Knock in bottom end at idle only, help with diagnosis

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lc-gtr-1969
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Knock in bottom end at idle only, help with diagnosis

Post by lc-gtr-1969 »

Hi everyone,

I have a straight six which has a dullish knock in the bottom end when at low idle only- like 600-700rpm. When idle is raised a touch, to say 850-900 it clears up. At 1000rpm I cannot really detect it at all. The motor is running as per normal. Revs to 7000rpm without issue (engine is relatively high spec).

I have good oil pressure at idle, about 40psi when cold and 20psi when hot at idle. The noise does not reduce whether hot or cold, only rpm. Currently has 20w40 oil- I have tried 20w50 and 20w60 with no real sign of improvement- seems just the same.

I am struggling to put my finger on the issue as all of the tests I have done have not really revealed anything.

I have removed each HT lead one by one and the noise does not noticeably reduce or go away. I did this to help see if the problem was located from one cylinder.

I have drained the oil and no real noticeable metal shavings etc. The engine hand cranks smoothly (hand turning crank).

Here is a short video which hopefully shows whats happening...
https://www.youtube.com/watch?v=vUJE5WU ... e=youtu.be

Any ideas?

Many thanks in advance!
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grandsport51
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Re: Knock in bottom end at idle only, help with diagnosis

Post by grandsport51 »

What engine ?
Standard diagnostics apply , stethoscope,Steelman chassis ear is my fave, works well on engine noises Ez to determine knock speed , 1/2 or full, or drive gears!!
Dave
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lc-gtr-1969
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Re: Knock in bottom end at idle only, help with diagnosis

Post by lc-gtr-1969 »

thanks for the reply!

Engine is an Australian straight 6, 202ci. Its extremely similar to inline 6 chev just a bit smaller... both GM designs.

ive used my stethoscope, and it has pinpointed that the noise is the bottom end, but it has also shown that noise is inaudable above 1000rpm. With the steph on the sump its quite clear, however thats all I have really been able to determine... that the noise is in the bottom end and reduces with rpm...

Any other ideas for further tests?
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superpursuit
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Re: Knock in bottom end at idle only, help with diagnosis

Post by superpursuit »

Have you checked for excessive thrust clearance on the crank? Try pulling and pushing on the harmonic balancer with your hands to check for any end float or play on your crank. If you can feel it - it's too much. Check with engine not running of course. Your video is set to private so we can't see it.
lc-gtr-1969
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Re: Knock in bottom end at idle only, help with diagnosis

Post by lc-gtr-1969 »

Cheers for the reply!

Yes, I have tried to feel for thrust play, and I cannot feel any by hand. I have tried to lever the HB back and forth and it all seems quite tight. I thought the same, possible thrust bearings too loose.

My apologies re video, here it is, fixed;https://youtu.be/vUJE5WUi6is
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Re: Knock in bottom end at idle only, help with diagnosis

Post by mag2555 »

It's bearing related , so pull that pan fast and find out what's going on now!!!!
The noise would seem to be stopping once that rpm increase has more oil volume and pressure moving into what I would say is a Rod bearing issue , or maybe a piston skirt issue
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
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Re: Knock in bottom end at idle only, help with diagnosis

Post by jed »

If it has stock cast Pistons it is a broken piston skir.
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Re: Knock in bottom end at idle only, help with diagnosis

Post by jed »

If it has stock cast Pistons it is a broken piston skirt.
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Re: Knock in bottom end at idle only, help with diagnosis

Post by MileHighMan »

Cut the oil filter open and look for metal....Other items that will knock at low speed are untorqued flywheell bolts or loose balancer bolt....If it uses camshaft gear drive instead of chain,,I've seen a burr on gear teeth cause knock very similar to main bearing knock....Dan.
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Re: Knock in bottom end at idle only, help with diagnosis

Post by lc-gtr-1969 »

jed wrote:If it has stock cast Pistons it is a broken piston skirt.
Thanks also for the reply... this crossed my mind, but as I didn't get any sign of metal in the oil, or shavings or glitter etc, I am started to question myself. Am i right in thinking a cracked skirt would show trace of metal in oil?
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lc-gtr-1969
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Re: Knock in bottom end at idle only, help with diagnosis

Post by lc-gtr-1969 »

mag2555 wrote:It's bearing related , so pull that pan fast and find out what's going on now!!!!
The noise would seem to be stopping once that rpm increase has more oil volume and pressure moving into what I would say is a Rod bearing issue , or maybe a piston skirt issue
Cheers... Unfortunately with the car its in I cannot pull the pan without removing the engine. I am happy to remove the engine but as it will be a month or so before I would have time to do this, I was hoping to narrow things down a little in the mean time. Aghh the fun!
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lc-gtr-1969
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Re: Knock in bottom end at idle only, help with diagnosis

Post by lc-gtr-1969 »

MileHighMan wrote:Cut the oil filter open and look for metal....Other items that will knock at low speed are untorqued flywheell bolts or loose balancer bolt....If it uses camshaft gear drive instead of chain,,I've seen a burr on gear teeth cause knock very similar to main bearing knock....Dan.
Cheers!

I have done this and no sign of metal so far...

The flywheel bolts could be possible, these engines are known to vibrate bolts loose from the fly... Would you expect the sound to come from the flywheel/ bell housing area when choked with a stethoscope (if this was the problem)? I'll check tonight.
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Re: Knock in bottom end at idle only, help with diagnosis

Post by DaveMcLain »

I was going to say loose flywheel too. I was also thinking about one of those that I rebuilt for a guy about 20 years ago, a 230 and it had a screwed up balancer and crank snout. It would knock at low speeds but you could see it wiggling around at the front of the engine.
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Re: Knock in bottom end at idle only, help with diagnosis

Post by Retroboy »

Mate it's standard Holden gudgeon noise. Every 202 Holden does it. Motor can only be a few weeks or months old and it will start. Don't worry about it that's just how they all are. Cheers Tony
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Re: Knock in bottom end at idle only, help with diagnosis

Post by Retroboy »

Just read a few other answers. Loose flywheel has a really pronounced sound. Piston skirts don't make any noise laying in the bottom of the sump. Haven't read everything but if it's rebuilt with cast pistons and it's a light noise that dissappears just off idle I'll put my left one on it.
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