or a heat shield. maybe trying with the starting handle to eliminate the starter(it could be loose)
    Hi, appreciate your suggestion, but the knock is clearly linked to the oil pressure building up. It stops very shortly after the oil pressure light going off - like in 1-2 seconds after firing up. It does sound very much like when you have leaking self adjusting tappets like on the VW engines. Little pods that fill up with oil to adjust the tappet clearance, until they start leaking and settle down and then take a few seconds to pump themselves up. I had a Golf like that once - not a very good car.

    Maybe one day I will remove the head and recover the followers to see if there is some signs of excessive play. Main point is that the knock is not present when running - idle or load - so no immediate concern.

    Just wondering if this was common, but seems there could be many causes.

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    Engineer holly76's Avatar
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    Default Re: knock on startup

    Quote Originally Posted by js5d View Post
    Hi, appreciate your suggestion, but the knock is clearly linked to the oil pressure building up. It stops very shortly after the oil pressure light going off - like in 1-2 seconds after firing up. It does sound very much like when you have leaking self adjusting tappets like on the VW engines. Little pods that fill up with oil to adjust the tappet clearance, until they start leaking and settle down and then take a few seconds to pump themselves up. I had a Golf like that once - not a very good car.
    remember this is a landrover forum. if you want to mention huydraulic tappets, talk about the V8! they are usually noisy for a while if left stood. sudenly quieting down once they warm up, given a bit of throttle.

    waht speed is the knock crank shaft or cam shaft speed?

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    Apprentice js5d's Avatar
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    Default Re: knock on startup

    Quote Originally Posted by holly76 View Post
    remember this is a landrover forum. if you want to mention huydraulic tappets, talk about the V8! they are usually noisy for a while if left stood. sudenly quieting down once they warm up, given a bit of throttle.

    waht speed is the knock crank shaft or cam shaft speed?
    ok thanks did not realise these darn little hydraulic tappets were fitted in the V8 too! Good idea when new I suppose.

    Good question about the engine rpm. Since I did have to listen to this VW Golf with the shot tappet many years ago, I would say it is 1/2 RPM, thus valve, but then these 2.25 idle so slow (500-600) I could be mistaken.

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