Lotus Elan

Sudden big end noise?

PostPost by: nigelrbfurness » Sun Jan 28, 2018 10:56 am

You need to be sure where the noise is coming from - top or bottom of the engine as other replies have said. A stuck valve can make you think you've got bottom end knocks as you get piston to valve noise and cam follower noise as a result. The fact it's only firing on three is suspicious but could be a red herring of course.

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PostPost by: NIGELAN » Sun Jan 28, 2018 11:40 am

Thanks NigelF,

Managed to get a brief look this morning.

Sadly I cannot feel any exhaust leak :(

Pulled each plug lead in turn, no change in noise.and same drop in revs so assume that all cylinders are firing and from what I have read this rules out a big end problem.

Frequency at 1100 rpm is about 4 beats per second, I assume this narrows it down to one cylinder but with a stethoscope I cannot detect any difference between cylinders when listening at cam covers or various points on the head, block or sump.

Have not done loose crank test or removed cam cover yet, not enough time today.

Noise seems to be just as loud whether I listen at top of engine or under sump.

I have made 2 recordings but do not know whether I can post these?

Thanks for all the interest.
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PostPost by: types26/36 » Sun Jan 28, 2018 12:08 pm

Dont know if its been mentioned but have you considered it may be a loose timing chain, if you dont want to remove the cam cover to check the adjustment, loosen the lock nut and turn in the adjuster slightly, you can usually hear the chain "whine" if it becomes too tight but then back it off slightly.
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PostPost by: NIGELAN » Sun Jan 28, 2018 12:38 pm

Thanks, will add to the 'to check' list for monday, a nice simple fix :D (if true!)
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PostPost by: types26/36 » Sun Jan 28, 2018 2:03 pm

NIGELAN wrote: a nice simple fix :D (if true!)

Yes providing it is only routine adjustment ...........(a slack chain) could have other implications with the sprockets/idler gear etc. as there is a "school of thought" that say a chain does not stretch :roll:
.......good luck anyway.
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PostPost by: oldelanman » Sun Jan 28, 2018 2:54 pm

A broken exhaust mount bracket on the back of the gearbox can produce alarming knocks and rattles which might appear to be engine/gearbox related. Check out this earlier post....
lotus-twincam-f39/help-identify-this-strange-noise-please-t40644.html
.... there is a video clip which might help you rule it out
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PostPost by: Chancer » Sun Jan 28, 2018 3:12 pm

As can a loose alternator bracket, crank pulley etc.

It would help if the OP could confirm that he does indeed know and recognise the sound of big end bearing failure. TBH I have my doubts as he drove it 100m and has not mentioned low o?l pressure, if a big end is shot enough to be heard at tickover only someone with zero mechanical sympathy could bear to raise the revs and drive the vehicle unless it was life and death.
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PostPost by: nmauduit » Sun Jan 28, 2018 3:26 pm

Chancer wrote:As can a loose alternator bracket, crank pulley etc.

It would help if the OP could confirm that he does indeed know and recognise the sound of big end bearing failure. TBH I have my doubts as he drove it 100m and has not mentioned low o?l pressure, if a big end is shot enough to be heard at tickover only someone with zero mechanical sympathy could bear to raise the revs and drive the vehicle unless it was life and death.

yes, the engine still running with oil pressure is what made me suggest to look into optimistic options... recordings of the noise may be of help, though I usually find it difficult to diagnose without facing the actual engine in the flesh.
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PostPost by: NIGELAN » Sun Jan 28, 2018 4:49 pm

Hi Chamcer, sorry should have replied to your earlier comment on "do I know what a big end sounds like" to which tne honest answer is "no". I did think I had said that the oil pressure is still very good.

Car was gently reversed back into garage at very low rpm after research on this forum seemed to indicate not a big end.

I did carry out some tests earlier today, see earlier posting, which seem to rule out big end.

Lots of things to check on monday, thanks.
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PostPost by: Chancer » Sun Jan 28, 2018 5:07 pm

I'm sure that it will turn out to be something very minor in the scheme of things.
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PostPost by: Mr.Gale » Sun Jan 28, 2018 7:29 pm

Drop a valve? You can buy a camera the looks through a probe that you could look in the cylenders through the spark plug holes.
https://www.harborfreight.com/digital-i ... 61839.html

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PostPost by: types26/36 » Sun Jan 28, 2018 7:39 pm

Mr.Gale wrote:Drop a valve? Mr.G


NIGELAN wrote: Pulled each plug lead in turn, no change in noise.and same drop in revs so assume that all cylinders are firing .


Unlikely to be a dropped valve if all cylinders are firing equally.
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PostPost by: MarkDa » Sun Jan 28, 2018 8:15 pm

Nigel
You could probably publish recordings on you tube and paste links to them here
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PostPost by: Mick6186 » Sun Jan 28, 2018 8:31 pm

Hi,
Four knocks a second at 1100rpm sounds like a valve or camshaft problem,
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PostPost by: NIGELAN » Sun Jan 28, 2018 9:17 pm

Okay.

I will check all ancillary possibilities tomorrow. Then timing chain, crankshaft movement and then off with the cam covers.

Will also try the YouTube suggestion - should be amusing!

Thanks again for all suggestions, very grateful.

Will report back.
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